Radio for high school students

Item Type text; Thesis-Reproduction (electronic)

Authors Ashe, John Lawrence, 1910-

Publisher The University of Arizona.

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Download date 04/10/2021 02:43:21

Link to Item http://hdl.handle.net/10150/553513 RADIO FOR HIGH SCHOOL STUDENTS

by

John L. Ash®

*******

A Thosis

submitted to ths faeulty of the

Department of Education

In partial fulfillment of

the requirements' for the degree of

Master of Arts

in the Graduate College

University of Arizona

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Approved: JLAj£ Director of Thesis -.5

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TABLE OF CONTENTS

Chapter Page

I. INTRODUCTION...... 1

General Importance of Radio..... r-i c\j oa Problem of this Study...... Limitations of the Study...... m c

Plan of Procedure...... if ^

II. THE PAST,PRESENT AND FUTURE OF RADIO

Purpose of this Chapter Brief History of Radio. Opportunities in Radio...... • 10 Broadcasting...... 10 Communication...... 11 Factories...... 13 Servicing...... 14 Distribution...... 14 Electronics...... 15 Television...... 15 Facsimile...... 15 Therapy...... 15 As a Hobby...... 16 Probable Future of Radio...... 10

III. TOOLS...... 31 Selection and Use of Tools...... 21 General...... 21 Soldering Iron...... 22 Pliers...... 24 Screwdrivers...... 25 Hand Drill and Bits...... 25 Miscellaneous Tools...... 26 Summary...... 27

IV. CONSTRUCTION OF A RADIO RECEIVING SET...... 28 General Discussion...... 28 Schematic Circuit Diagrams...... 29

i 1 3 2 U i. 1 Chapter Pag©

Construetion of a Crystal Set...... 31 // Circuit Diagram...... 32 Antenna Construction.... U . i Coil Construction...... //./ir-vv The Variable Condenser.. The Crystal...... The Fixed Condenser.... The Phones...... Other Parts...... --- Arrangement of the Parts

V. HOW A BROADCAST STATION WORKS

Wave Propagation...... f Water Waves...... } ' Mechanical Waves...... Picturing Alternating Current...... Relation of Radio Waves to Other Waves.... The Broadcast station...... Modulation...... Parts of the Station......

VI. HOW THE RECEIVING SET WORKS ? Crystal set...... Antenna Circuit...... How the Coils Work...... The Tuned Circuit...... Transformer Action...... How the Crystal Works...... How the Bar Phones Work.... Summary...... Vacuum Tube Receiver...... Comparison with Crystal sot VII. ELECTRON THEORY AND ELECTRICITY Need for Theory...... Electron Theory...... Molecules...... Atoms...... Protons and Electrons...... Electricity in Terms of Electrons Electric Current...... Conductors and Insulators..... Current vs. Electron Flow.....

ii Chapter Page

T i l l . ELBCTROiacafBTISM...... 62 Permanent Magnets...... 68 Lines of Force. Electromagnetic Fields...... Field surrounding a Wire...... Field Surrounding a Coil...... Inductance...... Inductive Couplings...... Effect of Coils on D. C. and A. C Impedance...... Summary......

IX. CONSTRUCTING A VACUUM TUBS RECEIVER...... Selecting the Type of set to be Constructed. Purpose for Which the Set is Designed.... Cost...... Frequency...... Volume of Output...... Construction...... The Chassis...... The Circuit...... -...... How to Operate......

X. ELECTRICAL UNITS, LAWS,

Definition of Terms. Amperes...... Ohm#...... Volts...... Laws...... OhmTs Law......

Heat Loss...... Length of Wire as Related to Resistance... Diameter of Wire as Related to Resistance. Magnetic Attraction and Repulsion......

XI. TWO ELEMENT VACUUM TUBES Importance of Vacuum Tubes...... How a Two Element Vacuum Tube Works Filament Emission... Action of the Plate. Detector Action....

Half-Wave Rectifiers 883822 2 3 SSSSSiSgSS 8 83SS82888 Full-Wave Rectifiers...... 91 Advantages of Each Type...... 92

ill Chapter Page

XII. THREE ELEMENT VACUUM TUBES...... 93 Importance of the Discovery...... 93 How the Triode Worka...... 95 Amplification*.*...... * * 95 Amplification Factor**...... 95 Characteristics Curves and Amplification.. 96 The Triode as a Detector...... 97

XIII. HOW THE ONE-TUBE RECEIVER WORKS...... 99 Circuits...... 99 Relationship of Tubes to Circuits... 99 Operation of the First.One-Tube Set... 99 How the Tube is Used...... 99 Purpose of the Various Parts of the Circuit...... 100 The Grid Leak...... 100 The R. F. Choke...... 102 Batteries...... 103 _____ Summary...... 103 Operation of the Second One-Tube Set. 104 Regeneration...... 104

XIV. CONDENSERS...... 106

Theory of Condensers...... 106 Definition of Terms...... 106 Capacity or Capacitance...... 106 Voltage and Battery...... 108 The Effect of the Size of the Plates..... 106 Effect of spacing on Capacity...... 108 Dlelectric Constants...... 109 Alternating Current and Condensers...... 110 Blocking and Coupling Condensers...... Ill Voltage Rating...... 112 Breakdown Voltage...... 112 Working Voltage...... 113

XV. THE NAMING, CONSTRUCTION, AMD USES OF CONDENSERS...... 114

Classification...... 114 As Fixed and Variable..... 114 By Nature of the Insulation 115 By Construction...... 116 By Use...... 116

It * Chapter Page Construction of Condensers...... 117 Air-Dieleetrlo Condensers...... 117 Paper Condensers...... 117 Fixed Mica Condeneera...... 118 Electrolytic Condensers...... 118 Dry Electrolytic Condensers...... 119 Uses of Condensers...... 120 Importance of Condensers...... 121

XVI. POWER SUPPLIES...... 122 Purpose...... 122 Theory...... 122 Schematic Diagram...... 122 Four Parts of a Power Supply...... 183 The Filter...... 124 The Bleeder-Voltage Divider...... 186 Construction...... 128 Selection of Chassis...... 188 Laying Out and Construction of Chassis.... 128 Mounting the Parts...... 129 Wiring...... 129

ADDING AUDIO AMPLIFIERS TO THE ONE-TUBE SET--- 131

Types of Coupling...... 131 Transformer Coupling...... 131 Theory of Transformer Coupling...... 133 Choke or Impedance Coupling...... 134 Resistance Coupling...... 13# XVIII. VACUUM TUBES 137

Triodes...... 137 Filament Structure..... 137 Cathodes...... 138 . Multi-Grid Tubes...... 139 Tetrodes...... 139 Pentodes...... 141 Beam Power Tubes...... 141 Multi-Purpose Tubes...... 142 Gas Filled Tubes...... 143 Mercury Vapor Rectifiers 143 XIX. RADIO FREQUENCY AMPLIFIERS 145 Reason for Radio Frequency Amplifiers in Receivers...... 145

v Chapter Page Sensitivity...... 145 L Selectivity...... 146 <* Types of Radio Frequency Amplifiers...... 147 Untuned Radio Frequency Amplifiers...... 147 Tuned Radio Frequency Amplifiers...... 147 Begenerative Pre-Selection...... 148

XX. OSCILLATION; RESONANCE...... 149 Oscillating Circuits...... 149 Definition...... 149 Coil and Condenser...... 149 The Vacuum Tube in an Oscillating Circuit. 150 Audio Oscillators...... 151 Test Oscillators...... 152 --- £j3eat Note Oscillators...... 153 Use of Oscillators in Transmitters...... 154 Resonance...... 156 Tuned Circuits...... 156

XXI. SUPERHETERODYNE RECEIVERS...... 157 Theory of the Superheterodyne...... 157 Block Diagram...... 157 Preselector...... 157 The Frequency Converter...... 158 The Intermediate Amplifier...... 169 The Second Detector...... 159 The Audio Amplifier...... 16# Necessity for the Preselector...... ISO Advantages of the Superheterodyne...... 161 • Sensitivity...... 161 Selectivity...... 161 -All-Wave Receivers...... 162

XXII. SELECTING A RADIO RECEIVER...... 163

How to Select a Radio Receiver...... 163 Purpose of the Chapter...... 163 Select a Superheterodyne...... 163 Select a Big Chassis...... 163 Speakers and Cabinets...... 164 Controls...... 166 Desirable Circuit Features...... 166 Preselector...... 167 All-Wave Receivers...... 167 Locating the Radio in the Home...... 168 Proper Placing in the Room...... 168

vi Chapter Page XXIICLASSIFICATION OF AMPLIFIERS...... 169 S' Three Classes of Amplifiers...... 169 \— Class A Amplifiers...... 16# Class B Amplifiers...... 170 Class AB Amplifiers...... 173 C __— ---— Class C Amplifiers...... 173

XXIV. SERIES AND PARALLEL CIRCUITS.. .. 176 Batteries...... 176 Dry Cells...... 176 Lead-acid Storage Cells...... 177 Size of Colls...... 178 Cells in Series and Parallel Circuits.... 178 Condensers...... 179 Condensers in Series end Parallel...... 179 Resistors...... 181 Resistors in Series and Parallel...... 181 Wattage Ratings...... 182

xxv. mmsMiTTiw...... — ...... — .... les Government Licenses...... 183 A License is Required for Transmitting.... 183 Classes of Licenses...... 183 Preparing for Communications Work...... 185 Where to Start...... 185 Advantages of an Amateur License...... 187 Requirements for Class B and C Licenses... 188

XXVI. THE INTERNATIONAL MORSE CODE...... 190 How the Code is Used...... 190 The Two Codes...... 190 Sound of the Continental Ced^...... 191 Sound of the International Morse Code.... 191 Learning the Code...... 191 The International Morse Code...... 191 Learning by Sound...... — ------191 Correct Sending,...... 195 Learning to Receive Code...... 195 Automatic Code Machines...... 197 The Code May be Hard for You...... 197

Til Chapter Page

XXVII. THE AMATEUR’S RECEIVER...... 198 Changing the *30 Tube Receiver for Short Waves...... 198 Importance of the Receiver...... I98 Short Wave Coils for the *30 Tube Receiver.. 199 Band-Spreading the *30 Tube Receiver...... 200 Adjustments...... 200 A Popular Amateur Receiver...... 201 Modern Two-Tube Receiver...... 201

XXVIII. SHORT WAVE RECEPTIONS...... 205 How the Short Waves Act...... 205 DZ,...... 205 The 1715-kc. Band...... 205 The 5500-lco. Band...... 206 The 7000-kc. Band...... 206 The 14,000-ke. or 14-Mo.Band ...... 206 The 28-Mo. Band...... 807 The 56-Mc. Band...... 807 The Ultra-High Frequencies...... 208 The Kennelly-Heaviside Layer...... 208 Fading and Skip Distance...... 206 Length of the Ground Wave...... 212 Time of Day...... 213

XXIX. TRANSMITTING EQUIPMENT...... 214 Planning the Transmitter...... 214 Should You Use Phone or Code...... 214 Advantages of the Phone Transmitter...... 815 Learn the Code Thoroughly...... 215 Phone Transmitters Cost More...... 216 C. W. Transmitters are Easier to Construct...... 216 Should you Build or Buy a Transmitter.... 216 Bow Much Power...... 217 Selecting the Type of Tubes to Use...... 218 Selecting the Circuit...... 219

XXX. ANTENNAS...... 222 Selecting a Transmitting .Antenns...... 222 Importance of the Antenna...... 222 Types of Antennas...... 222 Long Wire Antennas...... 223 Directors and Reflectors...... 224 Other Array...... 225

viii Chapter Page XXVII. THE AMATEUR15 RECEIVER...... 198

Changing the *30 Tube Receiver for Short Waves...... 198 Importance of the Receiver...... 198 Short Wave Coils for the *30 Tube Receiver.. 199 Band-Spreading the *30 Tube Receiver...... 300 Adjustments...... 200 ■ A Popular Amateur Receiver...... 201 Modern Two-Tub© Receiver...... 201

XXVIII. SHORT WAVE RECEPTIONS...... 205 How the Short Waves Act...... 205 DI...... 205 The 2715-rkc. Band...... 205 The 3500-kc. Band...... 206 The 7000-kc. Band...... 206 The 14,000-ke. or 14-Mc. Band...... 206 The 28-Mo. Band...... 207 The 56-Mc. Band...... 807 The Ultra-High Frequencies...... 208 The Xennelly-Heaviside Layer...... 208 Fading and Skip Distance...... 208 Length of the Ground Wave...... 212 Time of Day...... 213

XXIX. TRANSMITTING EQUIPMENT...... 214 Planning the Transmitter...... 214 Should You Use Phone or Code...... 214 Advantages of the Phone Transmitter..... 213 Learn the Code Thoroughly...... 215 Phone Transmitters Cost More...... 216 C. W. Transmitters are Easier to Construct...... 216 Should you Build or Buy a Transmitter.... 216 How Much Power...... 217 Selecting the Type of Tubes to Use...... 218 Selecting the Circuit...... 219

XXX. ANTENNAS...... 222 Selecting a Transmitting Antenns...... 222 Importance of the Antenna...... 282 Types of Antennas...... 228 Long Wire Antennas...... 223 Directors and Reflectors...... 224 Other Array...... 225

viii Chapter Page The Half-Wave Doublet...... 225 Advantages of the Doublet...... 226 Height Above Ground...... 227

XXXI. COMCLUDINO STATSMSMTS...... 228

BIBLIOGRAPHY...... 250 APPENDIX I...... 233

Abbreviations...... 233

APPENDIX II...... 234

Index...... w.. 234

ix CHAPTER I INTRODUCTION

General Importance of Radio During the past thirty years radio has grown from a scientific curiosity to an industry of major importance.

The size of the industry can be realized when we consider that it employs over one hundred thousand persons and has an annual payroll of more than two hundred fifteen million dollars, or a daily payroll of about two thirds of a million dollars. During 1938 radio sales alone amounted to about

$210,000,000. $16,728,533 were taken in by broadcast stations and chains during 1938.

Some forty-five million receiving sets are in use in the United States today receiving programs from eight hun­ dred broadcast stations in the United States as well as from many short wave stations in foreign countries. Any house­ hold of average size and income which does not have at least two receiving sets of modern design is considered by radio dealers as a good prospect to purchase one or more radios.

But broadcasting is only a small part of the radio field. There are fifty-five thousand licensed amateur stations in the united States, there are radio beacons along all of our important air lanes, there are hundreds of ship stations and stations aboard airplanes, there are press 2

stations throughout the country, weather reporting stations, coast guard stations, and forest service stations for pro­ tection against forest fires, as well as experimental tele­ vision and frequency modulation stations.

With all of these transmitting and receiving sets scat­ tered throughout the country it is obvious that many men are needed who are trained in the building, servicing, and oper­

ating of this equipment. It is with this phase, the

training of technicians, with which this study is concerned.

Problem of this Study

The purpose of this study is to combine modern theories' and practices into an elementary text book which will be

suitable for use In radio classes of the tenth grade level,

or in radio clubs sponsored by the high school.

Limitations of the Study

The text is designed only for use as an introductory course in radio for use in radio classes of the tenth grade

level and in clubs sponsored by the high school. No attempt

is made to completely train individuals for work In any of

the various fields of radio, nor is an attempt made to qual­

ify the student for an amateur operator's license.

Plan of procedure

These data have been gathered from a large number of

technical books, periodicals, experiments, notes taken in 3

college radio class, and experiences of the author in teaching of radio and in five years of operating an amateur radio station.

The general plan of the course is to divide the time evenly between class and shop work. A basic design of radio receiving set has been selected which progresses from a crystal set to a three tube receiver as the course advances.

The electron theory and wave propagation are taken up early in the course and used as a basis for explaining the oper­ ation of the various parts of the receiver.

The reader should bear in mind that the following chap­ ters are addressed primarily to students of the tenth grade level. CHAPTER II

THE PAST, PRESENT, AND FUTURE OF RADIO Purpose of the Chapter The purpose of this chapter Is to give the reader a broad view of the opportunities in radio. Most of the chap­ ter is concerned with conditions as they exist today, with

Just enough of the history and future of radio to give a complete picture.

Brief history of Radio The real beginning of radio came in 1887 when Heinrich

Hertz discovered that electrical waves could be sent through

space. His equipment was not very efficient, so he was only able to send these waves for a few yards. Later these mew found waves were named "Hertzian waves" in honor of the man who first discovered them. It would have been interesting

to watch Hertz trying to make sparks jump across a small gap in the loop of wire which he used as a receiver. His

transmitter or broadcasting station consisted of two metal

balls on the ends of rods and arranged so that he could change the spacing between them. By using high voltages

Hertz made sparks jump across from one metal ball to the

other. This was the first "spark gap transmitter". We get

the same effect today la setae of the older types of auto­ mobile radios. Each time the spark Jumps between the points 5

of the spark plugs of the automobile it acts as a miniature spark gap transmitter. When this happens we hear a sound in the radio. If the motor is running at an ordinary speed this sounds very much like machine guns in sound pictures.

If you have an all-wave radio you can pick up this "Ignition noise" by tuning the radio to about fourteen megacycles, or fourteen thousand kilocycles, and waiting for a car to pass on the street near the house.

An Italian boy of high school age, Guglielmo Marconi, became very much interested in Hertz*s experiment. He set up an experiment similar to the one Hertz had performed, and improved on the method. Marconi got an idea that the

Hertzian waves might some day be useful in sending messages through space. His greatest fear was that some famous sci­ entist would perfect a system for sending messages before he (Marconi) would be old enough and know enough about science to succeed in doing it himself. Marconi invented the anten­ na, or aerial, when he connected a long piece of wire to his sending set and put the. other end as high above the ground as he could get it. He called this wire an antenna because it looked like a long feeler on some giant insect. Another important improvement by Marconi was the use of the coherer for receiving signals. The coherer was a small glass tube containing metal filings. Whenever a signal was received the filings would Jump to the upper end of the tube. At first these filings had to be knocked down by striking the 6

coherer with a pencil or similar object. Then an automatic striker was invented which would automatically knock down the filings after each signal was received. The coherer could not be used to listen to a broadcast station because it makes only a buzzing sound when a signal Is received. It did work well enough that Marconi succeeded in sending sig­ nals for distances of several hundred yards, a mile, then across the English Channel, and finally in 1901 across the

Atlantic Ocean.

By the time Marconi succeeded in sending the letter "S" across the Atlantic, there were a number of people inter­ ested and experimenting with radio. In 1906, General

Dunwoody of the united States Army, discovered that a mate-r, rial formed by heating sand and coke In an electric furnace could be used to receive radio signals^ This material is known as carborundum. About the same time Dunwoody was ex* perimenting with carborundum, G. W. Packard discovered that a common form of lead ore, galena, could be used for

receiving radio signals. Several other materials were soon

found which could also be used. The general name "crystal

detector" was given to these materials. Because it was

cheap and performed fairly well the crystal detector became

the standard receiving sot for several years.

While the crystal detector was finding favor with most experimenters, a still better form of receiving device, the

vacuum tube, was being developed. In working with his 7

electrio lamp, Thomas Edison made a discovery which meant very little to him at the time but which led to the devel­ opment of radio as we know it today. Edison discovered a field of negative electricity surrounding the carbon fila­ ment of his electric lamp. By placing a small metal plate in his electric lamp, Edison found that current could be made to flow across the space inside the glass between the plate and the filament. Also he found that current flowed when the plus pole of his battery was connected to this plate and the negative pole to the filament. Current would not flow when he reversed these connections. This discovery is now known as the "Edison Effect". Later when we are studying how the vacuum tube works you will realize the importance of Edison’s discovery.

The action of any device used in receiving radio sig­ nals depends on its ability to let current flow in only one direction. In 1896 Dr. A. J. Flemming in experimenting with the Edison Effect realized that he had the same action found in the coherer but in a more efficient form. He then introduced to radio the first vacuum tube and called it the

"Flemming Valve". The name valve Indicated its ability to let current flow in only one direction.

In radio the various parts inside of a vacuum tube are known as elements. The Flemming Valve contained two -ele­ ments, the filament and the plate (See Fig. 1). About the time the crystal detector made its appearance in radio. 8

Dr, Lee Deforest got an idea that started radio on its present course. His idea was to place another element in the vacuum between the filament and the plate (Fig. 2). By

©hanging the voltage on this third element, called the grid, he could control the amount of current flowing between the filament and the plate of the tube. The name grid came from the shape of the element used in the Deforest tube.

In modern tubes the grid completely surrounds the fila­ ment and the plate surrounds both filament and grid. The structure can be seen by looking down through the top of a tube or better yet by breaking the glass of a burned out tube and examining the elements.

The addition of the grid made it possible to control the flow of current by email changes in voltage on the grid.

For the first time we had a method of Increasing or building up the strength of a signal. This building up processe known as amplifying, makes it possible to hear signals which are too weak to be heard with a crystal set. Amplification 9

also makes possible the use of loud speakers on radio sets and public address systems. Because of the high cost of vacuum tubes, they did not find wide spread use for several years. Better methods of manufacturing developed with increasing demand, causing prices of vaeutm tubes to drop rapidly. As prices came down vacuum tubes completely replaced crystal detectors in com­ mercial receiving sets.

Radio first found commercial use on ships and soon proved its value in the saving of lives. The entertainment value of radio next received consideration, in 1920 the first permanent broadcasting station, KDKA, was established in Pittsburg, Pennsylvania by the Western Electric & Manu­ facturing Company.

Have you ever noticed that the call letters of all broadcasting stations in the United States begin with the letters "K* or "W"? When stations came on the air In large numbers each selecting his own call a great deal of confu­ sion resulted. Representatives of the important governments of the world got together and assigned certain letters to each country, to be used as call letters by all stations in that particular country. The letters "K", «W", and "N" were assigned to the United States. W has been reserved for use of stations belonging to the united States Navy, leaving "l” and "K” for broadcast and other stations, it is now possible to tell in what country a station is located by the 10

call letters of the station. For example all Mexican calls begin with "XE", stations In Japan begin with "JM, and Cana­ dian stations with wVI*.

The limited number of frequencies which are available to each type of radio transmission makes it necessary that we have some form of control over the use of these frequen­ cies. In the United states it is the Federal Communications

Commission that licenses and regulates radio stations.

Other countries have different methods of controlling radio, but all abide by international agreements so that confusion Is avoided. ■

Opportunities in Radio Most of us are more familiar with broadcasting and radio servicing than any of the other phases of radio, but any person who is interested in radio work should have some idea of the possibilities in other fields even though he may not expect to contact them directly. Some of the major divisions are described briefly so that you may get a broader view of the opportunities offered in radio.

Broadcasting: Broadcast stations and networks require large numbers of men trained in operating broadcast equip­ ment. The Federal Communications Commission (a Commission appointed by the President to control telegraph, telephone, and radio communications) will not allow any radio station, regardless of its type, to be on the air without a licensed operator being present at the station. This means that u

where stations are broadcasting most of the twenty-four hours each day several technicians must be employed and work in shifts. Usually more than one technician Is on duty at a time in all except the smallest stations. :

To secure a license to operate a broadcast station, the technician must know not only the theoretical and practical sides of operating a station but must also know the laws and regulations governing the operation of such a station. He is directly responsible to the Federal Communications Commis­ sion for what goes on the air. In case there is something in a program contrary to the rules and regulations of the

Federal Communications Commission or to national or inter­ national law it is his duty to see that it does not go on the air<

ThnvG are many other jobs around the broadcasting station that make use of the knowledge of the technician even though they do not require a license. The announcer with.technical training can best place his microphones to pick up the event to be broadcast. The program manager can do a better job of selecting and planning his programs if he knows some of the fundamentals of radio. The architect who designs the studio should certainly know some theory in order to do his job well. Often the program managers and station managers start out as technicians and work into more important and better paying jobs.

Communication: The first important use for radio and still a very important one is radio communication, the sending of messages, from place to place by radio. First we think of ship-to-shore stations used in keeping the ships in contact with land bases and with each other. The importance of radio on shipboard was first shown in its ability to save lives, and this is still an important factor. Nearly all ocean going ships carry radio transmitting and receiving equipment requiring trained men to operate them. The larger passenger liners print newspapers on board with news received via radio. The business man can keep contact with his office by the use of radiograms even though he may be half way around the world.

The telephone system makes use of radio for overseas communication. Xf you wanted to talk by telephone to some one in London all you would have to do is call the local telephone operator, give her the phone number, or name and address of the person wanted, make arrangements to pay for the call, amd leave the rest to the telephone company. As

soon as the person called is located you can carry on a

conversation just as though he were across town. Your voice

is picked up by the telephone and relayed by wire to Bound

Brook, New Jersey. There it is put on the air by high powered radio transmitters and sent to Deventry, England, where it is picked up by short wave receivers and relayed by

telephone to the person to whoa it is directed. His voice

is picked up by telephone, sent to Deventry by wire, relayed 13

fey short wave radio to Bound Brook where it is again put on the wire and carried to your telephone.

Point-to-point Radio Communication is the name applied, to short wave stations owned by newspapers and other large industries for rapid handling of messages* Each of these is manned by a crew of teehnieians holding government licensee of various grades.

Aircraft radio is becoming more and more important every day. Co-pilota on large transport planes are radio operators as well as pilots. Radio-beacons, weather broadcasting stations, land-to-plane and station-t©-station communication all require licensed operators. Many radio service men are required in this branch of radio to keep the equipment oper­ ating properly.

Police and harbor radio are becoming increasingly important. The United States is rapidly becoming a network of police radio stations with station-to-station and two way station-to-car radio combining to form a complex, well organised system.

Factories; Seven hundred million dollars worth of new radio sets are manufactured each year. Replacement parts are needed for the forty-five million radio receiving sets now in use. There are fifty-five thousand licensed amateurs, and probably that many more unlicensed experimenters throughout the United States all requiring parts for their experiments. Meters and testing equipment must be manufae- 14

tured for thorn and for the thousands of radio service men.

Many men are required to work in the factories produelftg and

asaeabling these parts. The man with technical training has

first chance at a job.

Servicing: One of the largest and most important

fields in radio is servicing of receiving sets. It is in

this field that most radio men get their start, then later branch out into one of the more specialized fields as the

opportunity arises. The wide variety of sets which come to

the service man for repairs make his day more interesting.

A wide background of training is necessary to be able to

repair this variety of electrical equipment which comes to

the radio service man’s bench. A field closely related to

radio servicing Is installation and rental of public address

systems. This is a profitable side line which may develop

into a full time job. *

Distribution: Thousands of men are employed in whole­

sale and retail stores selling radios and radio parts.

Again training plays an important part, for the successful

dealer must be a “jack of all trades** as far as radio is

concerned. He must be able to talk intelligently about each part that he sells and be able to help the service man or

experimenter to select the part best suited for his partic­

ular purpose. Many men who start their radio careers ser­

vicing receivers build up good businesses as dealers without

investing very much money at any one*time. 15

Electronics: Another field growing in importance dally is electron!# controls. Probably the most familiar use of electronic control is the photoelectric cell, or electric eye, need in burglar alarms and for counting objects which pass a given point. A familiar sight along our main high­ ways is a pair of black boxes, one on each side of the road, for counting the vehicles using the road. Television: At the present time television is still in the experimental stages although several of our large cities have television stations which are operating on a regular schedule and several thousand television receiving sets have been sold. Probably it will be only a few years until the service man, the dealer, and the technician will have to include the problems of this growing industry in his daily work.

Facsimile: This field, the sending of pictures by radio, is closely related to television but will in all prob­ ability outgrow television. By facsimile transmission whole newspapers can be transmitted and received in a few seconds time so it is possible that this very new form of radio transmission may some day completely revolutionize the news­ paper industry. Not many men are employed in facsimile today, but its future has great possibilities.

Therapy: The manufacture, sale, maintenance, and oper­ ation of machines which are used in treatment of diseases is fast becoming an important field for men trained in the 16

as© of radio circuits and equipment.

As a Hobby: To some fifty-five thousand people in the

United States, and twenty theaeand others throughout the world, radio is the most fascinating and most exciting of all hobbies. Not only does the amateur radio operator listen to others in far away cities and countries, but actually talks to them using his own personal and often home made transmitting equipment.

Most amateurs are attracted to the art by the thrill of

DX but only a comparatively few find this phase of amateur

radio to be their chief Interest. DX is the term used by radio men to mean transmission over long distances, in the

early days of radio, it was the thrill of hearing voices at

a distance rather than the entertainment value of radio that

held the Interest of listeners. The wide spread sale of

"all-wave" radios during the pest few years has shown that

DX still has an important place in radio. Many are experi­

menters who like to build end rebuild receivers and trans­

mitters. For the experimenter there are endless circuits

and combinations of parts which he can try, always with the

possibility that he will uncover a new idea which may be of

value to the industry. . :

The "rag ehewer", the fellow who likes to visit, can find plenty of others on the air who enjoy nothing better

than to spend their time visiting. There is even a club,

called the "lag Chewers Club" which has a large membership • 17

of fellows interested in this phase of radio. .

The collector finds a fascinating hobby in collecting

QSL cards. The QSL card is a verification and a "thank you" card, usually in two or more colors, sent to the station after the first contact showing appreciation of the sender for the visit via air.

The builder has his chance in constructing much of the equipment used in an amateur station and then trying it out on the air to test his handiwork.

The student can always find something more to study in radio. There are dozens of theories that need better explan­ ations, there are many problems to be solved in circuit design and operation of transmitters, receivers, and antennas used in the amateur station. For example, these questions have not been satisfactorily answered: How do sun spots cause interruption of radio communication? Why is there more static in summer than in winter? Why is it that in some localities radio reception is better than in others?

Can we some day send power by radio economically? Can we design radio sets which will be free from static?

The traffic man, the man interested in communications involving message handling, can find plenty of messages to send, relay, or deliver, and can find other amateurs who will cooperate with him in this interesting field.

The man who is interested in public service can find a great deal of pleasure in the building and operating of 18

equipment for use in case of emergencies sueh as floods, tornados, and earthquakes when land lines are down end radio is the only means of communication available. There are Army Amateur nets and naval Reserve Communication Nets which he can join so as to be of assistance in time of emergency. . - . - ■ . ■ : . . ' . - . : Aside from the personal satisfaction of amateur radio the experience gained forms a valuable background in case the hobbyist ever decides to enter any of the commercial fields.

Probable Future of Radio

It is always difficult to predict the future of an industry, and especially of a comparatively new industry.

However it is fairly safe to say that there will not be any radical changed in the basic theories of radio although there are still many points which need to be cleared up.

There will no doubt be a steady expansion of the Industry as a whole for many years. The saturation point for the sale of radios in the United States has not been reached and probably will not be for several years even if nothing new comes along to stimulate sales. Revival of the battery set in the form of a portable, and the wide spread of automobile radios have greatly increased the market. In all probability other similar ideas will cause still further increases in possible markets. 19

Television will probably b@ perfected within the next few years so that it nay become an Important Industry in

Itself, fhe addition of vision to sound in broadcast

stations would doable the size of the staff necessary for

operating the station. It is not too much So predict She

transmission of colored scenes by television in the fairly new future.

Facsimile^ the sending of pictures by radio, is sure to see rapid growth within the next few years. It is even

possible that in a few years the morning paper will be delivered by air, broadcast from a central station and re­

produced In the home receiving set. Facsimile transmission

has even greater possibilities than its companion, television. Ultra-high-frequency equipment will no doubt be perfected

opening up many more channels, thus making room for more stations.

Frequency modulation is the newest field for the exper­

imenter, This new use of an old. idea seems to give promise

of true fidelity, natural sounding reproductions, with

frequency modulation, the striking of a match sounds like

"striking of a match" not like tearing of a window screen as

it does in the ordinary broadcasting system.

One other improvement needed for making sounds come out

of the receiver as they were when they went into the trans­ mitter is in loud speakers. At the present time only a very

small part of the power supplied to a loud speaker is 20

converted into sound waves and this is far from being a per­ fect reproduction of the original sound. Probably much will be done soon to improve the quality of sound reproduction.

Perhaps frequency modulation will solve the problems of television causing that field to begin a very rapid expan­ sion.

Some of the leaders in the field of radio have even predicted a personal wave length for each person but at the present time this does not seem possible in the near future.

In conclusion radio has had an exciting past, a very rapid growth, is now big business, and in all probability will continue to grow for many years. CHAPTER III

TOOLS Selection and Use of Tools

General: In general the better selection of tools you have to work with the better job you can do. Also, better quality and better condition of your tools shows up either

in time used or in the appearance of the project. Obviously

it is not practical for each of us to have a complete metal

shop and a complete wood shop even though we probably could

use all of the tools in both shops in the construction of

radio sets. Somewhere short of this ideal we must find a

compromise, it is the purpose of this chapter to help you

find the compromise best suited to your needs.

The selection of your tools will depend a great deal on how much you expect to use them, if you are a confirmed

experimenter you may already have some of the tools necessary.

In this case you will probably find it more economical to buy fewer tools and get better quality, in this way you can

build up, a little at a time, a kit of high grade tools which will last for a long time.

On the other hand If you plan to use your tools only

occasionally you probably would not be justified in buying expensive tools. In this case you probably would be better

off if you buy your tools at the "five and ten", mail order

house, or chain auto-supply store. An example of this 22

selection is in buying on electric soldering iron. Radio service men who use their irons continuously day after day often use irons costing in the neighborhood of three and. a half dollars. But for the fellow who uses his iron only four or five hours a week will find that a cheaper one will serve the purpose just as well and will give him more money for other things. Soldering irons, which will last the occasional user several years, can bo purchased at chain auto-supply stores and five and ten cent stores for twenty- five cents to a dollar.

There are a few tools which are absolutely essential for the beginner. These will be dissmssed individually, and then a few others which may be considered as optional to be added later if desired will be listed under miscellaneous tools. ' :

Soldering Iron: The first essential tool in radio work is a good soldering iron. Whenever electric power Is avail­ able an electric soldering iron is far bettor than one which depends on some outside source of heat* Since the electric soldering iron is practically universally used among radio men the discussion is limited to this type. It is more convenient to have two irons available, one of about

150-watt sise for heavy work and another 75-watt iron for light t o rk and for getting into tight places. However, an iron of 100-watt rating is a good compromise and will be found suitable for most soldering jobs. As mentioned before, the quality and prices of electric soldering irons vary greatly, hut probably one of the cheaper irons will bo moat satisfactory for the beginner.

Most soldering iron tips are made of copper and will become coated with a black scale If not properly "tinned". \

By tinning ve mean covering the tip completely with solder so that it will stay bright and clean. To tin a new iron* plug it in and let it warm up. As soon as it is warm enough rub the point first in rosin and then in solder until the entire point is covered with solder. If no rosin is avail­ able some rosin core solder can be melted and allowed to form a puddle on a tin lid or on a scrap of metal. By rubbing the tip carefully in this puddle of molten solder, tinning can be accomplished readily. Here is a trick used by service men to keep the tip clean and bright. After each soldering job, before laying the iron aside, give it a flip to throw all excess solder from the iron. If you do this you probably will have no trouble keeping your iron in first class condition. If the tip does become coated with the ■ black scale or oxide it can be removed by scraping the point with a knife blade or by rubbing It on a piece of flannel tacked to the top of the bench. In case the point becomes pitted, it should be filed down to a new point and retinned.

The secret of success in making soldered connections is to get the Joint hot and then apply solder to the heated joint.

Often this is best accomplished by holding the soldering iron 84

oa the under side of the connection and applying the solder to the top. As the wire gets hot enough, the solder melts and runs down over the wire forming a good connection. Do not use more solder than necessary and make your eonneotions neat. Electrical efficiency is almost directly proportional to the"neatness of the soldering job. :

Another Important thing to remember in soldering is that the surfaces must be clean before the solder will stick.

When using enameled wire the wire must be scraped to remove the enamel. An"old pocket knife makes an excellent scraper, or a small piece of emery cloth, or sand paper may be used.

It is a good idea to form a habit of cleaning each piece of wire before soldering. Always use rosin core solder for radio connections to insure good connections and freedom from noise." ' ' . .: - ' . ■

Make or buy a metal stand to hold the iron when it is not in use, and form the habit of laying your iron on its stand each time you finish using it. This will help to pre­ vent getting burned accidentally on your iron, and will help to keep your bench from getting burned spots.

Pliers: Even the beginner will find use for two types of pliers. The first of these, and the one used most often, is a pair of long nosed pliers. The long nosed pliers are used frequently when soldering to hold wires and small parts in place until the solder hardens. They are also very use­ ful for reaching Into tight places to hold a nut or to ss

recover small objects wbleh have been dropped into Inaccess­ ible places.

The second pair of pliers should be of the flat nosed type. A pair of ordinary pliers such as used around auto­ mobiles will serve nieely. The chief use of this type is in holding bolts emd turning nuts in mounting parts, if it is necessary to buy a pair then you should select a pair of side cutters rather than ordinary pliers.

In case you do have a pair of ordinary pliers, then you should buy a pair of diagonal cutters instead of the side cutters.

Screwdrivers: You will need at least two screwdrivers and possibly three. One should be of medium.size, about five or six inches long, and the other should be a small .

screwdriver about two or three inches long. Later on you will:probably want a larger screwdriver and possibly one or two more intermediate sizes. But two are enough for the beginner. Screwdrivers of the correct size are not as likely to ruin the slot in the screw. .

- Hand Drill and Bits: Another essential tool for begin­ ners and experts alike Is a good hand drill with a set of assorted round shank drills. It is belter not to buy the cheapest hand drill as these are usually too small to drill efficiently and are likely to slip when using any but the smallest bits. The drill bits may be purchased from a radio-supply house in assorted sizes most suitable for radio 26

work. Later on if you eontimie to do much construction work you may want an electric drill, but the cost la not justifi­ able for the beginner. - , .

When drilling in metal, the center of the hold should be dented with a center punch before attempting to start the drill bit. If this is not done, the bit will have a ten­ dency to "walk" or move away from the place where the hole is desired. If a drill bit becomes too hot it is likely to lose its hardness or temper. A good rule to follow is to never get your drill bit so hot that, you cannot hold It in your hand immediately after finishing the job. When drill­ ing through metal a light oil, preferably cutting oil, may be used to keep the bit cool. The speed and pressure varies with the material used and will soon be learned as you begin to use your drill. Be sure to hold the drill in the same position all during the drilling operation to prevent the bits from bending or breaking. This is especially necessary when using the smaller bits.

Miscellaneous Tools; other tools which are essential

but do not require special technique to operate are: a hammer (any kind), steel rule, center punch, file (medium

coarseness), and a knife.

Other tools which may be considered as optional and

which may be added later if desired, are a vise (four inch

size), hack saw, square, carpenter * s saw, carpenter *s brace

and hole cutter, set of socket hole punches, tin shears, 27

eleetrle drill, additional screw drivers and drill bits, and dozens of other tools best suited to your individual needs.

Summary; The essential tools are; soldering iron (elec­ tric if possible), two or three pairs of pliers, two or more screwdrivers, a good hand drill and assortment of bits, a hammer (any kind will do), steel rule, center punch, file (medium coarseness), and a knife for scraping. Other tools are useful but not essential. If you feel that you cannot buy all of the essential tools at once for your shop, and do not have any to use then start with a soldering iron, a pair of pliers, and a medium size screwdriver. Add to them as you can afford to.

Good work demands good tools. A few minutes spent reg­ ularly oiling, cleaning, polishing and sharpening tools will pay you well in time, appearance, and satisfaction. CHAPTER IV CONSTRUCTION OF A RADIO RECEIVING SET

- - r '> - ' •■■■ . General Discussion

If the radio engineer wore faced with the problem of securing a radio receiving sot and had his choice of building it or buying a factory made set he would probably buy the factory made set. His decision would be based on the idea that the saving in cost would not be worth his time. If your entire interest in radio is the securing of a good receiving set then you would probably be better off to follow the example of the radio engineer and buy a factory made set.

Mass production has brought the price of radios down until there is very little difference in price whether the set is ready to operate or gathered up as a kit of parts to be put together.

However, if you want to learn what makes a radio work and how it is put together, then build it yourself. If you want to learn the fundamentals of radio there is no better

/ . / ' \ ' ' •- ■ ...■ .•■■■.; , ■ - - way than to build radio sets and make then work, of course you can not learn all about how a set works by merely put­ ting it together. If you study each part as you are using it, if you follow your theory carefully as each part is used in a new way, if you keep your construction and your study going along hand in hand, then you will get to know the fundamentals of radio.

Do not get discouraged if your set does not work the first time— it may not. Cheek your wiring carefully to see that you have all wires in the proper places. A good way to insure getting all of your wires in place is to mark each connection with a pencil on your diagram as soon as you have soldered the connection in your set. After soldering each connection try to pull it apart, if you have failed to make a good connection, the wire will pull out easily.

Although the crystal set has heen replaced by vacuum tube receivers this is a good set for the beginner to build first. It is-the simplest type of receiver, and yet a thorough understanding of the construction and operation of a crystal set is a big step toward understanding other types of radio equipment. Build your set and study until you are sure you know the purpose of every part.

Do not get discouraged if you are not able to under­ stand completely seme of the theories as you come to them.

Radio is too large a subject to understand all of its phases the first time you meet them. Have patience and after you have come across the same type of situation several times you will begin to understand it more clearly.

Schematic Circuit Diagrams

In studying radio we find that the plans for nearly all sets are drawn using symbols to represent the various parts. This type of plan is known as a schematic circuit diagram or schematic diagrams. There are several reasons for using the schematic diagram. Before you have worked with radio very long you will want to make drawings yourself. Perhaps you will want to copy dircuit diagrams from a library book.

Unless you are a pretty good artist you would have a hard

time drawing pictures of each part. Sven if you could draw the pictures successfully it would take too much time. It

is much easier and much faster to use symbols to represent parts. Another reason for using symbols to represent parts

is that many parts are the same electrically but different

shapes or sizes. The symbol shows the electrical connections.

Another very important reason for schematic circuit diagrams

is that the experienced radio man can tell more about a

radio set by looking at the schematic diagram than he can

from a picture diagram or even from looking at the set it­ self. Once you have become familiar with the symbols used

you too will like schematic circuit diagrams. In Fig. 3 you

will find the symbols used in construction of your crystal

set. A good way to learn what a new part looks like is to

refer to a catalog from a radio-supply house. These catalogs

will be sent free upon request and are invaluable in esti­

mating costs of a new project. a

entenna "'3 coil (air core) T -9 induetanoe ©oil a rtr ground ' ^ or3 Ctransformer (air core)

Z p fixed condenser variable inductance

r c flr “i*J _ variable condenser 9)...... phones . 7| movable plates v

no electrical connection ■ ■ ■■ V - f

Fig. 5 — Symbols used in Radio

As you have already seen in Figs. 1 and 2, a vacuum tube is represented by a circle, in some of the never diagrams the circle has been lengthened into an ellipse in order to have room to put in all of the elements. In some oases only the elements are shown without the circle. Other symbols will be introduced as needed.

Construction of a Crystal Set

As we mentioned before, the- construction and under- standing of the operation of a crystal set is a big step toward understanding the operation of other radio equipment.

Ordinarily, crystal sets are good for broadcast reception for distances not greater than twenty-five miles, if you live more than twenty-five miles from the nearest broadcast X i- ~ ? n u Xe ” prrrc

f > 7 7 ^ 2 Raa^fa* I ‘ ^ ^4 f

J 3 10 ^ M l'o t s

SY -«• &f» co be*; i %f_ 4 . T # o .-vVAAA_yyy)'v— | H eld sl'iiiv . 4— # e f 3 j : e jevom ^j | \

— 5 • B-^‘i

e t&***X5 asrid yov " e vd St X30«? ed end eXai:ie 5HEuais si )b /yq )# aoei sreri )# xebxd ►•i*: e^aasieXe »M4 %ljso @&##& foo^boilal ed XXXs Biodtissps

S'dO S 0*1 j clod ceaoi#*** ew ba

j) :z v 'fS <8 le .^oidaidqc 3d3 to sai&aafe to mi#**t? -:t; sdd ^^bnfiXeiebajy bxenoi *d9& ladeYio eXiX*arus*0

X tf& B P B X SUBad . .daeas roa seonafaib xol ft &#!&* ^vll-/jneiij ssrij eioe avil 32

station it will probably be better not to attempt tie oon-

•*raction of the crystal set, but be sure you find out l#w if-works. However, if you do live near a broadcast station you will find the experience gained well worth your time and effort.

Circuit Diagram: Since you will be using schematic diagrams In practically all of your construction, we will

start with a schematic diagram of the crystal set. (See

Fig. 4). By looking at Fig. 3 we can find the names of the previous parts. Referring to a catalog from a radio-supply house we find there is rather a large assortment of parts all having the same ratings but made by different companies,

end having different sizes and shapes. If makes very little

difference what size or shape the parts are in this set as

long as they have the same ratings as those given in the . • . >■ / diagram. ■ ■— ...... _ ... . < , - ,

Fig. 4

Antenna Constmetion: Suppose we start with the antenna

and discuss the parts as we come to them in the diagram. 33

Next ?/e will discuss parts not shown but nesessary such as the chassis (base), panel, and terminals. Lre shall leave the discussion concerning the way in which, they operate until a later chapter. The Important thing now is to get started with the construction. Beginning at the left we find the symbol representing an antenna. In the case of a crystal set there is no power used except that which comes through the antenna. For that reason we should make sure that we have a good antenna in ' ■ - . /'- ; .... - order to get our signal In the phones loud enough to hear without effort. The antenna should be from seventy-five to one hundred feet long and as high as possible above the ground. Fig. 5 shows the construction of a typical antenna.

One end is fastened to a short pole nailed to the house, and

the other to a tree. If you do use a tree be sure to fix the ’ • ■ . antenna so that it will not break when the tree sways in the

wind, in the diagram, a spring similar to those used on 34

eereen doors is placed between the last insulator and the tree. Fig. 6A shoea another method of allowing for seay, A pulley is fastened to the tree. Then a piece of rope is tied to the insulator and passed throu^i the pullay. A : • ■' ■' - ■' #••’/ ■ ' - weight of fifty or seventy-five pounds is then tied on the end as shown in the figure.

Fig. 6

The lead-in wire should be well insulated and should

come through the window or wall through a porcelain or glass insulator as shown in Fig. 6B. In place of this Insulator you may use an insulated copper strip designed to come

through the top or bottom of the window. These strips may be

purchased at the five and ten cent stores for a nickel. If

you do use an insulated lead-in strip, be sure to solder the

lead-in wire to it so that you will have a good electrical

connection. All antenna connections should be soldered.

Coil Construction: Coming back to our diagram, we find

that we have an air core transformer. This transformer is made up of two Inductors, or coils, wound as shown in Fig.

— ...... ;—; — — _ Fig. 7

The core is two inches in diameter and four inches long. It may be either a cardboard or bake!Ite tube, solid wood or any other material not made of metal. The primary consists of thirty-two turns of No. 28 wire wound so that each turn touches the next (close wound}. When you come to a place for taps put a twist in the wire. This will make it easier to adjust your taps. By tap we mean an electrical connection between the ends of the coil. A center tap is a connection half way between the ends of the winding. Fasten both ends of the coil securely by passing the ends through holes bored in the form. If the form is of solid wood drive a tack at each end of the coil to hold it in place. Leave about an eighth of an inch apace between the primary and secondary windings. The secondary is wound with No. 28 D.C.C. (double cotton covered) wire and consists of ninety turns, close wound, and tapped as shown. The numbers indicate the num­ ber of turns from the top of the coils marked "T". The secondary must be wound in the same direction as the primary. 36

When the coil is finished it should be given a coat of "coil dope" or clear lacquer to keep out moisture and to help hold the windings in place. Make sure all insulation (including the lacquer) is scraped off the taps.

The Variable Condenser: The variable condenser' is the ordinary size used in broadcast receivers. If you have a condenser in good mechanical condition it will probably work just as well as a new one. These variable condensers have many shapes and sizes so that it is impossible to tell you just how to fasten yours to the base. All of them provide for mounting solidly either on the base or on the front panel. The purpose of the coil Lg and variable condenser C2 is to form a tuned circuit which will select the station you want. It is a variable condenser, or set of variable con­ densers, which you turn each time you tune in a station on any radio set of modern design.

The Crystal: Referring to the diagram again, we find the symbol for a crystal and next to it is written the word xtal. Xtal is the abbreviation used by radio men to mean crystal.

The crystal may be of any of the popular types, but it is better to select one of the fixed type, open type crystals such as the galena have spots which are not sen­ sitive so careful placing of the contact is necessary to find a good spot. The contact used for this type of crystal is a fine wire spring, and is commonly known as the "cat whisker". m

The cat whisker usually has to be moved each time you want to listen to your crystal set, and at times it is hard to find a good spot oa the crystal. Fixed crystals do not have cat whiskers and are always ready to operate.

The crystal usually works better one way than it does the other, so when you get your set hooked up try reversing the connections to find out which way it works best. The Fixed Condenser: The purpose of the fixed condenser is to "by-pass” the radio frequency current so It will not get into the phones. By-passing is used often in radio work, and will be discussed more thoroughly when we study the oper­ ation of condensers. For this set, just remember that the by-pass condenser allows radio frequency ourrent to pass through it but does not allow direct current to pass through. The cheapest condenser you can get haying the.proper rating will be satisfactory since there is very little voltage in a crystal set. > ; ......

The Phones: The phones may be any of the common types used in radio work. A telephone receiver does not work very well in this circuit because of its low resistance. The phones should have a rating of two thousand ohms or more.

Remember the word ohms — we will hear of It. again. Other Parts: We have discussed all of the parts shown in the schematic diagram but we need a few more parts to / hold all these together. The base on which a radio is built is known as the chassis. If you will look at your house 38

radio from the back side you will find that She chassis is made of metal. Nearly all commercial receivers do have a metal chassis. However, your crystal set and first vacuum tube sets can be built on a wooden base. This type of con­ struction is known as breadboard construction. A board about six inches square and a half inch thick will serve nicely as a base for your crystal set. The front panel may be a piece of "presdwood” or bakelite six inches long and five or six inches high. This may be fastened to the base with wood screws. . ; :

Other parts which you will need are terminals, or binding posts for the antenna, ground, and phones. These are not essential but improve the appearance of your receiver.

They may be taken from an old radio or from certain types of old batteries, or may be purchased for a few cents.

Arrangement of the Parts: It makes very little differ­ ence how the parts are arranged in this set except that the condenser should be mounted so that the knob will come through the panel at the same distance from each side. This makes the panel look balanced. The coil should be placed so that it will be easy to adjust the taps. The terminals for connecting the antenna, ground, and phones should be located near the back edge so that the set will be easy to hook up. Mark each terminal as you wire the set so that you can look the set up easily even after you have had it put away for some time. m

Mount all of your parts before you start to wire the

set, arranging them so that the wires will be as short as possible without destroying the neatness of the job. Neat wiring not only looks better but is much easier to trace in

ease something goes wrong with the set.

Cheek your wiring carefully before trying the set to

see if it works. Turn the condenser a complete turn each time you make a change in the coil taps. Continue to adjust

the taps on the coil, and reversing the taps on the crystal

until you get a signal, and then adjust for the loudness. You have a real thrill in store when you hear the first sig­

nal on a set you have built. You will get thrills again and

again as you build more complicated receivers, but none

quite like hearing a signal on your first set. CHAPTER V

HOW A BROADCAST STATION WORKS Wave Propagation

Water Waves: If a stone is thrown into a pool of still water, waves spread out in all directions along the surface of the water from the point where the stone strikes. If we place a cork on the water and then drop in the stone we find that even though the waves are moving rapidly away from the point struck by the stone the cork merely bobs up and down rather than following the motions of the wave tops. This experiment shows that the particles of water are not moving forward but that the wave is merely a wave of energy.

Mechanical Waves; Here is another example of this wave movement, or wave propagation, as it is called. Line up a set of dominos standing each domino on end close enough to the next one so that as we push the first one over it will strike the second domino knocking it over. The second domino knocks down the third, the third knocks down the fourth and so on down the line until all have been knocked over. The movement of each domino was small and yet the wave moved for several feet.

A wave motion similar to radio waves as we often think of them can be seen by use of a long rope. Tie one end of the rope to a tree or post. Stand back far enough to hold the rope fairly rigid. By a quick up and down motion of the 41

hand holding the rope a wave can be started which will travel to the other end of the rope. (See Fig. 8A). By continuous up and down motion of your hand you can get a series of waves similar to those shown in Fig, 8B. This

Fig. 8 wave motion is similar to the drawings which we use to show a.c. (alternating current) electricity. Alternating current electricity is an electrical current which changes its direc­ tion often. The distance between the tops (crests) of two waves is known as the wave length. In Fig. 8B the distance from "C" to "Dn is one wave length. Wave length can also be measured between the troughs as between ”1" and "F" in Fig. 8B or between any two corresponding parts of the waves. The number of waves which pass a given point in one second is known as the frequency of the wave, in the case of the cork on water, the frequency is the number of times per second that the cork rises or falls.

Picturing Alternating Current: As we mentioned before. alternating current is current which changes its direction of flow frequently. If we start with the average voltage as our zero line in Fig. 9, wo find that our voltage builda

Fig. 9 from the average "A” to a high positive voltage as at "B".

Next the voltage decreases to zero as shown at "C”. From. "C" the voltage becomes negative, (flows in the opposite direction), to a peak "D" which has the same value as the peak MBM. The negative voltage then decreases to a zero at a point "IS" and the cycle or wave is complete.

All of the crests above the line marked zero Indicate current flowing in one direction, while all of the waves formed below the zero line represent current flowing in the other direction. The height above the zero line at any

point indicates the voltage. A more detailed discussion of

alternating current will be found in a later chapter.

Relation of Radio Waves to other Waves: Most of the

electric current used for houses is of the alternating type. . - ; : : V ■. . ■ ■ ; ■ ■ ■ ■ Current flows through an electric light bulb first one way

and then the other sixty times per second. Bach time it

passes through the bulb It causes light and heat to be given 43

off. in this sixty cycle electricity, current flows in each direction sixty times per second. If Instead of having sixty wave crests pass a given point we should insrease the number to about ten thousand cycles per second, we would find that we no longer need a wire to carry these electrical waves. When the frequency reaches this point we call the waves radio waves.

The frequencies useful for radio communication over very great distances"extend from ten thousand cycles per second to about thirty million cycles per second. Higher frequen­ cies than thirty million cycles per second are used for short distances up to about twenty-five miles.

If we continue to increase the frequency of our waves they begin to lose the characteristics of radio waves and become heat waves and infra-red rays. Further increase in frequency gets us into the visible light rays, still higher in frequency we find the ultra-violet rays, x-rays, gamma rays (given off by radium), and the highest frequency waves known, called cosmic rays. Cosmic rays come from out in space, and their source is not known.

Sound waves are vibrations of the air rather than elec­ trical waves. The frequencies of sound waves which can be heard by human ear range from sixteen cycles to about twenty

thousand cycles per second. These frequencies are known as audio frequencies. You cannot hear sixty cycle electrical

current until it is changed into vibrations of air. An example of this in the humming of a transformer^ Often ' '-I? the plates in the core of a transformer can move slightly as the current changes direction. This moves the air

slightly and sets up sound waves in the air which strike the ear drum as a sixty cycle hum. In building all-ole ctrio

sets you will become familiar with sixty cycle hum. --# • The Broadcast station

. . Modulation: Radio waves used for broadcast purposes

begin at five hundred fifty kilocycles or five hundred

fifty thousand cycles per second. The ear is sensitive up

* to about twenty thousand cycles and so cannot hoar these

radio frequencies even if they were changed into vibrations

of ttTe air. Since it is necessary to use these high fre- - quencies in order to send waves without the use of wires- we must have some way to change them so that the- listener can '

hear the grograrnu This bflngs us to a big word, "Modulation". Amplitude modulation, the most common form of modulation, means the

changing of radio waves by increasing and decreasing the strength of the radio signals at the sane rate the sound waves strike the microphone. This can»be seen by referring to Fig. 10.

a The illustration, Fig. 10A., shows the wave form of the radio frequency wewe generated in the broadcast station.

This frequency is known as the carrier frequency. It is this carrier that determines the poeitioa of the station on your radio dial. Fig. 10B shows the wave form of the audio frequency to be used in controlling the power of the carrier.

In Fig. IOC we have the modulated carrier wave. The dis­ tances above and below the line "X" show the amount of power being transmitted. It increases and decreases at the same rate and the audio signal strikes the microphone. Parts of the Station: The block diagram, Fig. 11, shows

Fig. 11

how the parts of the broadcast station work together. The

radio freojitacy generator is a high frequency generator .f- - .. using vacuum'tubes and generating the carrier frequency.

This is then amplified in the radio frequency amplifier and

power amplifier. The-microphon© picks up sound waves and

changes thgm to electrical waves. These electrical waves

are then amplified in the speech amplifier and modulator. 46

The modulator ie the part which supplies the power to in­ crease and decrease the strength of the carrier wares. The power amplifier supplies the modulated carrier ware to the antenna where it is radiated out into space.

In the next chapter we shall see how these waves are picked up by your antenna and changed back into sound by the crystal set. Later we shall see just how each of these parts does its job. CHAPTER 71 HOW THE REClIVmO SET WORKS

Crystal set Antenna Circuit; The purpose of the antenna of the

crystal set is to pick up the radio signals sent out by the broadcast station. The radio waves strike the antenna and

cause an electric current to flow in the wire. The current

flows down the lead-in wire and through the coil L^, of Fig. 4, to the tap and from there to the ground.

How the Coils Work: As the current flows through the primary coil a magnetic field is set up around the coil

L^. Since the secondary winding of the transformer Lg is on

the same coil form, it is also in the magnetic field. As we

shall see from experiments suggested in a later chapter,

whenever a wire or coil of wire is in a magnetic field which

is building up or breaking down current flows in the wire or

coil. The radio waves are alternating current so the field

builds up around L, first one direction, and then breaks . v : . . down and builds up in the other direction. This causes cur­

rent in Lg to flow first one way and then the other. The

direction of current flow changes as often as the frequency

of the carrier wave of the broadcast station. For example,

if you are tuned to six hundred twenty kilocycles then the

current flows in each direction six hundred twenty times per ■eeond.

The Tuned Circuit: Signals from many stations are striking the antenna aad causing current to flow. This makes it necessary for us to have some method of selecting the station we want to hear. This is done by the coil Lg and condenser C^. For any given size of coil and condenser in a circuit of this type, alternating current can be made to flow at only one frequency. If we change the size of either the coil or condenser we change the frequency at which the current will flow in the circuit. in this set, and in most others, wo change the effective size of the condenser,by- turning the plates of the rotor in and out of the spaces between the stationery or stator plates. If we want a lower frequency we turn the plates in and for a higher fre­ quency we turn them out. The capacity of a condenser is determined partly by the distances between the surfaces or plates. As we turn the rotor plates out, parts of each plate become much farther from the stator plates. This decreases the capacity. Turning the rotor in brings more of the surface of each rotor plate close to the stator plates, causing the capacity to be increased. This type of circuit used to tune to the desired frequency is known as a tuned circuit. By proper setting of the condenser we can turns to the carrier frequency of the station to which we want to listen. Tils causes radio waves of this particular fre­ quency to set up currents in the tuned circuit Lg and 49

Allows any other frequencies to flow to ground without effecting the receiving set.

Transformer Action: We could have hooked the antenna to the top of Lg and made It work but there Is a definite advantage In having the two coils. In any transformer, the ratio of the voltages on the primary and secondary windings is the same as the ratio of the number of turns on the two windings. In this transformer Lg the primary coil has thirty-two turns (when all are being used) and the sec­ ondary has ninety turns. This means that their turns ratio is nearly one to three, or that the voltage will be three times as great in the secondary as in the primary. By this ■» transformer action we have increased the loudness of our

signal three times, or we can hear a signal only one-third as loud as before. ,

Here you might be wondering why we do not use an even greater turns ratio and increase the signal even more.

There are good reasons for not doing this. First, we cannot

increase the number of turns on the secondary without

decreasing the frequency to which it will tune. This means

that if we want to increase the turns ratio we must decrease

the number of turns on the primary. If we decrease the num­

ber of turns in we find that the strength of the magnetic

field which couples the two coils is not as great as before. » . ' - ' - So all we can do is find a compromise between field strength

and voltage gain. We shall study more about coils and so

transformers in later chapters.

How the Crystal Works: The current e oMng Into the antenna and into and Lg is alternating current at radio frequency. The modulated carrier picked up by the antenna may be represented by the wave form shown in Fig. 12A.

A B C

Fig. 12

All of the waves above the zero line indicate that the cur­ rent is flowing in one direction. All waves below indicate current flowing in the other direction. Usually we say that the top half is positive and the lower half negative, although strictly speaking alternating current has no positive or neg­ ative. Or rather, each end becomes positive and then nega­ tive as often as the frequency of the wave. For convenience we pick out one end and coll it positive. In the diagram showing wave forms we let the top half represent current flow in a positive direction.

Suppose we say that current flowing in the direction from "X" to ”Y" in Fig. 12B is positive and from "Y" to "X" is negative. A crystal has the ability to allow current to flow in one direction and keep it from flowing in the other 51

direetion. For convenience let us assia® that the current can flow in the direction from "X" to "Y" In Fig. 12B. le agreed that we would call this direction the positive direc­ tion. Each time Gurr#a#.., flows - through the circuit in this direction it can pass through the crystal and through the phones. Bach time the current flows in the opposite or neg­ ative direction in the tuned circuit it cannot flow through the crystal because of its ability to pass current in only one direction. The result is that we get a wave form as shown in Fig. 12C. Only the positive half of the wave is left after the current has passed through the crystal. We have changed alternating current to direct current, it flows in only one direction.

This ability to pass electric current in one direction only is known as rectification. A device capable of recti­ fying current is known as a rectifier. / ' ' , '■/ • y , How the Sar Phones Work: The phones consist of small electro-magnets which cause the diaphraa to move toward the magnet. The amount of movement depends upon the strength in which the current flows through the coil, (See.Fig. 13). The signal causes the diaphram to vibrate. The vibration sets the air in motion producing sound waves which are similar to the sound waves striking the microphone at the broadcast station. After we study coils and transformers you will have a clearer idea of just how the magnet works. QtafAn'M EleCTre

C*-Se. C e p e r

Fig. 13

The signal as shown at in Fig. 12 is a radio fre­ quency, It is much too high a frequency for us to hear, in the crystal set the phones change radio frequency to audio frequency. The diaphram of a phone is too heavy to vibrate at radio frequencies so follows an average signal. This is shown by the heavy line in Fig. 14A. This.average produces

A B

Fig. 14

a sound wave (Fig. 14B) similar to the sound wave which

entered the microphone at the broadcast station.

Summary; Briefly, the signal is picked up by the

antenna and brought to the set through the lead-in wire. In

the transformer L«, the signal Is amplified to about three m

times its original strength. The crystal changes the radio frequency alternating current to direct current by allowing it to pass through in only one direction. The ear phonos change the radio frequency to audio frequency by following the average value of the rectified radio frequency signal.

The movement of the diaphram in following this signal sets up sound waves in the air similar to the sound waves striking the microphone at the broadcast station.

Vacuum Tube Receiver

Comparison with Crystal Set: _The Vacuum tube.receiver works very much like the crystal set except that vacuum tubes have the ability to amplify or increase the strength of the signal. Vacuum tube receivers have antennas, transfor­ mers, tuned circuits, and phones or loud speaker®. The action of these parts is the same as in the crystal sets. In studying vacuum tubes we shall see how the vacuum tube rec­

tifies and amplifies. Vacuum tube receivers do not have to depend entirely upon power picked up by the antenna. Addi­

tional power is supplied by the batteries or by the one hun­ dred ten volt line. CHAPTER VII HZCTROM THEORY AND ELECTRICITY

Meed for Theory

In order to thoroughly understand the workings of radio sets it will be necessary for us to understand the fundamen­ tals back of this operation* Whenever possible we explain observed actions in terms of scientific laws, but laws do not always explain all of the things which happen. This is especially true in radio and electricity, we cannot tell what is happening inside a vacuum tube by looking at it.

The difference between a theory and a law is that the law has been proved and theory has not. Theories seem to be true but until we can prove them to be true In all oases they do not become laws of science.

In order to understand electricity it is necessary for us to understand both laws and theories which concern the workings of electricity.

^ ■■ ' " Electron Theory - . ' : . . .. -

Back of all of our theories In radio as well as in

Chemistry and Physics is the electron theory. This theory of the structure of all matter seems to explain many things which take place in radio as well as in the other sciences.

Since we cannot prove the truth of the theory it cannot be classed as a law. Nearly all of the new ideas in radio are 55

explained in terms of the electron theory because this seems to be the most logical explanation.

Molecules: If we could take a drop of water or a par­ ticle of scene other material such as common table salt and break it up into very small pieces eventually we would get the smallest particle of water or salt which could exist.

This particle is known as a molecule. It may be defined as the smallest particle of a substance which can exist and still be the same substance. Molecules are very small. It would take millions of then to form a particle large enough to be seen with a high powered microscope. According to this theory all matter is made up of molecules.

We can think of air as being made up of billions of molecules all moving rapidly in different directions. If we confine a certain part of this air in a basketball the mole­ cules are continually striking the walls of the container.

If we put in more molecules by pumping in more air the ball soon becomes very hard. The reason for this is that there are billions of molecules striking each square inch of the bladder each second. A molecule is veryrsmall and the force exerted by a single one would be to© small to measure, but when we get enough molecules confined in the ball the com­ bined force of all of the billions of molecules striking each square inch will give us pressures of several pounds.

Molecules of all substances are moving rapidly but some are not free to move as far as others. There are forces • 56

which hold them together. These molecules which can move only in certain areas form what we know as solids. Molecules

of liquids are more free to move than those of solids but

not as free as the molecules of gases. Atoms: If we take the molecule of water and break it

up even more we find that we no longer have water but in its

place we have two tiny particles of hydrogen and one of oxygen. If we take a molecule of common salt and break it up we find that we have a tiny particle of metal sodium, end a

tiny particle of a gas, chlorine. However, when we break

these down still further we do not get more common materials.

If we could break up many kinds of substances we would find

that there are ninety-two substances which cannot be broken

up into other substances. These ninety-two substances are - called elements. All other substances are made up of combi­

nations of those elements and are called compounds*

In breaking up a molecule of water we would get a tiny

particle of oxygen and two particles of hydrogen. These

smallest particle® of an element are known as atoms. In the

case of many elements an atom is also the molecule. In

others two or more atoms of the same kind combine to form a

molecule of the element. Two or more atoms of different

elements combine to form a molecule of a compound.

Protons and Electrons: Mow suppose we take an atom and

break it down still further. When we do we find that we no

longer have particles of the substance but have broken it up Sf.

into electrical charges. There apparently are several kinds of electrical charges but the ones in which we are most interested are protons and .electrons*. protons are positive particles and electrons are negative particles of electric­ ity.

Each atom is made up of a heavy central part and is surrounded by electrons revolving about the central part called the nucleus, much as the planets revolve about the sun. The atom of each element has a definite number of pro- tone and electrons. For example, hydrogen has one proton and one electron circling around it. Oxygen had sixteen protons and eight electrons in the nucleus and eight addi­ tional electrons circling around the nucleus each in its own orbit. In any kind of atom there are the same number of protons and electrons under ordinary conditions. The proton is about one thousand eight hundred forty-five times as heavy as the electron. Since the electron is so small, and since many of them are circling in orbits about the nucleus, they may be knocked out of the atom under certain conditions.

Electricity in terms of Electrons

Electric Current: This gives us a definite basis on which we can explain the flow of electric current. Suppose that we could enlarge some of the atoms of a copper wire so that they would look like the diagram, (Fig. 15). There are twenty-nine electrons circling around the nucleus of a > -

Fig. 15 copper atom, but only one in the outermost orbit. For con­ venience, we will consider only this one electron, if some force should cause the electron "A" to move to the right as indicated by the arrow it would force electron "B" to move, and in turn electrons "C", "D", and WE" would each be forced to move along the wire. Each electron has moved only slightly but the wave of electricity has moved the full length of the wire.

We can define electricity as the flow of electron®.

Electric current always flows at a speed of 186,000 miles per second. This also is the speed of light, and the speed at which radio waves travel through space. Electricity flows at the rate of 186,000 miles per second, but this does not mean that the electrons are flowing at that rate. The electrons actually are moving only a few inches per minute. It is the wave motion that moves with the speed of light.

The above explanation of current flow applies to direct current. In alternating current the electrons move first one way and then the other, causing the waves to flow back and forth from end to end of the wire. The number of times per second that the electrons and waves move in each direction is the frequency of the wav®.

Current secured from batteries is always direct current.

Current secured from a generator may be either d. c. or a. c. depending upon the type of generator. Current secured from a transformer is always a. c. D. o. may be flowing in a steady stream or it nay come in spurts. D. c* secured from a battery comes in a steady stream and is known as "pure d. c.". D. C. which comes from a generator or from a vibrator comes in spurts and is known as "pulsating d. o.".

Conductors and Insulators: Some materials such as silver and copper have electrons which are easily knocked out of the atom and moved along in the direction of ourraat flow; These materials which allow current to flow easily are known as conductors. Silver is the best conductor, but is too expensive to find many uses. Copper is the second best conductor and is used for nearly all electric wires.

Other materials such as glass, porcelain,-rubber, sad bake-llte have atoms which hold securely to the electrons.

Since the electrons do not move very readily, these mater­ ials are not good conductors. Materials which are very poor conductors are called insulators. Insulators find many uses in radio and electricity in preventing the current from escaping into parts of the circuit where we do not 60

want it.

toe property of preventing or limiting current flow is known as resistance. Objects which allow current to flow

easily are said to have low resistance. Those through which

current will not flow or flows with difficulty are said to have a high resistance, toe unit of measure of resistance

is an ohm. toe ohm.may be defined as the amount of resis­ tance which will let one ampere of current flow when forced

toy a pressure of one volt. As indicated toy the preceding

sentence an ampere is the unit of measure of volume of elec­

tric current. A volt is the unit of measure of pressure of

electricity^

Current vs Electron Flow; Fig. 16 shows the relation of

current flow as compered,with electron flow. In the early

- Suit-#

3 A Current flow

Fig. 16

days of electricity it became necessary to decide upon a

definite direction of current flow. This was necessary in

order that all experimenters and electricians would have a

common understanding of terms used. It was agreed that for 61

convenience we would think of current as flowing from the plus pole of a battery through the circuit and back to the minus pole.

After the invention of the vacuum tube and general acceptance of the electron theory It became apparent that electrons did not flow from plus to minus. Experiments proved that electrons flow from the minus pole of the bat­ tery through the circuit and back to the positive pole.

When we study the operation of the vacuum tube you will readily see that the electron flow must be In this direction.

In radio work it is usually easier to think of the flow of electricity In terms of electron flow. However, we often have to use current flow, and for that reason you should be sure you know the direction of both current flow and electron flow. Current flows from plus through the Circuit to minus; electrons flow from minus through the circuit to plus. CHAPTXR VIII ELECTROMAGNETISM

Permanent Magnets Lines of Faroe: If we touch permanent magnets to a pile of iron filings w® find that many of the filings stick to the magnet, Most of the filings will stick to the ends, or poles, of the magnets showing that the attraction is stronger at these points. Fig. 17 shows the shape of the

field surrounding a bar magnet. This can be demonstrated by ' ■ ■ . '■ .... - f ' : '' - placing a bar magnet on a table and over it place a piece of

stiff paper. Sprinkle iron filings on top of the paper covering an area larger than the area of the magnet. Tap the paper gently and you will find that the filings arrange them­

selves in lines showing that the field is as shown in Fig. 17.

The lines used to diagram a magnetic field represent points

in the field which have the same strength. Whenever we' speak of the lines of force we mean the imaginary lines surrounding the magnet and joining the points of equal strength. Another method of determining the direction of the magnetic field at any point is to take a small compass and hold it near the magnet. The needle indicating north points in the directions shown by the arrows in Fig. 17. Of course our figure shows only a cross-section of the field which completely surrounds the magnet. Permanent magnets find many uses in radioj especially in ear phones, loud speakers, and certain types of micro­ phones.

Electromagnetic Fields

Field Surrounding a Wire: If we connect a piece of

number ten or twelve wire to a six volt car battery as shown

in the diagram (Fig. 18a ) and place some iron filings near

■ s r i 4 r ? 9*TUry Cu,er±*T «w*y fr»*i «4i: *-5, ' : B

Fig. 16

the wire we find that the filings will stick to the wire

just as they stick to a permanent magnet. We have created a

magnet similar to the permanent magnet. If we disconnect 64

one end of the wire so that the current stops flowing, the filings will drop off, .We have a magnet only as long as the current is flowing. If we hold a small compass near the wire the direction the needle marked "N" points gives the direction of the field. An easy way to determine the direc­ tion of the field around a wire is to hold your right hand partly closed, and with the thumb pointing in the direction of the current flow. The direction of the field is the same as the direction pointed by your fingers. This is known as the "right hand rule". Check this rule with the diagram, Fig. 18B.

If instead of using the entire battery we connect the wire across only one cell or two volts we find that the wire will not hold as many filings. The strength of the field is determined by the amount of current flowing through the wire.

Field Surrounding a Coil: If we make a loop of the wire as shown in Fig. 19A then follow the wire applying the

Fig. 19 right hand rule we find that all of the lines of force

Inside of the loop run in the same direction, and all of the 65

lines of force outside of the coil run in the opposite direction. If we continue to add turns, form a coil, we

find a field of force surrounding the coil in the directions

shown by the arrows. The direction of current flow is also

Indicated by arrows. The winding of a coil has concentrated the lines of force forming an electromagnet with the poles as shown in Fig. 19B. If we should wind the coil around an iron core instead of air we could still further concen­

trate the lines of force. In making electromagnets for ear

phones, loud speakers, relays, etc. iron cores are always used. : ■ -

Inductance: Colls seem to have the ability to store

electrical energy. If we send a surge of current through a

coil a magnetic field builds up. This building up of the

field takes time and energy. The length of time required,

as well as the strength of the field, depends largely upon

the number of turns. As soon as we stop the current from

flowing into the coil the magnetic field starts to break

down. As the field breaks down the lines of force cut (pass

through) the windings of the coil and induce or cause cur­

rent to flow in the same direction as the original surge.

This gives us the same effect in electricity that a flywheel

does in a machine. It takes time and energy to get a

flywheel started, but once it is started it will continue to

turn the machine after the power is shut off.

This ability of a coil to store energy is known as d*

inductaace. Inductance nan be increased by increasing She diameter of the coil, the number of windings, or by using iron or alloy cores. Inductance makes the tuned circuit possible. " ■ ' ' ' ' ' ' - : ‘ ' ■ - ' • .. '

When direct current come" in spurts this storing of energy in coils can be used to smooth out the humps and give a more even flow of current.

Inductive Coublina: When two coils are pisced near each other as in the cyrstel set (See Fig. 4 and Fig. 7) ' each coil is in the magnetic field of the other. The radio

signal in causing current to flow in sets up a magnetic

field which includes Lg. whenever a magnetic field builds

up or breaks down around a coil"it sets up currents in the -

coil. By sending radio waves (a. cY current j through the

coil we build up and break down a field around Lg and

cause current to flow in Lg. The amount of current is pro­

portional to the strength of the field so we get the same

kinds of current in Lg that we had in L^. We have trans­

ferred energy from one coll to another without any direct

connection between the coils. This coupling through a

magnetic field is known as inductive coupling. Winding the

colls on an iron core increases the coupling between the

colls by concentrating the field. Power transformers and audio transformers all have iron or alloy cores. Radio fre­

quency transformers do not use iron cores. The reason for

this is that iron cores increase the inductance. Increased 67

inductance makes the coil tune to lower frequencies. Coils must have a low inductance in order to tune to radio fre­ quencies.

We use the term ’’iron core" to mean any core of iron or alloy which concentrates magnetic lines of force. "Air core" means any non-metal core such as air, wood, bakelite, card­ board, and insolantite forms.

Effect of Coils on D. C. and A. C.: Take a coil with fairly high inductance and pass pure direct current through it. As the current starts to flow a field builds up around the coil. The building up of this field tends to retard the flow of current. However, as soon as the field has been built up there is no longer any holding back (resistance) of the current other than the resistance of the wire itself. '-

If in place of the-d. c. we pass alternating current through the coil we find that there is continual holding back of resistance:to the flow of current in addition to the resistance of the wire. This is because the magnetic field must break down and build up in the opposite direction each time the current changes direction.

Impedance: By selecting coils of the proper inductance the proper impedance (holding back effect) can be obtained which will stop completely the flow of alternating current at any given frequency. This property of the coll, imped­ ance, is used often to keep radio frequency currents from getting into parts of the circuit where they do not belong. 68

"Choke ©oils" or "chokes", as they are called, are used to keep both radio frequencies and audio frequencies out of the power lines in all-electric receiving sets.

The term impedance includes both the resistance of the wire and the holding back effect due to inductance. The term reactance which does not Include resistance of the. cir­ cuit is sometimes used In radio work.

Summary;- Fields of force surround permanent magnets at all times. Fields of force surround wires and coils when current is flowing through the wire or coil. This field is useful In transferring energy from one coil to another.' This is known as inductive coupling. A coil has the ability to store energy and supply a flywheel effect to a circuit.

This is useful in smoothing out d. c. current which comes in spurts. The "holding back effect" of a coll is khown a® impedance. Impedance is useful to hold back alternating current from getting into parts of the circuit where it is not wanted. Coils used for this purpose are called choke coils or chokes, iron core chokes are used to stop audio and power line frequencies; air core chokes are used to stop radio frequencies. CHAPTER IX

CONSTRUCTING A VACUUM TUBE RECEIVER , Selection of the Typ® to be Constructed

By this time you should have completed your crystal set and had time to try out some of the experiments suggested In the preceding chapters. Probably you are anxious to get

started building a receiving set using vacuum tubes so that

you can hear stations other than locals. In selecting a set

to be constructed there are several things which must be considered. Perhaps we should take several of these consid­

erations and discuss them.

Purpose for which the Set is Designed: The first con­ sideration in selecting the set to be built is the purpose

for which the set is to be used. If the sot is to be used

for communications work there are certain requirements which

it must meet which are not necessary in a broadcast receiver,

and often are not even desirable. For example, communica­

tions receivers are usually built in metal cabinets to

shield the circuit from the stray r. f. currents. The

transmitting equipment is usually only a few feet away.

The broadcast receiver on the other hand need not be in a metal cabinet but should be placed in a cabinet which will

compare favorably with other furnishings in the house. " • '... ' • ' : - _ - ' : . • . : : . The person who is to use the set must be considered. *0

file airway operator will want one thing in a receiver, the amateur operator demands something different, and the broad­ cast listener demands something from either of them.

The othlef purpose in selecting this set is to help you learn something about the fundamentals of radio. The set is designed by the author so that you can begin with the sim­ plest of vaeuum tube sets and add to it as your knowledge and experience increases. The set is conventional in all details. Each step is laid out so that the least possible number of changes will have to be made with succeeding addi­ tions to the set.

In spite of the simplicity of the set you will be well pleased with its performance as you learn to operate it properly......

Cost: Cost Is usually an important consideration. As in any other field you get Just about what you pay for in a radio set. The commercial operator pays more for his receiver than the average broadcast listener. He m e t have certain features that the broadcast listener does not require, and so must pay for the extra time and parts necessary to construct a receiver to meet his needs.

For our purpose we. can get along without many of the latest developments in order to keep down cost as well as simplifying construction. The set is designed for low cost tubes, a minimum of parts, and battery operation. The addi­ tional cost and complications of building a set to operate %

on 110 volt a. c. current, is probably not justified until you have had more experience in construction. This parti#- ular set is well adapted to: portable use and later you may like to rebuild it into a convenient carrying case. it is suggested that you not attempt to make the set for portable use the first time you attempt its construction.

Frequency: A radio receiving set should be designed for a particular group of frequencies. When this is done it will probably work on other frequencies, but best performance is to be expected on the band of frequencies for which the set is designed.

For the beginner the broadcast band is best because of the large number of stations operating at all times. It gives you more chances to find out how well your set works.

Also you are already familiar with conditions on this group of frequencies.

This set is designed primarily for broadcast operation.

Colls may be wound for short wave bands so that you may try your luck on the short waves.. The only change necessary to change this set to a fairly good short wave set is to pull­ out one set of coils and insert another. Better results could be obtained in the short waves by using a .00014 con­ denser in place of .00055 a. e. but this condenser would not cover the complete broadcast band.

Volume of Output: The strength of the sound which oust be produced will effect the design and cost of the set. This 9*

set is designed to operate ear phones only until the third tube has been added. Then it should operate a loud speaker producing sound enough for the average room. Extremely strong sound signals should be reserved for a. c. sets . because of the comparatively large amount of power needed to produce loud sounds. : . :

-S'.: Construction

The Chassis: Again we select the bread board type of construction because of its simplicity. The base %bard : should .be about twelve Inches long, t#n inches videg? and a : half inch thick. This, will give plenty of room, since the *-> reoelver is designed prlnmrily; for tb# bMaddast ba6& ; can make our leads several inches long withoug seriously affecting its operation. The parts for the three tube set will be located as shown In Fig. 130A. As we add each part

C»-V « u.4’6

COAfbti B

Fig. 20 refer to this diagram for proper placement of the various parts. The panel may be of plywood, presdwood, or bakelite, s M may be of any convential size. Seven by twelve. inches would make a nice appearing panel. The tuning condenser, regeneration control, and filament rheostat should be in the general location shown, but should be located so that they will look best with the knobs to be used. (See Fig. SOB).

The Circuit: The schematic circuit diagram is shown , in Fig. 21A for tho first one tube set. The performance of

s=

this set will be very little better them the crystal set, hut should be a^d® to operate before adding"L* and as shown in Fig. 21B. The socket connections for the tube

(type ’SO.)’ are shown in Fig. 22A. nils is a top yifi.JQn- looking up the socket connections of a # tube b# sure to ; notice whether the top or bottom view is shown. .Mbst:tube charts give bottom views for use when metal chassis are used

Tubes are numbered with three digit numbers such as

230 or 345. The first digit indicates the manufacturer. 74

while the last two show the type. The first of these numbers means a *30 type tube made by one manufacturer. Hie 545 indicates a type '45 tuba mad® by a different company. iThis is not true of tubes using letters as part of the designs* tion -examples 2A5 and SL6d). -

Fig. 22

Fig. 22B shows the connections for the coil and for the socket into which the coil plugs. . : . . . -

The method of winding the coil is shown in Fig. 23.

n - * _ - s fHT+vms A*. 3-Y “t*

& ~ 4 y/ Lt = az?***: J/*' ptS'c‘ Ln ~ ¥X TarnS Atm, 3 6 D*S>C. Wtnnd otmr- 4a. A\] tmih are c/#xe. V in TAe. Sclmc

Fig. 23

P. E. means plain enameled, b. S. C. means double silk covered. The coil form is the standard size five prong coil 78

form and may be.purchased from any radio-supply house. If,, you prefer, you may make a form by mounting a cardboard mailing tube of the proper size on a five prong tube base.

The ends of the wire are brought down inside the form and soldered into the prongs. It is possible to buy coils already wound, but you will learn more by winding them your­ self. You also save money by winding your own coils.

How to operate: There is a special technique to oper­ ating a regenerative receiver (receivers with the third coil on the same form as in Fig. 21B). After the set has been turned on, rotate the regeneration knob slowly until the set begins to oscillate. As you pass the point of oscilla­ tion you will hear a single click and then a continuous hissing sound. As you tune in a station with the set oscil­ lating you will hear a distinct whistle. This decreases in frequency as the station is tuned in more carefully. For receiving voice transmissions-the regeneration control

should be set just below the point of oscillation, and for receiving Morse Code signals the set should be operating

just above the oscillating point. Regeneration makes a set very sensitive so you will be able to pick up many stations

as you learn to operate your one tube set.

In buying a regeneration control you can buy one that

has a switch built into it so that by rotating the knob to

the extreme left a click is heard which indicates that the

switch is off. This is the switch indicated by Sw^ and 76

makes it unnecessary to dlaoonneet any of the batteries when not in use. CHAPTER X

ELECTRICAL UNITS, LAWS

Definition of Terae We have reached the point in our study where it will be necessary to get an idea of the value of the various units of measure used in electricity, and of their relationship to each other. The three terns most commonly used in electric­ ity are ohms, amperes, and volts. Ohms and amperes are defined legally In terms of conditions which can be dupli­ cated in any country of the world and at any time. This makes possible a standard language In electricity which knows no nationality. Ohms, volts, and amperes mean the same thing in any language. Other terms will be defined as needed.

Amperes: An ampere is the unit of measure in electric­ ity which corresponds to gallona-per-minute in measuring water. It is a measure of quantity of electricity. For a universal standard the ampere is defined as the amount of current required to deposit .001118 grams of silver per second from a solution of silver nitrate in water, and under certain special conditions. We seldom think of amperes in this manner, but it does give us a standard.

An idea of its practical size can be gained by thinking of an ampere as the amount of current which would flow through 96

a 110 watt lamp In an ordinary house lighting circuit*

Better understanding of its value will be gained by using the various units.

Another common unit used in radio is the milliampere, meaning one thousandth of as aaperej.

Ohms: The unit of resistance is known as an ohm. The legal definition of an ohm is the amount of resistance , ■ offered by 14.4R21 grams of mercury stretched put in a uni­

form column 106.3 centimeters long and at zero degrees cen­

tigrade (temperature of ice). ; ; 156.6 feet of number 18 common bell- wire has a resist­

ance of one ohm. Also 1000 feet of number 10 copper wire

has a resistance of one ohm.

Volts: The unit of pressure in electricity is called a

volt. This unit is defined as the amount of pressure

required to force a current of one ampere through a resist­ ance of one ohm. - , ....

A dry cell (battery) such as found in flash lights, old

type telephones, and in some battery radios, will supply

about one and one half volts. Car batteries usually measure

six volts. "B" batteries used in radios contain a number of cells similar to flash light cells. These one and one half

volt cells are connected in series to give forty-five,

ninety, or one hundred thirty-five volts. The size of a

cell, makes no difference as far as voltage is.concerned but

larger cells will last longer. 79

Ohm's Law: In 1826 Georg Ohm, a German physicist, pub­ lished a paper showing the relationship between volts, amperes, and ohms. . This law known as Ohm's Law may be ex­ pressed in three different ways, all of which mean the same thing. They are: R = | ; E = I x R; and I = where «E" stands for. electromotive force (e. m. f .) expressed in volts. *1" stands for current expressed in amperes, and "R" for ; resistance expressed in ohms. These letters are often used with the same meaning in electricity and should be remember­ ed, Stated in words these three forms read as follow;

(a) R « the resistance in ohms of a circuit is equal to the number of volts divided by the number of amperes of current; (b) E “ I x R, the number of volts is equal to the product of the number of amperes times the number of ohms;

and (c) I * amperes of current which will flow in a cir­ cuit is equal to the number of volts ' divided by the ntmber

of ohms of resistanee.

Ohm's Law is used often in radio and should be'memo­

rised so that it can be used without having to look up the

relationships each time you need to figure voltage drop,

current flow, or resistance, where the other two are known.

Suppose we have a resistor of unknown else which when

we connect it across a six volt battery in series with an

ammeter it allows two amperes of current to flow. Referring

to Ohm's Law we see that resistance in ohms is equal to volts divided by amperes. Six divided by two equals three.

The resistance is three ohms. Now suppose we have a resistance of three ohms and want a current of two amperes flow, h o w many, volts would be ’ necessary to force two amps, (amperes} through three ohms of resistance? Using the second form of Ohm’s Law (E * I x R) we find that by multiplying amps, times ohms we will get volts. Three times two equal six. Therefore, six volts will be required to force two ampe. through three ohms.

If we have a six volt battery and a three ohm resistance how many amperes can we cause to flow through the resistor?

Using th© third.form of Ohm’s Law (1 * g ) we see that the

current in amps, is equal to the number of volts divided _

by the number of ohms of resistance. Six divided by three

gives us two amps, ©f current.

Power: The electrical unit of power is called a watt. A watt is equal to one ampere of current at a pressure of

one volt. The formula for power then is U = E x I. Expressed

in words this means that the number of watts ia equal to the

number of volts times the number of amperes. Briefly, watts

equal volts times amps. Six volts times two amps, would equal twelve watts of power.

746 watts equal one horse power. This means that a quarter horse power motor would require 186& watts of elecr

trical energy. ;

If we compare electrical power with water-power volts at

would correspond to pressure and amps, to amount of water,

A small strem of water amder high pressure will generate as much power as a much larger stream with very little pressure.

Heat Loss; The heating effect of an electric current may be expressed by this formula: - 1 % . gy represents the heat produced in watts. I2 means the number of amps, squared or multiplied by itself, and R is the resistance in ohms. For example, suppose w© have a resistance of twenty ohms and force a current of five amps, through the resis­ tance, Substituting in the formula we have = (5)2 x 20 or Hw " 26 x 20. Hw * 500. The heat produced would be equivalent to five hundred watts of power.

The chief use for this law In radio is in deciding how much heat a resistor must be able to stand without burning out when used for certain purposes.

Length of Wire as Related to Resistance: The resistance of a wire is directly proportional to its length. That is, the longer the wire the greater its resistance. This same relationship Is true with pipes carrying water, it Is five times- harder to force water through a fifty foot pipe than through a ten foot piece of the same size. A wire fifty feet long has five times as much resistance as a ten foot piece

of the same size and kind of wire.

Diameter of IFire as Related to Resistance: As the diameter of water pipe is increased it becomes easier to force water through it. The same is true of wire used to 88

carry electricity. If we douMs the diameter of the wire we decrease the resistance to one fourth. If we increase the diameter three times we decrease the resistance to one ninth. Multiplying the diameter by four decreases the re­ sistance to one sixteenth.

In general we say that: The resistance of wire is - inversely proportional to its diameter. '

By an inverse proportion we mean that as one term

Increases the other decreases. In this case the diameter is increasing and the resistance decreasing. A direct pro­ portion means that both terms increase or decrease at the same time. In the case of the length of wire as related to resistance both length and resistance increased or decreased at the same time. Magnetic Attraction and Repulsion: I f we take two bar magnets or horse shoe magnets and hold the north pole of one near the south pole of the other we find that they attract

each other strongly. Now if we turn one of the magnets around so that we have two north poles together or two south

poles together we find that they try to push each other away.

From this experiment we get two laws of magnetism which help

us to explain many of the things happening in radio:

Unlike charges attract each other.

Like charges repel each other.

These two laws are useful in explaining the actions of

protons and electrons. Protons have a plus charge and 83

attract electrons which are negative. Protons repel protons and electrons repel electrons. From this law it can be seen that electrons do not have to be in actual contact with each other to cause electric current to flow. If one electron Is moved toward the second the repelling force causes the second electron to move away. This happens all along the wire so that we get current flow without many of the elec­ trons actually hitting each other. CHAPTER XI

TWO ELEIdSNT VACUmi TUBES Importance of Vacuum Tubes

DlscoVery of the principle of the vacuum tube was with­ out doubt the greatest single discovery in the history of radio. Without the vacuum tube we could not have the high powered broadcast stations as we know them today. The radio signal picked up by the antenna is very weak, usually only a few thousandths of a volt. It is necessary to increase or amplify the strength of this signal hundreds of times before it will be strong enough to operate the loud speaker. This amplification is done by vacuum tubes. Television, facsimile, electronics, and therapy would not be possible without the vacuum tube.

How a Two Element Vacuum Tube Works

Vacuum tubes are used in all modern receiving sets and in all broadcast stations- It is eoeenSSal that you have some definite idea of how a vacuum tube works before you go very far in radio.

In looking over the large number of types of vacuum tubes it may seem hopeless to the beginner to attempt to learn how they all work. However the general principle® - ; are the same for all tubes so that once you understand how a few types of tubes work you can readily understand any other tube by merely looking at its characteristics; Manu- fsotazers eutice available to all users charts which show how each tube should be used, including all voltages and currents needed for its operation. . ;

Filament Emission: Operation of the w @ u u m tube depends directly on the ability of a hot object to emit or give off electrons. This emission of electrons was first diesowered by Edison and is known as the Edison Effect.

The Edison Effect may be explained in terms of the elec­ tron theory. All matter is made up of molecules which in turn are made up of atom. Atoms are made up of protons and electrons. In addition to the electrons present In the atoms, experiments seem to prove that there are large numbers of : . ; - . /. V M \ ; free electrons in some materials, especially certain types of ^ if i metals. All of these molecules and free electrons are moving very rapidly in various directions. Under erdinax^ condi­ tions neither the electrons nor the.molecules have energy enough to get past the surface of the metal.

According to the electron theory the speed at which the molecules and electrons are moving is directly proportional to the temperature.

If we could cool any material to minus 273 degrees Cen­ tigrade or about 459 degrees below zero Fahrenheit, all motion of the molecules and electrons would stop. If Instead of cooling a metal we heat it, the speed of the molecules and electrons increases. When the temperature is raised until a metal becomes red hot, the speed of the electrons becomes so great that many of them break through the surface of the metal and out Into the space scpnrouadiag the metal.

If wo heat the metal enough, the molecules also will eseape and the metal will evaporate or change to a gas. The molecules do not speed up as much as the electrons when we add heat because of their greater weight. It takes much more energy to speed up the heavy molecules. . . fig. 24A shows the vacuum tube with the filament hot, but no voltage on the plate.

0l*t.TttnS s*rr*»**>'*f

It makes no difference how the filament is heated in order to

make the electrons escape. However, the only convenient method of heating the filament Inside of a glass container

from which the air has been removed is by means of an elec­

tric current. For this reason, all modern vacuum tubes

employ electric currents to heat the filament.

As soon as the filament becomes hot electrons escape 87

into space surrounding it. If there Is no voltage near to - affect the field, the electrons stay near the filament and many return to the filanont as shown in Fig. 24A. The only purpose of the three volt battery is to heat the filament so that electrons will be emitted. ' :

This giving off of electrons by the filament is known by several different names all meaning about the same thing.

It is known as filament emission, electron emission, and thermionic emission. Thermionic emission means emission of electrical charges due to heat*

Some metals and compounds ere better than others for emitting electrons. Barium oxide and thorium are most com- mohly used. Alloys of thorium are often used and are known as thoriated filaments.

Action of the Plate: If we place another element such, as the plate (Fig. 24)-within the glass container, and con­ nect it to a battery as shown in Fig. 24B, we can get some eleotrons to leave the filament and go to the plate. Notice

■ " ;...... : : ' ' that the plus side of the battery, is connected to the plate

and the negative to the filament. Unlike charges attract

each other so the electrons, negative particles of electric­

ity, are attracted to the plate which has a positive voltage.

The higher the voltage on the plate the more electrons are attracted to it, and therefore more current flows. The

forty-five volt battery supplies the voltage for the plate

and the additional electrons to replace those emitted by the filament. The electron flow is from the -filament; aeross ^the vacuum to the plate, then to the plus of -the battery, through the battery and- from the minus of the battery back to the filament. , . ' ; r ; Now suppose we reverse the comae®tl#a# to;the forty-. five volt.battery (called the "B" battery) so that the minus is connected to the plate, and the positive is connected to

the filament. Electrons try to flow from the filmmeat to.-.

the plus' of .the battery, through the battery, from the minus

to the plate, and across the vacuum to the filament. The plate is cold and will not emit electrons; therefore, elec­

trons can flow in only one direetioa through a vacuum tube. Detector Aetioai This ability of a vacuum tube to pass

current in one direction only is similar to. the action of a

crystal detector. This may be used as shown in Fig. 25A'. \

Fig. 25

Allowing current to pass through the circuit gives us the

wave form shown in Fig. 12G. This type of detector Is used

in many modern receiving sets, and is known as a diode detector. Any tube containing only a filament and a plate is called a diode. Triodes contain three elements; tetrodes

Contain four; and pentodes five elements.

Half Wave Rectifiers: Although diodes are often need as detectors, their greatest use is in changing alternating current from the power line to direct current for use la radio sets. ■ •" In Fig. 25B we have a diode connected to an a. c; cir­ cuit to change the alternating current to direet current. '

The five volt winding on the transformer supplies the - current necessary te heat the filament. V,'e may represent the alter­ nating current ©oalng into the circuit from the light cir­ cuit by the sine wave as shown in Fig. 26A. All above the

A

Fig. 26

zero line represents current flow in on# direction, while

flow in the opposite direction is represented by the part of

the wave below the line. We usually say that the top is

plus and the bottom negative. If it were not for the vacuum tube in the circuit alter­ nating current, induced in the secondary of the transformer, would flow through the load. «• .. . . , . ;v

Suppose with the vacuum tube in the circuit that we could stop the flow of current at the instant the top of the high voltage winding is negative. (See Fig. 25B). „At that instant the electrons are flowing toward the filament and away from the plate; Since the filament.is hot it can emit electrons. The plate at this instant has a positive voltage on It so the electrons are attracted to the plate. When this condition exists, the vacuum tube is offering very little resistance to.the flow of electrons and they flow as indl- ?; \ i r u ' . - - ■ ' ' : r-; ^ oated by the arrows. > ,

low-suppose we consider the circuit when current is flowing in the opposite direction in the 110 volt 11##. When this happens the end of the high voltage coil which is con­ nected to the filament will have the positivevoltage, arid so will hold any electrons emitted by the filament. The end of the winding connected to the plate is negative and electrons will try to flow in a direction opposite the arrows in

Fig. 24B. They cannot flow from the plate across the vacuum t® the filament because the plate is ©old and cannot emit electrons. The result is that for this half of the wave

cycle no current can flow in the circuit.

As the direction of the current keeps changing we get

first a.flow of current, then an.equal time interval with no #1

current flow. This in turn is followed toy another flow of eurrent in the same direction as the first. The results may be pictured as shown in Fig. SSlw We have changed alternating current to.pulsating direct current.

This type of rectifier ie known as a half wave recti­ fier because It uses only half of the alternating current wave. The type ’81 tube is a half wave rectifier.

Full Wave Rectifiers: In order to take advantage of the full cycle, two half wave rectifiers are often connected as shown in Fig, 27A, or a tube sueh as the type *80 which

=

Fig. 27

contains two plates may be used as in Fig. 27B. At the

instant when the top of the high voltage winding is positive electrons will flow from the filament to the plate of Y^,

through the upper half of the coil, out through the center

tap, through the load, and back to the filaments as shown by

the arrows. During this half of the cycle no electrons flow .

across the tube Y^ because the plate has a negative voltage

on it, and therefore repels electrons. 92

When the direction of the current in the 110 volt line changes or as we say in electricity, when the phase changes, so that the lower end of the high voltage/winding becomes positive electrons will flow through the tube V- and out - y- through the center tap. At this instant no electrons can flow across Y-,because of the negative voltage on its plate.

The result is that, regardless of the direction of cur- rent flow in the 110 volt line, the electrons will always flow from the center tap of the high voltage winding through the load and back through either or Vg to tho plate cir­

cuit and back to the winding. In this way we get more spurts of d. c. current than from the half wave rectifier. The

wave form from a full wave rectifier is shown in Fig. 260. Advantages of Each Type: Hie only real advantage of

the half wave rectifier over the full wave rectifier is that

only half as many turns are required on the high voltage

winding of the transformer.

Oh the other hand, this is offset by greater efficiency

of the full wave rectifier in usihg both halves of the cycle.

The full wave rectifier gives a more continuous flow of cur­

rent. Twice as much current can be obtained from a full wave

rectifier as compared with a half wave rectifier. CHAPTER XII THREE ELEMENT VACUUM TUBES Importance of the Discovery

It is the third element or grid sbloh makes possible amplification in a vacuum tube. Diodes may be used as detectors and rectifiers but it is necessary to include the grid in a tube before we can build up the strength of sig­ nals to operate a loud speaker, or to transmit a radio pro­ gram at the broadcast station.

How the Triode Works

Amplification; m experimenting with a magnet, we notice that the closer together the poles of two magnets are the more they attract or repel each other. This force is inversely proportional to the square of the distance between the two poles. This means that if we double the distance between the poles we decrease the force to one-*fdu3rbh of its original value. If we increase the spacing to four times the original, the strength of the attraction or repulsion is

decreased to one-sixteenth its former value.

This fact is useful in the construction of vacuum tubes

In the diode as shown in Fig. 24 a grid may be added between

the filament and the plate as shown In Fig. 28.

Suppose that we have a triode (three element tube) with

the plate four times as far from the filament as the grid. 94

In this case, since the plat® is four times as far away from the. filament the foree of attraction or repulsion by th© plate for electrons will be only one-sixteenth that of the grid. One volt o b tb® grid would have the same effect on current flow as sixteen volts on the plate.

If we put 160:volts on the plate of the triode described above it would have the some attraction for electrons that '

10 volts on the grid would have. Also if we have a 10 volt negative charge on the grid the minus 10 volts would have the same amount of repulsion for electrons as the attraction of the plus 160 volts on the plate. In this case some elec­ trons would be attracted to the plate while others would be repelled back to the filament by the grid. We would get some current flow but not nearly as much as if there were no grid. -This situation la shorn in Fig. 28A. -

-t It»y- •j.iiiK

Fig. 28

Instead of minus 10 volts, suppose we put minus 15 volts on the grid of the tube. This minus 15 volts on the grid 95

would have repulsion equal to the force of attraction of plus 840 volts on the plate. Since we still have only plus

160 volts on the plate, electrons will be .repelled by the grid rather than attracted to the plate (See Fig. 288). • If we put a negative charge of only 5 volts on the grid and keep the plus 160 volts on the plate, we find that the attraction to the plate is much greater Shan the repulsion by the grid, and, therefore, many electrons will flow to the plate. (See Fig. 88C). The minus 5 volts would have a repul­ sion equal to the attraction of only plus 80 volts on the plate.

•; , ' By changing the grid voltage from minus 15 volts to minus 5 volts wo have caused the same changes in the tube that would have been caused by changing the plat® voltage from plus 80 volts to plus 240 volts. A change of 10 volts on the grid is equivalent to a change of 160 volts on the plate. This means that any signal affecting the grid voltage will give us sixteen times as much change in the plate cir­ cuit. The signal has been increased or amplified sixteen times. ■'

Amplification Factor: The number of volts change on the plate necessary to have the same effect as one volt change on the grid is known as the "amplification factor" of the tube. The amplification factor of the tube we have been discussing is 16. The Greek letter (mu) is often used to represent "amplification factor". This tube has a 96

[4 Of 16. - ' ' " V- ' • ;-r ^ ' '' The possibilities of amplification are very great. . For example, if we should, take the signal from the plate circuit of this tube and amplify it through another similar tube the resulting signal would be 16 times 16 or 256 times as strong as the original. Amplifying it through a third similar tube would bring our signal up to 256 times 16 or 4,096 times its original value. ' '''' -/ Characteristic Curves and Amplification: Fig. 29A shows

the relationship between plate current and grid current.

This curve is known as the characteristic curve of the tube.

The higher the amplification factor of the tube the faster the curve goes up. The distance of any point "X" on the curved line above the zero line show# the amount of plate current which will flow when the grid voltage (bias) on the tube is represented by the point "Y" directly below. 97

The point "W” represents the amount of fixed bias (volts on the grid) used on the tube when used as an amplifier. The points and "Z* represent the limits of changes in the grid voltage (grid swing) caused by the incoming signal. The bias *fn for ah emplifier Is fixed somewhere near the center of the straight line part of the curve so that the entire signal is amplified eouelly.- ’ ' - v

The Triode as a Detector: Triodes have an advantage over diodes as detectors in that they can act as detectors and at the same time amplify the signal.

Instead of biasing the grid of the triode as shown in

Fig. 29A if we increase the negative bias to the point *8* in Fig. 29B, we can make the tube act as a detector. The half of the grid swing from "S" to "T” is amplified just as in the regular amplifier. This part of the swing is toward the positive voltage and causes more current to flow in the plate circuit. The bias at "S" is of such value that any change in bias toward the negative stops the flow of current completely. The result is that as the signal swings the bias more negative no current flows and this half of the signal does not come through the tube. We have secured, by proper bias of the triode, the same

effect found in the crystal and diode detector. At the same time, we have taken advantage of the amplifying ability of the triode to increase the strength of the half of the sig­

nal which remains. 98

The grid bias voltage at which plate current ceases to flow for any given plate voltage is known as "cut-off" or

"cut-off voltage". For detectors the fixed bias is at or near cut-off. For Glass A amplifiers (the type discussed) bias Is set far enough above cut-off so that plate current can increase and decrease the same amount as the incoming signal swings the grid more positive and more negative. . ■ : ' . ' 1 ■ ; _ . ' ^ : v .. . - - - • *■' CHAPTER XIII ' ; ■- ’ ' ' .-r • HOW THE ONE TUBE RECEIVER WORKS Circuits 3 i.rs; p - Relationship of Tubes to Circuits: In order for a ■ • . _ : • ■ ' . ■ ■ ■ ■ , : f;. „ , vacuum tube to operate properly there are two requirements which must be met. Without either of those two essentials vacuum tubes are worthless. First, the tube must have the proper voltages on all of its elements. This voltage may be supplied either by batteries or from the 110 volt line through rectifiers and filters where necessary.

The second requirement is the proper circuit, it is the circuit in which a tube is used that determines the

results obtained from the tube. As we saw in to® last chap-

ter, merely changing the bias of a vacuum tube can change it

from a radio frequency amplifier to a detector-amplifier.

Some of the purposes in radio for which the same tube may be

used, depending on the circuit, are: radio frequency ampli­

fier, intermediate frequency amplifier, audio frequency

amplifier, detectors, oscillators for generating both radio

and audio frequency signals, and similar uses in video

(television and facsimile) transmission and reception.

Operation of the First One Tube Set •

How the Tube is Used: In our first vacuum tube receiver 13291-1 100

(Fig. 21 A) the vacuum tube is used as a detector-amplifier as explained in the preceding chapter. Since the ampli­ fication factor of the type *30 tube is 9.3 it is possible to hear a signal which is one^ninth as strong as could be heard in the crystal set. Or y/o should be able to hear the same signal nine times as loud as with the crystal set. Purpose of the Various Parts of the Circuit: Some of the parts of the one tube set are the same as used in the crystal set. since they serve the same purpose here they will be mentioned only briefly. Some parts were not used in the crystal set and need more detailed explanation. Several of these parts are discussed individually. (Unless other­

wise indicated all references in this discussion refer to

Fig. 21A). V ; ' : ' ' " ■ - :• v ’ ‘ . •

The Srid Leak: Beginning with the antenna and following

the signal into the set we find first a radio frequency

transformer similar to the one in our crystal set. Also we

find one winding of the transformer and a variable condenser acting as a tuned circuit.to select the desired signal.

The next part of the circuit differs from the circuit

of the crystal set. we find a 2 meg. (two million ohm)

resistor and a .00025 mfd.(microfarad) condenser connected

in parallel between the tuned circuit and the grid of the tube. : • ' ' ' ^ • . •. • -

As the electrons flow from the f ilament to the plate

(See Fig. 28) some of them strike the grid and stick to it. If the grid mere left "floating" (not connected to anything)

•eon, enough electrons would collect on the grid to bias the grid to cut-off and stop the flow of oleoSrons tb»B<•Sorting the tube from operating. Electrons are negative particles of electricity and as they collect on the grid the negative foliage increases. ^ - ' . . . ;

Mow suppose that we should connect the grid directly to the filament through the tuned circuit leaving out the "grid leak"* The resistor and condenser in parallel as used in this circuit are known as the ;"grid leak". In this case there would be very little resistance to the flow of electrons from the grid and, therefore, the grid voltage would be practically saro* - .. - ..

: Referring to the characteristic curve of the f50 tube similar to the ones in Fig; 29 we find that zero bias would act give us out-off:bias needed for detector action of the tube. z • _;v; -r v - 1

If we connect a resistor of the proper size in the grid circuit we can control the number of electrons escaping from the grid. By holding back:electrons in the proper numbers we can keep the negative voltage on the grid a t .the proper value for cut-off bias. Again referring to data furnished by the manufacturer, we find that the proper value of the grid leak resistor should be between one and five megohms.

Adding this resistor to the circuit gets us into more trouble. Wot only does it provide resistance to the electrons trying to escape from the grid, but it provides resistance to the very weak radio signals which we want to get to the grid. A condenser allows waves of alternating current to pass through it but does not let direct current flow. The electrons trying to escape from the grid are always moving in the earne direction and so is direct current. Radio waves are alternating current. By using a condenser of suitable size we can provide a path of low resistance for the incoming signal, and at the same time force the electrons leaving the grid (grid current) to go through the grid leak resistor. - ' • . . - -

Negative voltage for the grid obtained by using a- resistor in the circuit is called "grid leak biasM. We could have used a battery or a rectifier filter system to obtain this bias but the grid leak is the simplest method, and is just as effective.

. The R. F. Choke: la the crystal, set the amount of radio freqneney current in the cireulS was very small, pro­ viding a by-pass condenser was enough to keep the r. f . out of the phones., m this set we have amplified the-r.f. cur­ rent several times, and so in order to make sure that no

r. f. gets into the phones we include a radio frequency

ehoke(RFC) between the by-pass condenser and the phones. The

impedance of this choke is such that.it offers h i # resis­

tance to radio frequency, but very little to the audio fre­

quency. This forces the r. f. to go through the by-^yss condenser.

* Batteries: Two batteries are used. One three volt battery is used to heat the filament of the tube. The 45 volt battery supplies electrons for the plate current and the positive charge on the plate to attract electrons emitted by the filament. A six ohm variable resistor (rheostat) is provided in the filament circuit to control the temperature of the fila­ ment and thus control the volume by controlling the number of electrons emitted. *' A switch is provided in the filament circuit to shut off the set when not in use. It is not necessary to have a switch in the plate circuit since no current can flow In that circuit when the filament is not heated.

Summary: Briefly, the operation may be summed up as follows: The r. f. signal is picked up by the antenna and brought into the set through the antenna coil and then to ground. Flowing through thecoil the signal sots up a field around and Lg . This induces an r. f . current in Lg

The desired signal is selected by the tuned circuit L^C^.

The r. f. signal then passes through the grid leak condenser to the grid of the tube. The tube amplifies the signal and

at the same time acts as a detector by rectifying the signal

so that the headphones can follow the average power, and

thus set up sound waves which are similar to the waves

striking the microphone at the broadcast station. Voltage is supplied to the filament by a three volt "A.” battery, to the plate by a 45 volt "BT battery, and to the grid by a resistor allowing electrons to leak slowly.from the grid.

Operation of the Second One Tube Set

Regeneration: The only change in this set (Fig. 21B) from the first vacuum tube set is the addition of the third coil L3 and the variable resistor Rg. For the moment let us disregard the resistor R and see why the ©oil L adds to . .. , , .. . . 2 . ; • . . _ ... _ 3 . . the performance of the set.

The signal on the grid of the tube causes the plate eur- • : ' . - ' - ' . ' f : ■■ ' ■ 1 ■■ : .. . rent to vary. The strength of the signal in the plate cir­ cuit of the ’30 tube is about nine times as strong as the signal in the grid circuit. The tickler coil Lg is wound on the same form as Lg. Plate current flowing through Lg sets up a field surrounding itself and the other coils. Since Lg is in the field of Lg current is induced in Lg. Both coils are wound in the same direction so that the field set up by

Lg adds strength to the original signal.

The result is as follows: A signal is picked up from the antenna, amplified in the tube. The plate current then sets up a field surrounding L_ and L which adds to the O 6 strength of the original signal. This stronger signal is then amplified again and sets up a still stronger field, again building up the signal. The effect is that the signal goes from Lg to tube, to Lg, to Lg, to tube, etc. at the speed of 186,000 miles per second. Each time the signal goes through the tube -it is amplified until it has increased to several thousand times its original value.

This process of feeding back powey from the plate cir­

cuit to the grid circuit by means of a tickler coll is known

as "regeneration". This type of circuit is known as a

"regenerative detector". Because of the high amplification, these sets are very sensitive. You should be able to pick

up any station that the average broadcast receiver will pick up.

The purpose of the resistor R_ is to control the amount

of current flowing through the coll L_. This controls the

amount of amplification by controlling the strength of the

field around L„ and Lp. " ' ‘ ’3 ... ' <5. ■ . . ■ ' . . . : - ' S - / . ■ CHAPTER. XIV ^ ' . ' ---- : ' _-Y ^ -- ' - r ' " joNDiaiKBawisBnRi; , .... ' ." „ ;.r -: Theory of Condensers

Definition of Terms: In studying the fields surrounding a ooil we saw how electrical energy could be stored in a magnetic field. There is another way in which electrical energy can be stored. This type of storage is known as ca­ pacitance or capacity. In very advanced radio theory capac­ itance is considered to be the proper term, but common usage has made the term capacity have the same meaning. Since in oaamon usage they do have the same meaning we shall use ca­ pacity to mean the ability of a condenser to store electrons.

Capacitor is the correct technical term for a device used to store electrons, but the term condenser is so widely used - ■■ ' - ■ ■' ■ . - f.."- ' • ' v that we seldom hear of capacitors. For that reason we shall use condenser.

Capacity or Capacitance: If we take two metal plates separated by some Insulator such as air and connect them to a 45 volt "B" battery as shown in Fig. 30a , we find that the condenser takes on a charge whose voltage is equal to that of the battery. Removing the connections from the battery and placing them close together as shown in Fig. 30B causes

a spark to jump. The size of the spark depends on the volt­ age of the battery and the size of the condenser. • .:..

4-r/. "f A

Fig. 30

The arrows in Fig. 30 indieate the direction of electron flow. Electrons flow from the minus pole of. the battery through the load and back to the plus pole. ia this case the electrons cannot flow between the plates because they are separated by an insulator. Since electrons are negative charges they repel each other. Forcing electrons on to plate

"X" repels the electrons away from "Y" and back to the bat­ tery. This means that we have an excess of electrons on plate ”XW, and a shortage of electrons on plate "Y". *

When we disconnect the battery free the condenser we leave the charge on the condenser. Touching the wires to­ gether as in Fig. SOB allows the excess electrons to flow from "X* through the wires to "Y". Since we no longer have more electrons on one plate than on the other we do not have any voltage across the condenser. The condenser has been discharged. Some high voltage condensers will hold a charge for several hours if not discharged by connecting the plates as in Fig. 30B. . • 10#

Voltage and Battery: T7e can now define voltage again, this time in terms of electrons. Voltage is merely a dif­ ference in the number of electrons between two different ‘ - points, then the condenser is charged there are many more electrons on "X" than on "Y". If we measure’the voltage - with a voltmeter across the eondenser we find that the plate

"Y" has as many more volts than "X" as we had volts in the battery. . __ - v '

We can think of a battery as being a container having a very large number of excess electrons on one side, the minus, and an equal shortage of electrons on the other side, the positive. •' ■ ■■.• v- v, ■:

: The Effect of the Size of the Plates: The capacity of

a condenser depends directly upon the area of the plates

exposed to each other. A condenser whose plates had a com­

bined area of twenty square inches on the sides facing each *

other would have twice the area of a similar condenser having

only ten square inches of area on the sides facing each

ether. - - ... • ■■ - : - ■ ;; '" . . r

If more than two plates are used they should be alter­

nated as in Fig. 300. In this case, plates WSW , wTn, and -

do touble duty since they each have two surfaces or twice as

much area used. In ealmilaStng capeeity we usually consider

only the overlapping sections of the plates as shown between the dotted lines in Fig. 30C. The rest of each plate may

have some capacity but it is so small in most ocndenwws " 10#

that we do not consider It*

Effect of spacing on Capacity: Capacity varies Inverse­ ly. -with the spacing between the plates. Two condensers sim­ ilar- in construction, but with spaeings of 1/8 inch and,1/16 inch respectively, will have different capacities. The con­ denser with 1/16 inch spacing between, the plates will have twice the capacity of the one with 1/8 inch spacing.

Dielectric Constants; The capacity is affected greatly by the dielectric (iA&ilatorJ which is used between: the plates. For example,;if we have two condensers which are similar except that air is used for the dielectric between the plates of one and mica is used as the dielectric in the other, the mica dielectric condenser will have about three times the capacity of the air dielectric condenser. A glass dielectric condenser would have from five to seven times the capacity of an air dielectric condenser of similar construc­ tion. '• ' ' " ' ■ r •

In order to compare various substances as dielectric# *e consider air as having a dielectric constant of one. Mica has a dielectric constant of 2.94, and glass from 4.90 to f .00. Transformer oil has a dielectric constant of 2,5, and quartz has a dielectric constant of 4.2.

T M s means that a condenser using transformer oil would have two and one-ha If times the capacity of a similar air condenser, while a condenser with quartz as an insulator •"•=*' wohld have a capacity of four and two-tenths times that of a 11®

similar air .dielectric condeaser. ' ^ <

1 ‘ Alternating Current and Condensers: :in discussing the operation of the crystal set, and of the one tube seta, we mentioned that a. o. could cross a condenser but that d. c. could not. Fig. 31 shows how a. c. can cross a condenser. ^

Fig. 31 ‘ .

- ; .: ; : \ In Fig. 31A the eleetr

by the arrows flow to plat* % e y cannot get across the

space from "X" to "Y", but .repulsion of like charges forces the electrons to l % w e plate ’’Y" and flow toward the

lamp, nils sends a wave of elestricity through the lamp end

back to the 110 volt line.

When the direction of the electron flow changes, the

electrons flow back throu#i the lamp and to plate ’•Y” of the

condenser. This excess of electrons on plate nYN forces the

electrons to leave plate "X" and flow toward the 110 volt

line. Electrons do not flow far in a. c. electricity, but

merely shift back and forth causing waves of current to flow first one way and then the other. Condensers offer very little resistance to this shift. The amount of resistance fteereases as the size of the ©oaSenser increases. The amount of resistance also decreases as the frequency of the - current increases.

Direct current cannot cross a condenser because in d. o.

there is a definite drift of electrons from minus through the load and back to plus. The condenser will not allow elec­

trons to cross and so effectively stops direct current.

Blocking and Coupling Condensers: A good example of the

use of condensers in stopping d. c. and in allowing a. c. to

go through is shown in Fig. 32. . In this ease we. want plus

Fig. 32

135 volts on the plate of. and minus 13 volts on the grid

of Vg. Obviously we cannot connect.these two direct currents

together or we will get a plus 122 volts, the difference of

the two voltages, od the grid of Vg. We must devise some

method of getting the.signal from the plate of the first

tube to the grid pf the second. Fortunately all signals

which we want to amplify are a. c. so that we sen take ad- 128

ftaiiaige of the properties or characteristics of the condenser.

‘ As shown in the diagram (Fig. 52) the plus 135 volts is kept away from the grid by the condenser C^. Since effec­ tively stops or blocks the d. c. it is sometimes called a blacking condenser. The electrons can flow from the plate through the choke to the battery. The Choke offers very little resistance to d. c. after the first surge whii^^build® up the field surrounding the coil.

The a. c. signal easily crosses the condenser 0^ as

explained in Fig; 51, The purpose of the choke is to pre­

vent the a. c. from going to the battery as explained in the

chapter on electromagnetism, since connects or couples

and Vg it is often called a coupling condenser. • >

By using a condenser which offers high resistance to

d. c. and a choke offering high resistance to a.c., we have

effectively separated a.:c. and'd. c. from each other.

, ;, . Voltage Rating Breakdown Voltage; When voltages of opposite charge

exist on each side of a material as is the case of the

dielectric of a condenser, considerable strain is placed on

the electrons attached to the atoms of the dielectric. On

one side is a plus charge pulling on the attached electrons,

and on the other side we have a negative charge repelling

them. When the voltage becomes high enough these electrons

break away and allow current to flow across the dielectric. 113

This is what happens when a condenser is used with too high a voltage and breaks down. The voltage at which these electrons break loose and allow electrons to cross Is known as "breakdown voltage".

The breakdown voltage Is different for different dielectrics.

For example, the breakdown voltages per .001 inch thickness of some common dielectrics are: dry air, 50 volts; mica,

8000-8000 volts; cotton (single covering), 260-340 volts; paraffined paper, 800-1000 volts; and glass, 150-340 volts.

Working Voltage: The working voltage is the d. c. volt­ age at which a condenser will operate for long periods of time. This rating is given by the manufacturer and is printed on many of the condensers. ’Then used for alternating current a condenser should have a rating of 1.4 times the a. c . voltage at which it is to be used. The reason for this is that for a. c. voltmeters indicate effective voltage rather than peak voltage. The peak voltage should never exceed the voltage rating of the condensers. : • ■ ' .... " ■ - "

CHAPTER XV ,4 THE NAMING, CONSTRUCTION, AND USES OF CONDENSERS

Classifications As Fixed and Variable; The tv?o main divisions into which condensers are divided are fixed and variable. If the structure of a condenser is such that the capacity cannot be changed without rebuilding, the condenser Is classed as a fixed condenser. There are many kinds, types, and sixes of fixed condensers. Probably there is a greater variety of condensers than of any other single kind of radio part. Some of these will be discussed under more definite classifies^ tions.

Variable condensers are condensers In which the capacity can be changed readily and often. In general they are of two types. In one type of variable condenser the plates are arranged so that the space between them can be varied. These usually have only two plates, one stationary and one movable.

Usually these are about the size of a postage stamp, end are adjusted with a screwdriver. See Fig. 33a .

Fig. 53 115

In the other type of variable condenser the spacing remains the same between plates, but the effective area is changed. Fig. 33S shows two plates, one rotor and one sta­ tor. Usually there are several rotor plates on the same shaft and several stator plates so that the rotor plates are all moving between stator plates at the same time. The shaded area shows the effective area which can be changed by changing the position of the rotor. This is the type of condenser most commonly used in tuned circuits for selecting the stationr in most cases more than one circuit must be tuned in order to change stations. This is accomplished by coupling the rotors together in such a"manner that they ill rotate at the same time and speed. Often the rotors of several condensers are built on the same rotor shaft. When two or more condensers are hooked together iri this manner they are called “ganged" condensers. Two-gang condensers and three-gang condensers are quite common, but we may have four or five-gang condensers as well. The ganging of con­ densers is shown in a diagram by joining the arrows with a dash or dotted line as shown in Fig. 33C.

By Nature of the Insulation: 'Condensers are often classified by the nature of the dielectric used. -For example, Condensers using air as a dielectric are known as "air dielectric" condensers or "air cbMansers"* Those having mica Insulation are often called "mica condensers". *

Some of the more common kinds are paper condensers, mica 116

condensers, air condensers, vet electrolytics, semi-dry electrolytics, dry electrolytics, and glass dielectric con­ densers . This is the most common method of classifying condensers other than as fixed and variable condensers.

By Constructions Another method of classifying conden­ sers is by their eonetruction. Condensers which are built into a metal can are known as "can-type" condensers. Some­ times we have several fixed condensers built into the same unit. These are known as condenser "blocks".

Some of the more common types are: cartridge type, tu- bular type, paper, paper block, metal can, and bakelite or molded condensers.

The block and paper block condensers are also classi­ fied by the capacity of the various units included in the block. A block containing an 8 rafd. condenser and a 4 mfd. condenser is known as an "8-4 mike" condenser. One con­ taining two 8 mfd. and a 4 mfd. condenser is known as an

"8-8-4 mike" condenser. A block containing two 0 mfd. con­ densers is known as a "double 8 mike" block. The unit of measure of capacity is a farad, but this Is too large a unit for radio work so we use a microfarad (mfd.) meaning one- all lionth of a farad.

By Use: Another very common method of classifying con­ densers is by the use for which it is designed. Air-dielec­ tric variable condensers are often called "tuning condensers". 117

High voltage condensers of from two to sixteen mfd. designed

for use In a. c. power supplies are known-as "filter conden­

sers”. -

The•common types are; filter, by-pass, tuning, coupling,

and trimmer or padding condensers, ife have used tuning con­ densers and by-pa»» condensers in the construction of the

crystal set, and in the one-tube sots.

Construction of Condensers

Air-Dlelectrlc Condensers: Most air-dielectric oonden- sers ere of the types described under the discussion of va­

riable condensers and shown In Fig. 33. Few fixed condensers are made using air as a dielectric because of the low break­

down voltage of air. The chief disadvantage of air-dielec­

tric condensers is the size necessary in order to get enough capacity and to stand high voltage. Air has a low dielec­

tric constant and low breakdown voltage. - '' . i- - - Paper Condensers; Most of the low voltage by-pass con­

densers are of the paper dielectric type. Usually paraffined

or waxed paper is used. These condensers are made by taking

two long strips of paper and two long strips of lead foil or

aluminum foil and rolling them into a compact cylinder;

These are rolled s© that there is a layer of paper between

each two layers of foil. Th# edge of one sheet of foil sticks

" ...... ■...••...... - out of one end of the cylinder. A cap is then soldered, or.. clamped to each end and leads brought out. In this manner U@

ve get two plates of the.condenser, separated by paper as a dielectric. The condenser is enclosed in a cardboard case or cartridge. This makes a very compact and easily manufac­ tured condenser* .. : , ^ Fixed Mica Condensers: Mica fixed condensers are usually built by alternating layers of mica and lead foil or aluminum foil. The entire eoMeftser is the# molded into a bakelite case, allowing only, the lead vdres to come through.

These condensers cost more than paper condensers but are sturdier and will stand higher voltages.

Many of the trimmer type variable condensers.(Fig. 33A) use mica for the dielectric when closed, and use mica and air for the dielectric Then open. ,

: Electrolytic Condensers: The name electrolytic comes from the use of a liquid conductor. Any solution which will conduct electricity is called an electrolyte.

When an aluminum rod is placed in a borax solution as

shown in Fig. 35, a thin coating of aluminum hydroxide and

.+

Fig. 33D aluminum oxide forms on the rod and acts as a dielectric.

The rod must always be connected to the positive side of the circuit, and therefore cannot be used with alternating cur­ rent. if we connect the minus, to the rod and' plus to the r can the film Is destroyed and thus the condenser is destroyed.

There are two advantages of the electrolytic over a paper condenser. One is the comparatively small space occu­ pied by the condenser because of the extreme thinness of the dielectric. -~ ; : r :u -v;

: The other advantage is that exceeding the breakdown voltage does not destroy the condenser. As soon as the excess voltage Is removed the coating is replaced by the chemical reaction of the current and the condenser is as good as be­ fore.. . : -: V - 1 - - --V , ■

The disadvantages are (a) there is danger of spilling and splashing of the electrolyte in shipping or moving; and

(h) it cannot be used for.a. c. The first disadvantage is partly overcome by adding a thickening substance to the electrolyte, making it thick like gelatin. This type is known as a semi-dry electrolytic.

Dry Electrolytic Condensers: The dry electrolytic con­ denser is even more compact and more convenient to use than the wet type electrolytic condenser. The construction of the dry electrolytic is somewhat similar to that of the

paper condenser. A strip of aluminum foil Is prepared with a coating of hydroxide and oxide covering its surface. This 120

serves as the positive plate of the condenser. The electro­ lyte which may be a thickened borax solution is soaked up by some material, such as gauze. The coated aluminum foil, ,

another foil for the negative plate, and two strips, of elec­

trolyte absorbed by the gauze are then rolled together and

placed in a metal can or in a cardboard carton. Wires are

fastened to each of tho foils and brought out through the eon-

When,a metal can is used for an electrolytic condenser

of any type, the center pole is usually plus and the can is

the negative. In cardboard encased dry,electrolytics flex­

ible wire leads are used.: The red lead is always positive, and the black is always negative. ; ...... ,

Like the wet and semi-dry types dry eleotrolytics can­ not be used for, a* o. but can be used for d. c. and pulsating

-• :• Uses of Condensers

Importance of Condensers: The underlying principle of

all radio circuits depends on the proper combination of

three things: Inductance, capacitance, and resistance. With-'

out any of these three factors radio would be impossible. We

need capacitance in various parts of the circuit for: by-

passing r. f.; coupling circuits; tuning; filtering; padding;

and blocking. We have already started studying some of the

■ • - . / . . uses of omadensers in the crystal set and the one-tube sets. m

We have seen that variable condensers are used for tuning and that fixed condensers are used for by-passing r. f. •-YX : current around the phones and in the grid leak. In Chapter

ZIT we saw Mow a condenser could be used in coupling two parts of the circuit.

We have mentioned trimmer or padding condensers several ' . - . ' - . - ; - : - . r i-"'L: i r-.i? '' times but have not defined them. If you will look carefully at a two or three-gang condenser you will find somewhere on each section a small variable condenser of the type ahcnrn In . ; /": -- \ y,. ' :/ V ' - I- - : ' f, ' - - Pig. 324. These small condensers are connected to the lar- gar sections and are part of the total capacity. The purpose

': : . •. • . . " - :. .. ' \ :• of these trimmer condensers is to take care of any small ■ ;• . . , . ' : v' ' ' ' . ' " : - . , . ' '. . : " . - ■ differences in capacity of the circuit so that both circuits will be in tune at the same time. Suppose the circuit in which the second section is used should not have q^uite as much capacity as the one in which the first section is used.

If we did not have a trimmer condenser we could not get the circuits for both sections in tune by turning the rotor. By adjusting the trimmer to make up for the lack of capacity we

can make both sections tune together.

These trimming condensers are also called "padding con­ densers" or "balancing condensers". •

Filter condensers will be discussed in the next chapter. , " ' .<■ • t . ' • “ V-*‘- CHAPTER XVI

POWER SUPPLIES Purpose , .

The p«fpbM:ror. this chapter is twofold. First, it . ^ . Y , ^ < . - . -i - ; \ - "v- f ; - . enables you to underatand how alternating current from the 110 V. line is changed to pure d. p. which can be used in ' ' ^ v - ■■■•■■. ' ' ' . ■ - the various parts of the radio circuit.

The second purpeae Is to give you plans for a power .

supply tiiich can b0 used In experimenting with various cir­

cuit s* It is not designed to be used with the *30 tube set

' ’ ‘ ■ • ' ; ' " ■ - " ' ■ ■ - ■■ ■■' - - ' -■ ' ' - - • - but can be used to provide plate voltage in place of the "B"

batteries.- . . « The filament voltage ^ will * ^ still - . ' have. Y .. to- . . come. . . from batteries, - : ... ; C - : .xs, : .,. - r % c r e are many circuit diagrams,available for sets which

operate from this type of power supply, n ,

" Theory ' ' '''t; . v ' ' Schematic Diagram: The schematic diagram of a typical

power supply is shorn In Fig, 34, Most of the symbols have been used before ia different circuits. The transformer is

an iron core transformer with the primary wound for 110 volts a. o. Three secondary windings are shown. One is the

high voltage winding to supply **3" voltage. The flv® volt

winding is to heat the filament of the *80 rectifier. The

third secondary winding is a 2.5 or 6.3 volt filament supply tor filaments of the tubes used in the set. Your ehoioe of 3.5 or 6.3 volts will depend upon the tubes used in the set selected. . ? j/- - i c.:- ;

=

; - :

:— ■>af<>v' *,c> 2e>’Y‘tc- l§9*

m

- ? :: *• 'tar-- 0 ^— liT *0OQ'~ « ' m -$ * il . Fig. 34 J . " : /.dr;:,'' / ' " ' .. • ‘ Four Parts of a Power Supply: Any- divided into four distinct parts: (a) the transformer ehsuiges the voltage from 110 volte to 350 volts for the "B” supply, to S volts for th. reotlfler, and. to 2.5 or 6.3 volte for the filament of the tubes in the set; (b) the rectifier, usually a vacuum tub®, changes the a. c. from the 350 volt winding to d. c.; (o ) the filter, consisting of chokes and condensers, smooths out the fluctuations of the pulsating d. c.; and

(d) the bleeder-voltage divider gives us taps for various . voltages and k e # # down extremely higi peak voltages.

The action of the transformer and rectifier have already been discussed in the chapters on Electromagnetism and Two

Element Vacuum tubes. The filter and voltage divider will 124

be considered htst# r, ; ^ : . T •- - The Filter? In,Fig. 54 the filter consists of two 30 henry chokes, and L2 , and two 8 mfd. condensers and Cg.

The unit of masure of inductance is a henry, often abbre­ viated, hy. or h. The operation of the filter system can best be understood by comparing it with a water system as shown in Fig. 35. : ;r ^ .-

vf t*»*r n*4e

Fig. 35

In this arrangement we have water coming in spurts from a piston type pump. s. and so are sections from an old inner tube which will expand under pressure to hold more water.

V1 and Vg are partly dosed valves or faucets. Water coming

from the piston type comes in spurts with intervals during

which no water is being supplied.

If both valves were open water would come from the end of the pipe, in spurts just as it entered from the pump.

With 71 partly closed the entire volume of water from ... . _. ... : , - . - :/ . + - ' , - ...... , the first surge could not get through as fast as it comes

from the pump. This holding bask of the water by V1 causes 125

the section of inner tube 5^ to expand in order to hold the excess water. As soon as the surge from the pump dies down the pressure on 8^ decreases and it begins to contract.

This contraction forces water to flow through the valve 7^ even after the surge from the.pump has stopped.

If we allow enough storage space in S1 and adjust 7^ properly we can get a continuous flow of water from the pipe.

Adding another similar section s2 and 7% gives us more smoothing effect to smooth out any slight surges which may get through 8^ 7^.

In the filter system, Fig. 34, and Lg correspond to the valves 7 and 7, and C2 correspond to the sections of inner tubes s_ and s The electrons flowing across the 1 . ■ *80 tube to first one plate and then the other correspond to the spurts of water from the pump.

A surge of electrons flows across to either of the plates of the *80 tube, through the circuit and back toward the filament. As the surge reaches Lg it is held back by the inductance of the coil. The building up of a field around Lg takes.time. This holding back of electrons causes them to go into the condenser Cg where they are stored. As the surge stops, the field around Lg starts to break down causing current to continue to flow. This breaking down of the field, plus the voltage across Cg,. causes the electrons to leave Cg end continue toward the filament. This gives us a smoothing out of the surges of electricity just as V1 smoothed out the flow of water.

4 second section continues the smoothing out effect giving us pure d. c. Fig. 364 shoes pulsating d. a.

f**k rtitles . *u. srerfcyc wW*f«d

h

Fig. 36 as it comes from the rectifier. Fig. 36B shows the current as it comes through the filter, practically free from vari- atioas. We have smoothed out pulsating d. c. into pure d. c. smoh as we obtain from batteries.

Bleeder-Voltage Divider: The resistor in Fig. 34 serves two purposes. First let us consider the bleeder

##ti

There are times when we want to use a power supply with little or no load on the d. e. section. If it were not for

R^ high voltages would build up across the condensers and

'8 mils not only puts a strain on the condensers but also gives us too high an instantaneous peak voltage on the equip­ ment wWLeh we |an eonneot to the power supply. By. placing w

in the power supply circuit we always have some load on the power supply regardless of the rest of the circuit.

A bleeder also allows the condensers to discharge after the power supply is shut off. This helps to avoid unpleas­ ant and sometimes dangerous shocks when working with the power supply* - " -- ' - - - ^ \

The sedend purpose of is to supply various voltages for the different tubes and parts of tubes used in the set. •

These 15,000 ohm resistors are obtainable with tape %V e # three thousand ohms. The exact voltage drop between taps may be calculated using Ohm's Law when we know how much cur­ rent is to be taken from each tap. For two and three-tube sets we can assume that the voltage drop is the same between any two taps. The load supplied by the plate circuit is ’ comparatively small and as much as 20$ changes in voltage make very little difference i n .performance of modern tubes.

-These.voltage dividers,may also be purchased with

. . ' * • sliders so that you may secure any desired voltage by moving the tap.l Using this type, and with the aid of a voltmeter, you can secure the exact voltage required. Be sure to measure your voltage under load as the voltage changes when more current is drawn through the resistor. (Ohm’s Law).

Notice that the highest d. c. voltage obtainable is

only 250 volts, while the a. c. voltage of the high voltage

winding is 350 volts. This voltage drop is due to resist#*##

in the rectifier and in the chokes. - 128

- Construction r Selection of Chassis: Suppose wo decide to build this power supply to be used with a receiver which we intend to build. Again we have a choice between breadboard construe^ tlon and' metal chassis. ■> , - ; :, ■ - ■: \1 .. :• - - - . . a . ' This time we decide to use the metal chassis, one rea­

son is to get experience in building metal chassis and the other is that we may want to use this power supply with a

short wave receiver. Short wave receivers require better

shielding. .• : r. • : . -

Laying Out and Construction of Chassis: A chassis measuring about 7* x 10" % 2" will give us sufficient room.

We will need a piece of sheet metal measuring 11&" x 14&".

About will be required to give a well rounded edge. Cut

out a notch 2in square in each corner as shown in Fig. 57A,

■ ' -

V ■ ' ■ ' Fig. 37 "

and bend the sides along the dotted lines. The bending takes

up about , leaving us an inverted tray two inches deep. 129

(Fig. STB)v Solder the corners together on the inside making a rigid chassis.: - - in the chassis will vary with the types of transformers, chokes, and condensers used. Round holes for tube sockets and inverted type electrolytic condensers may be made with a circle cutter or with a cold chisel. If cut with a chisel the edges should be filed or reamed to improve the appearance, and to get rid of sharp edges.

Square openings for the transformer and choke may be cut with a cold chisel. When using a cold chisel put a block of wood directly under the point being cut to prevent the chassis from being bent out of chape.

Mounting the Parts: All parts should be mounted secure­ ly to the chassis so that there will be no chance for them to become injured or to cause short circuits. Transformers, chokes, and electrolytic condensers usually are provided with bolts or holes for bolts to be used in mounting, Dry-elec- trolytios provide more of a problem. Some of them, provide for mounting but others do not. The ones which make no pro­ vision may be mounted to the under side of the chassis by taking a strap of metal, bending it to fit tight over the condenser or condensers and bolting to the chassis.

Wiring: As much as possible of the wiring should be ddne under the chassis. This improves the appearance, and protects the.wiring. The ohasais should be considered as ground and all negative leads connected to the chassis. 130

lhen#rer a positive lead must come through the chassis it should be well insulated. Make the hole considerably V"/ - - larger than the wire and use a type of high voltage insulation known as "spaghetti" over the wire where it comes through the chassis. Make sure all connections are properly soldered, and tape all bare places in the wire. CHAPTER XVII'

ADDING AUDI0 AMPLIFIERS TO THE ONE-TUBE SET

When the signal comes through the detector tube it does not set up very strong vibrations in the earphones. Usually one or two tubes are arranged to build up the strength of the audio frequencies when they leave the detector. A e W tubes are known as audio amplifiers.

Types of Coupling

. In general, there are three ways in which audio ampli­ fiers are coupled to each other and to the detector. They are:known as: transformer coupling; impedance coupling; and resistor coupling. Many variations of these three types are found but an understanding of the general principle enables you to understand the variations. , t ,

-Transformer Coupling: The achematic diagram. Fig. 38, shows the one-tube set with two stages of audio amplifies- 132

tlon added, -uxunax parts required are the audio fro,uo.oy transform. T, ana Tg, ana the adaitional

,a° ,ebeS VS ^ ^ 386 Fle" 20A the proper placement of 72 , V3 , and T2 . . .

: > = s t audio frequency transforaiers or auaio tranaiornera as they are usually callea, have the terminals marked vB„ '

"F», and The terminal marked -P, connects to the plate Of the preoedin* tube. Ho changes are necessary in

the original circuit. ffe merely co^ect the primary of the

audio-transformer in place of the phones. The other end of the primary, ^ . connects to the plus 45 volts just as the othsr Bide Of the earphones did before. The connection

marked "G" connects to the gridhf the audio amplifier, and the other end of the secondary connects to the minus side of the filament.

in case the audio transformer is not marked, we can

find which terminals are connected inside the transformer This can be done by connecting a 1J volt battery in series

,1th k.W,h..es and one of the taps. Touch the other lead of the Phones to each of the other three taps. m e one

Which makes: a click in the phones when touched is the other end of the winding. Since there are only two windings the

Other two poles must go together, m e s s wo have an ohm

meter for measuring the resistance in the two windings we

will have to connect the transformer up first one way and then the other to see which way works the best 133

' If we do have an ohmeter we can determind the secondary hy finding the winding with the most resistance. Most audio transformers have three times as many turns on"the secondary as on the primary. This means that there is three times as much wire'on the secondary, and the resistance will be about three times as high. This type is known as a "three to one" audio transformer, meaning that the turns ratio is three to

Theory of Transformer Coupling: The theory of trans­ former coupling is very simple. Variations in the plate current of- the detector or preceding audio tube cause vari­ ations in the current flowing through the primary of the audio transformer. These variations in current through the primary cause a field to build up and breakdown surrounding both windings. This field building up and breaking down esuses currents to be induced in the secondary which are similar to the currents causing the field. In a three to

one transformer we get the signal amplified three times in the tremuaforaer# This; is one of the advantages of trans- " ' " v: ^ 1 ' VP- r % --/ / former coupling. » • ; ■ ' . 1 " " % e signal is then impressed on the grid of the ampli­ fier. The amplification by the tube was described in the

chapter on three element vacuum tubes.

The third stage is exactly like the second with. T„

coupling V2 and just as T^ couples and V^. See

Fig. 20A for placement of the parts. The first audio aitould 134

be made to operate properly before adding the second stage.

With two stages of audio this receiver should drive a magnetic or permanent magnet dynamic speaker loud enough for the average room. Raising the plate voltage of the audio tubes will liter ease the volume of output, but you may have to connect a

VO* battery in the grid circuit between the filament and the audio transformer. The plus of the "C" battery connects to the filament and the minus to the "F" tap of the transformer.

*" "C" bias of minus 4.5 volts should be used for 90 volts on the plate, minus 9 volts for 155 volts on the plate, and minus 13.5 volts for. 180 volts on the plate of the '30 tube.

Choke or Impedance Coupling; Another common type of coupling is shown in Fig. 39A. This is known as choke coupling or impedance coupling. The circuit is the same as

Fig., 39 . for any other eirouit until we get the signal through the

MFC. The choke prevents the signal, alternating current, from going to the battery and forces it through the coupling condenser to the grid of the amplifier. The resistor serves the double duty of supplying grid bias to the tube and keeping the signal from escaping to ground or minus of the

supply. .

This type of coupling has an advantage over resistance coupling in that almost the full plate voltage reaches the plate because the resistance of the choke to direct current is very low. It has a disadvantage as compared with trans­ former coupling in that there is no amplification in the coupling.

Resistance Coupling: Fig. 391 shows a common form of resistance coupling. The resistor keeps the signal from going to the "B” battery or power supply, forcing it to go through the coupling condenser C^. Again we have the grid leak resistor keeping the signal from going to ground so that it is impressed on the grid of the amplifier tube.

The advantages of resistance coupling are that the parts are cheap and take up very little room.

There are some disadvantages. "First, the resistance of

Rg must be fairly high to prevent the signal from going through it. This causes considerable drop in d. c. plate voltage which must flow through the resistor. This means that the batteries or power supply must be able to produce higher voltage in order to get the rated voltage on the tube.

Another factor which is of importance in battery sets is Shat power Is lost in dropping this voltage. The power lost in heat can be ealetilated by the formula Hw s I®R. This loss is not great but does cause batteries to run down more , ' : ■ - quiokly. This loos can be neglected in sets operating off the 110 volt a. c. line.

• i d-W - ^ OffiVPTEHmil 3:: '' ' . ' ', : "t - -,f VACUUM TUBES .. - A '-' - ^ "■•1 . v / -.... Triodes ^ - . ' ' - ' . r_ * - f . r- J t . : ' " '' Filament structuro: We have already discussed the ■ '.•■■■■ ■ ■ : : •' t *' r . \ ' theory of operation of the three element vacuum tube or tried®. There are several types of triodes with the dif- ferenoe due to the purpose for which the tube is to be used.

The chief differences are in the electron, emitting element.

% # filament of a battery operated tube is of rather . - .. . : r - " L.-' ' . . . ' - -v, : light construction, heats up rapidly, and uses a minimum of filament current. Tubes designed to operate with a. o. heated filaments have filaments of heavier construction so a : that they will not cool off while the direction of current ‘i.v. ■ ■ *- - . . ■ ■ ■•. ■>*,. :r,: ' - - ' "' " ■ ■- is changing; Using 60 cycle a. o. to heat the filament of a battery type tube such as the *50 tube produces one hundred twenty.cycle hum. Sixty cycle current changes direction one huMred twenty times per second, allowing the filament to heat up and cool off that many times per second.. % i s heating and cooling allows the electrons to flow in waves

with the result that we get hum in the phones.

; ■ ' ... ■ : ■' : ; ■. . . '' " ' ' • ' - The larger wires of a. c. filaments do not cool rapidly

wough to cause h w . The a. c. filament can be heated satls- . ' t. : • ...... ■ < : C" . ' •-- .' factorily with d. c. except that the current required for the ' " ■ ■ . ■ 1 . ' V' ' r : . . ■ . . .- ■ • . ■ - - larger filaments causes a rather heavy drain on the "A" 138

tettery. - - V:-:\ : " ' • " r " ■ Cathodes: A better method of getting away from hum

when using a. c. to heat the filament is by the addition of

a cathode. The cathode is a metal cylinder surrounding the filament and close enough to the filament to be kept red hot.

The cathode may be insulated from the filament by some good insulating material such as "isolantito" or may be insulated

by the small space between the filament and cathode.

When cathodes are used the electron emitting substance,

barium oxide or thorium compounds, is coated on the cathode Y- ■■ - : ' . -- - . . . rather than the fllament. The cathode is the electron emitter and is heated indireotly by the filament. The ca­

thode is comparatively heavy so that it heats up slowly and

cools off slowly. This slow heating and cooling completely

eliminates hum. Receiving sets using cathode tubes usually

require a warming up period of thirty seconds or more, before

they are ready to play.

Vacuum tubes containing a filament, cathode, one grid,

and a plate are classed as triodea even though they do actu­

ally have four elements. The cathode and filament are con­

sidered as one element because together they perform the

same work done by the filament along in most battery type

tubes. The minus of the nBn battery or power supply connects

to the cathode instead of tho filament when there is a ca­ thode in the tube. This part of the circuit from the minus 1S9

of the power supply to the cathode or filament is known.as the cathode circuit. The part of the circuit from the.plus of,power supply to the plate of the tube is known as the plate circuit, while the circuit from the grid of the tube to the cathode or filament is known as the grid circuit.

Tetrodes: The amount of attraction for electrons depends

upon the spacing between the positive electrode, the plate,

and the electron emitter, the cathode or filament. By

decreasing the spacing we can increase the attraction, hut at _• ' : : ' * Vs.'.;:;-'.- ^ : :: ' \ ‘ • the same time we would decrease the amplification factor of

the tube as seen in the chapter on "Three Element Vacuum

Tubes". At the same time decreasing the spacing would lower

the breakdown voltage across the tube making it necessary to

use lower plate voltage. In order to keep the plate spacing great enough to have : ; - ' ' ■■ :: j ; ::- ' ' ' . • - ' - a high amplification factor, and at the same time have a

positive charge near enough to the cathode to attract large ,

numbers of electrons, we may add another element between the * • < . ' - i . : . . V . ' : ■■ : 3 ; . r v 3 ' grid and plate as shown in Fig. 40A. This second grid is ' . . ... / - . ' . " 3 :: / ' ? 3 : ' / "'3, -.r :: 3 . .< S,T*.,. ' . known as the screen grid and is shown as grid number two on . •- . 3 - ' ' ' '' uc 3 ' . . ■ V ': ' " ^ .. 3 ■ ;■ diagrams of tube connections. Grids are numbered conseeu- tlvely beginning with the one nearest the. cathode or fila­ ment. ' Grid number one (with rare exceptions) is used as the control grid. ' ,

? 'V5- - ' j;,™ i- a

A ■ l-S

Fig. 40

positive voltage to attract the electrons from the filament.

When the electrons reach the screen grid most of them do mot

stay but are attracted by the higher positive voltage of the plate. In this way we have placed a positive voltage nearer

to the cathode, but at, the same time are able to keep high voltages on the plate and keep the amplification M o t e * high.

The positive voltage for the screen may be obtained .

ilveetly from She battery or voltage divider of the power

supply, or may be obtained by using a "screen grid dropping resistor" as shown in Fig. 40A. Some of the electrons

flowing from cathode to plate are attr&oted to the screen .

grid and stick to it. Enough of these are kept on the screen

grid by the resistor (R in Fig. 40A)-to keep.its voltage less th# voltage. .r f-: . Pentodes: Addition of a screen grid in certain tubes caused the tube to be noisy. Experiments showed that the 1 electrons were attaining extremely high speeds when attracted by the voltage, on the screen and plate. Electrons traveling at these high speeds were striking the. plate with such force that many electrons v/ere being knocked out of the atoms in the metal plate. This "secondary emission*’ as it was called caused the noise, imat was needed then was; some way to slew these electrons down before they hit the plate so that secondary amission would mot take place.

A solution was found by adding a third grid (See

Fig. 40B) between the screen grid and the plate. This third grid is operated at a very low voltage so that the electrons lose part of their high speed.

In soro tubes the suppressor grid, as this number three grid is Called, is connected to the cathode or filament in­ side the tube so that there are no external connections to make. ' In other tubes the suppressor lead is brought out

separately bo that the voltage may be changed depending upon

the use to which the tube is being put.

In general, the more grids a tube has the higher the amplification factor. "

Beam Power Tubes: A new type of tube for use in driving

loud speakers has become popular during the past few years. ■■■ ' - .. '■ ■ ' '. : . ■ ■ ' ■ : a V- ■ - This tube is known as a "beam power tube"" because it uses beam forming plates instead of a suppressor grid. These plates are placed betweed the screen grid and plate in such a manner that they allow the flow of electrons to Strike only the desired portion of the plate. these beam forming plates also set up a negative Mspace charge" which s l W l the electrons down Just as the suppressor does.

The beam forming plates have a separate connection brought out through one of the prongs so that they may be connected either, to the cathode or the shell of metal tubes or both depending upon the circuit. The shell may also be . left "floating" in some circuits.

This type of tube is capable of. higher amplification of rather large amounts of power.

;' Multi-Purpose Tubes

There are a large number of tubes designed for special purposes which are really two- or three-tubes-in-one. A duo-diode contains two diodes in the same glass envelope.

The type *60 tube is a duo-diode used as a full-wave recti­ fier. Twin-triodes consist df two separate triode sections in the same envelope. Twin-triodes are often used as power tubes to drives loud speaker.

Another common type contains two small diode plates in addition to the triode or pentode elements. These are known as duplex-diode trlodes dr duplex-diode pentodes depending upon the structure. The most common use for this type is as 143

r* diode detector, automatic volume control, and audio ampli­

fier. The one tube is made to perform three duties. Each

section works Just as though it were a separate tube.

:,, The tubes mentioned are the most common types, but

there are many other types which have other special uses.

Gas Filled Tubes

Mercury Vapor Rectifiers: Most tubes work in a vacuum,

but there are a few which use a gas inside the envelope instead of vacuum. The most common of these are the mercury

vapor rectifiers. In the manufacture of mercury vapor recti­

fiers all of the air is first pumped out and then a small

amount of mercury is added, some of the mercury evaporates

filling the tube with mercury vapor. Electrons flowing from

the filament to the plate strike the atoms of meroury causing

them to ionize or lose electrons. This ionized vapor makes a path of low resistance for the electrons to follow.

Because of the low resistance of mercury vapor the

voltage drop in a mercury vapor rectifier is fifteen volts as

compared with several times as much in vacuum rectifiers.

Because of this low voltage drop, and their ability to carry

heavy currents, meroury vapor rectifiers are widely used in

transmitting equipment and public address systems.

There is one disadvantage of the mercury vapor rectifier

In radio receivers which almost completely eliminates it from

this field. Meroury vapor rectifiers often produce a hum in 144

receivers.

Mercury vapor rectifiers are not able to stand as high peak voltages as vacuum rectifiers with the sane spacings, but will carry more current.

■.f " L'y.

y.

,x«: v ' '

;.e:- f . , • . ' . • .1

r c . : " . .'V - . '-.ti ' " ;: . . .': .

, . . i-- : . CHAPTER XIX

: . , , " ■ . - ■ ■ ■■ ‘ ' . . RADIO FREQUENCY AMPLIFIERS

Reason for Radio Frequency Amplifiers in Receivers

Sensitivity; The sensitivity of a radio receiver is its ability to pick up radio frequency signals. A receiver , -■ • ■ .. . • ' ;■ • ,• ..z.,' r ‘ . which dan pick up weak signals from distant stations is said to be very sensitive, or to have a high degree of sensitivity. Sensitivity depends largely upon the number of times that the signal is amplified. There are several ways of amplifying signals. We have seen how signals were amplified in the one-tube detector, in the regenerative detector, and in audio amplifiers.

There is a limit to the number of times that a signal can be amplified at any one frequency. For example, if we used about four stages of audio amplification we would find that noises originating inside the tube, and background noise from other sources, had been amplified as well as the signal. Above a certain level this background noise becomes very disagreeable to the listener. We must find some way to keep the "signal-to-noise ratio" high. The signal-to-noise ratio is the name applied to the comparison of the signal strength as it comes from the receiver with the noise which comes from the receiver. The volume of the noise is. known as the "noise level". The noise level varies greatly with different receivers and in the same receiver at different times. Keeping down the noise level makes it necessary to ' limit the number of times the signal is amplified at audio frequencies. / - v , ,'v . ■

Increasing the amplification of a receiver without fur­ ther increase in audio amplification can be done by adding am amplifier ahead of the detector designed to amplify the signal at radio frequencies. Radio frequency amplification also is limited by the noise level, but this noise is ampli­ fied at a frequency different from - the audio section. This makes possible amplification at radio frequencies up to the point where noise becomes troublesome and then changing the frequencies to audio frequencies in the detector. The audio '' - ' V . ■ • "d" ■ ' amplifiers can then take the signal and amollfy it without . ,d / . ./d:dd;d. . • - ' - amplifying radio frequency noises which do not come through the detector.

By using two different frequencies, r. f . and a. f ., we have greatly increased the ability of the set to amplify a

signal without raising the noise level. The sensitivity of the set is increased by radio frequency amplification.

Selectivity: Selectivity is the ability of a radio

receiving set to tune in the desired signal and tune out all

others. Sets which can easily tune out undesired signals and

tune in the desired signals are said to be very-selective or to have a high degree of selectivity.

Tuned circuits are usually used in radio frequency amplifiers. This gives us two or more, tunedeircuits.

Suppose that we have a receiving set with two stages of tuned radio frequency (T.R.F.) amplifiers aW#(L of the dateetor. ,

The first tuned r. f . elrouit selects the desired signal and amplifies it. The second.tuned r. f. stage again selects the desired signal and amplifies it. The detector circuit makes a third selection so th#t we have three successive t m m d cir­ cuits selecting the desired signal and discarding the other unwanted signals. . .

Tuned radio frequency amplifiers, increase the sele.ctiv- ity of a receiver by adding tuned circuits ahead of the detector. . • . . , : ... ■ , . : ?

Types "of Radio Frequency Amplifiers

Untuned Radio.Frequency Amplifiers: In some of the older type sets a stage of untuned radio frequency is used ahead of the detector. An untuned r. f. stage is coupled to the antenna through a condenser or untuned transformer, or to a preceding stage through an untuned transformer. This type of r. f . amplifier adds sensitivity but not selectivity to the set.

Tuned Radio Frequency Amplifiers: The tuned r. f. ampli­ fiers have almost completely replaced untuned r . f. ampli­ fiers because of the added selectivity of the set using addi­ tional tuned circuits. The coils and condensers of the t. r.

f . amplifier are exactly like the detector coils and condenser 140

so that they can be tuned by ganging the condensers and tuning with one dial. Regenerative Pre-Selection; Tuned r. f. amplifiers are .* ...... " tn ! ' ' '' . - \; • ■ • I-* \ often called pre-selectors or stages of pre-selection because of their tuned circuits selecting the signal ahead of the detector circuit. Regenerative pre-selectors are t. r. f . amplifiers which have the amplified signal from the plate circuit fed back to the grid circuit. Circuits similar to our regenerative detector (Fig. 21B) may be used except that the tube is biased as an amplifier rather than as a detector.

"Feedback", as this coupling of plate, to grid circuit of the

®Mae tube is called, may also be accomplished by using con­

densers instead of tickler coils to feed back thje signal.

Vr CHAPTER XX . ' . - : : /' J ^ ... .H ' \ OSCILLATION; RESONANCE ' - . . --- r" ' v' % ' Oselllating Circuits .■ . : . ■ . v .. ■ ■ ■ 1 . / . : : - ? '■ ' :tr\ : ;* ' ' ' . :■ : _ Definition: To oscillate means to swing back and forth.

The pendulum of a clock oscillates. The striker on an elec- ' - -' ^ ~ \ - . ' '-1 / v. . - . ' - ^ :: trie bell or alarm clock also oscillates. In radio and

electricity we say that a circuit is oscillating when the current flows first one way and then the other. A circuit

which produces these electrical vibrations or oscillations is

called an oscillating circuit or oscillatory circuit. . .. . ; ■ ■■■,. ■.. .. y.,i,, - c,: . - .- : . • Coil and Condenser: The fundamental oscillatory cir- rr~- 7" ; ; " ■ " ■ ■. * :• . ; •. ... . ■ . cult consists of a coil and condenser in parallel as shown in Pig. 41A. If we should suddenly charge the condenser as ■ .. - ' '...... ' ■ y.. •' .. ' vr :

Fig. 41

shown in the figure the excess of electrons on plate "X" would try to get to plate "Y" where there is a shortage of electrons. Since the two plates are separated by an insula­ tor, the electrons are forced to go through the coil L. This flow of current sets up a field around, the coil L.

W e n enough electrons have reached plate "Y" to balance

up the circuit, the current attempts to stop flowing. As soon as the current begins to decrease the field begins to break down. This breaking down of the field induces current

in the coil forcing the electrons to continue flowing,until

the field has completely broken down. This is the familiar

flyWeel effect of a coil in electrical circuits.

At the instant the field has. .completely broken #own we find that.w® have the same condition which existed at the

beginning except that this time the plate "Y" has an excess

of electrons and plate ”X" is lacking in electrons. This

difference in voltage across the condenser causes the elec­

trons to start back through coil L to plate "X". Again the

field builds up and. breaks down forcing an excess of elec-

trons on to the plate "X*.

If it were not for the resistance of the circuit this

oscillation or shifting back and forth of electron® would

continue indefinitely. If no power is added the resistance

of the circuit,and absorption of power by objects in the

field of the coil soon "kills" the oscillation.

The Vacuum Tube In an Oscillating Circuit: Since power

is being lost from the oscillating circuit by radiation and

absorption some method must be used to replace this power if 151

we wish to keep the circuit oscillating. This can be done by connecting the plate of a vacuum tube to one end of the oscillating circuit as shown in Fig. 41B. - The electrons charge up the eomdemeer plate connected to the tube and start back toward the opposite plate. Just as they are starting b a d we give them a push by sending a shot of electrons through the tube. In this way oscillation is easily maintained just as it is easy to keep a person oscillating in a swing by giving him a push each time he starts moving away from you. In order to -apply this push at the proper time we usually couple t h e ?amplified plate current back to the grid by using either a tickler-coil or condenser. The regenera-

tive one-tube set is constructed so tiiat turning the regen­

eration control so as to force all or most of the plate cur-

rent to flow through the coil will cause the set to oscillate

The point of oscillation is detected by a single click in the

phones. If the set is tuned to or near a station while

oscillating a whistle will be heard. Oscillation is regenera

tion carried to the point Where the energy feedback from the

plate is enough to keep the tube working even after the ori­

ginal signal is removed from the grid.

Audio Oscillators; Vacuum tube circuits can be made to

oscillate at almost any frequency from a few cycles per

second to several hundred million cycles per second. Cir­

cuits designed to oscillate at audio frequencies find many 15#

uses in radio. One important us® for them is in testing

audio amplifiers, headphones, and speaker®, service men use them often for this purpose, they are also used in a musical

instrument called a "Novachord" to produce musical notes.

Audio oscillators are sometimes used to modulate a trans­ mitter. The code which is often used to introduce news * broadcast is produced in this manner.

irH-.S’f. -IKS*

Fig. 42

used by beginners to learn the International Morse Code. The

transformer should be the smallest and cheapest one available.

Larger and better transformers give too low a note.

Test Oscillators: Radio frequency oscillators are often

used in service work for lining up a set. It is easier to

get all circuits properly tuned if a strong steady oscilla­

tion is used instead of trying to use a modulated signal

from a broadcast station. This also makes possible the

lining up and testing of a set when no strong local station

is operating.. Usually for service.work the oscillator Is built with sevoml oeili which can be switched, thus changing the frequency of the signal generated, •; - : ,

Beat Mote Oscillators: It is often desirable to beat two signals together in order to get a third signal of a. different frequency.’ This is shown in Fig. 43. Two signals

...... - ...... ■; i ...... ris. 43 , ...... ; of different frequencies are shown in the diagram! At cer­ tain points: a, b, and c the twd'signals add togettiefso that we get a strong beat note at these points. Suppose we have two signals, one at 110,000 cycles and the other at

111,000 cycles. Each of these signals is far above the audible range as well as being too high a frequency for the phones to follow. The difference between the frequencies is the number of times per second at which the waves will coin­ cide. This gives ue 1,000 extra loud signals per second when both peaks arrive at the same time. The phones cannot follow either the 110,000 cycle or the 111,000 cycle, but dan. and will follow the 1,000 cycle beat note. The 1,000 cycle note is well withid the range of hearing so that by combining two high frequency notes we get one we can hear. This beat note makes it possible to hear code signals 154

with the one-tube set which cannot be heard on most broad- - cast receivers. Most code stations do not have any audio frequency variations, modulation. Only the carrier fre- quenoy, at radio frequency, is broadcast. We cannot hear them In the average' broadcast receiver because there is no beat note oscillator In the set. Our regenerative one-tube set can be made to oscillate and the strength of oscillation * j « .- - , - < controlled by the regeneration control. By tuning the set so that it is oscillating at about 1",000 cycles;more or 1,000 cycles less than the incoming signal we can get an audible beat note. It is this beat note that causes the whistle when tuning in a broadcast station with the set oscillating.

Neither the frequency of the carrier aor the frequency of

oscillation in the sot are of audio frequency, but the boat note is audible. >.

The beating of one note against another is known as a

heterodyne. This heterodyne is used In the meat popular of

radio receivers, the superheterodyne. Operation of super­ heterodyne receivers will be discussed in the next chapter.

Use of Oscillators in Transmitters: Radio transmitters use stable vacuum tube oscillators to generate the high fre­

quency carrier wave. This signal is then amplified through

several stages and then radiated from the antenna.

Fig. 44 shows two types of oscillators used to generate

radio frequencies. The one at the left is a Hartley Oscilla­

tor, which was very popular at one time. This type of oscillator has been almost completely replaced for transmitt­ ing by crystal oscillators of the type shown at the.right.

------...... - ...... • ------—-—

Hartley Circuit Crystal Oscillator

' " ' Fig. 44 ' " ... -

In the Hartley circuit the coil is divided Into two parts by tapping it, The lower part acts as a pick up coil to feed back the signal to the grid and thus maintain oscil­ lation. > • -

In the crystal oscillator the oscillating circuit is from the grid to ground through the crystal (Xtal). The cry­ stal is a thin quartz plate which serves as the oscillating circuit. The crystal will oscillate at only one frequency and so makes a very stable oscillator. The plate circuit then is tuned to the frequency of the crystal in order to amplify the signal. The frequency at which a crystal will oscillate is determined by its thickness. The thinner the crystal the higher the frequency of oscillation. Resonance

Tuned Circuits: For any combination of coil and con- denser in an oscillating circuit there is only one frequency ' " . at which the circuit will oscillate. In tuning in a station we change the capacity of the condenser until the tuned cir- cult would oscillate at the same frequency as the carrier * v,.. : '• ' ■: ’v'-; . . •- - wave being broadcast. In turning to this desired frequency we say that the set or the tuned circuit of the set is tuned’ to resonance with the/station, or that the .circuit is resonating at the frequency of;the station. ' : " ■ ^ !

When a circuit la tuned to resonance with an oscillating ■ ’ ■ '■ . ' *' " : circuit, the tuned circuit will absorb power from the field of the oscillating circuit. .There, are thousands of waves of different frequencies striking the antenna at all times. 1

The set takes power from the field of the station with which ' ." - . - : ' v: ' - : ..y. ' ' : : ■ . ■ it is resonating, but not from the fields at different fre- quencies. It is this property of resonance that makes tuning possible. 1 f-s

■■ ' . : • ; i;--; - If- ' . ' .-.f. CHAPTER XXI ^ 'v:-' -- SOPERHEfflRODYNE RECEIVERS , . . -• ■ .. ^ v i ■: Theory of the Superheterodyne

Block Diagram: Fig. 45 shows a block diagram of a . - : . . ' ■ • . . : i. i •- - : '• superheterodyne reoeiwer.- % 1 # type of receiver is by far

Fig. 45 ; •- the most popular on the market today. Some of the advantages

can be appreciated when we understand the operation of the

"euperhet". The signal is picked up by the antenna and

amplified by the preselector. The frequency converter con- eisting of the mixer-first detector and the high frequency

oscillator changes the signal to a frequency between the

• : ■ ■' . ■ .• - ' . - . ' ' • r. f . signal picked up and the audio frequency. The signal

at this intermediate frequency is amplified in the interme­

diate frequency amplifier. The second detector changes the 1. f. signal to audio frequency. This audio frequency is

then amplified and delivered to the speaker.

Preselector: The simplest of preselectors is merely a 158

tuned circuit ahead of the detector. This is sometimes used in some of the low priced receivers. It is better to have at least one stage of amplification in the preselector so that the signal will be amplified enough to override the tube noises which arise in the mixer-first detector. Pre­

selectors are not absolutely essential to superheterodyne receivers but if omitted undesired signals are likely to get into.the'i. f. circuit. - ' ; - t'--r .

: The Frequency Converter: In order to get a signal at

the desired intermediate frequency (usually 465 k. c.) a local oscillator known as the high frequency oscillator is

included in the set to beat against the incoming signal.

This signal from the h. f . oscillator is combined with the

incoming signal in the mixer-first detector. This gives us

a beat note as shown in Fig. 43. In order to get an inter­ mediate frequency of 465 k. c. the frequency of the oscil­

lator must be either 465 k. c. higher in frequency than the

incoming signal or 465 k. c. lower in frequency.

The oscillator must always be exactly 465 k. c. more or

465 k. o. less than the incoming signal regardless of fre­

quency of the station tuned in. For this reason, the oscil­

lator condenser is usually ganged with the detector tuning

condenser. The h. f. oscillator must be very stable in

order to keep the beat note at 464 k. c. If the oscillator

signal should-change 10 kilocycles this would Change the

i. f. by 10 kilocycles. If the drift is very great the signal will not go through the i. f. amplifier which is timed to 465 k. c. , ,

The mixer-first detector gets its name from its douhle duties of combining the incoming signal with the local eeeilletor and then selecting only the one desired frequency, the beat note. ;

The Intermediate Amplifier: The i. f. amplifier con­ sists of one or more stages of amplifloatloh. It is at this frequency that the superhet does most of its amplifying. , The addition of an i. f. amplifier gives us three different fre­ quencies at which the signal can be amplified. Each tube in the i. f. amplifier has its grid and plate circuits tuned to 465 k. c. This makes the -superhet more selective than other types of receivers. These circuits may be tuned no that the receiver is very selective or they may be tuned broadly for high fidelity. Too high a selectivity cuts down on the quality of the music or speech. .

The Second Detector: The second detector takes the signal at the intermediate frequency and changes it to audio frequency by cutting off half of the wave and allowing the

grid of the first audio amplifier to follow the average sig­

nal. This action is similar to that of the detector studied before, except that the signal coming into this detector is

always at the intermediate frequency, usually 465 k. c.

The Audio Amplifier: The audio amplifier then amplifies

the signal again and drives the speaker just as in any other 160

type of set. - :■ : ; -■ • , '

Necessity for the Preselector: Suppose that desire to tune In a signal having a frequency of 550 k. o. In this ease the h. f. oscillator would probably be turned,to 1,015 k. c. since the oscillator is usually tuned to a frequency

465 k. c. higher than the incoming signal. In case there should be another station operating on a frequency of 1,480 k. c. it also would produce a beat note with the h.. f. oscillator at a frequency of 465 k. o. This would give us programs from both stations at the same time since both could come through the 1. f . amplifier. A tuned circuit - ■ w ■ ' ■; ■ ■ ■ • j . ahead of the mixer would help to keep out this undesired signal. Several tuned circuits would be better. This type of interference is known as "imago Interference?.

There are two types of noises originating in the mixer- first detector which can be overcome by using a preselector.

Free electrons are always moving at random in any conductor causing miniature pulses of current to be set up. In tubes used as amplifiers this "thermal agitation" is very small compared with the plate current. In a detector the grid bias is such that the plate current is very low. Thermal agitation then becomes a much higher percentage of the sig­ nal. The hissing or frying noise heard in the loudspeaker when the set is tuned between stations is duo largely to thermal agitation.

Converter noise causes even more trouble than thermal 161

agitation. Converter noise is caused by electrons from the

•ftthode of the mixer-first detector striking the plate in spurts or shots rather than in.a.steady stream. These waves are amplified In sueoeeding stages and come out of the speaker sounding like shot pouring on a tin roof.

Thermal agitation and.converter noise can be largely overcome by providing one or more stages of r. f. amplifica­ tion ahead of. the detector. This builds up the signal so ; that it overrides the tube noises and pushes them far into the background. _

Advantages of the Superheterodyne

Sensitivity; The sensitivity of the superhet is much better than a tuned r. f. receiver because of the third fre­ quency, i. f . at which the signal is amplified, since amplification in the i. f. stage is always at the same fre­ quency the sensitivity is much more uniform over the entire tuning range than in the T.R.F. receiver. Amplification can be greater at intermediate frequencies than at either radio or audio frequencies without other unwanted noises becoming troublesome.

Selectivity; selectivity is the ability of the set to tune in the desired station, and tune out the unwanted sta­ tion. Selectivity is almost directly proportional to the number of tuned circuits through which the signal must pass.

Each grid circuit and each plate circuit of the i. f . 1#*

amplifier is tuned so that we have more tuned circuits in the superhet than in other types of receivers. There is danger of getting the sensitivity of a superhet so great that fidelity suffers. : • ' ... _ 1 , - * :■ . U ‘ ? f ". 1 " r - All-Wave Receivers: In addition to the gain in sensi- tivity and selectivity there is another advantage of the superhet itiien used as an all-wave receiver. Each r. f . amplifier must be tuned to the frequency of the incoming signal. This means that if we used three stages of r. f. ahead of the detector in a tuned radio frequency set, all three of these stages as well as the detector would have to : - v ' • : -■ .: . : ^ : •••-.; ' v „ "... . . - ... \ have coils switched each time we changed wave bands. This ' G- 7 .7 -7 '' - ’ T : '. ; '.7 ' --- : ” would require four coil switching assemblies. '■ ..' ' ■ . .' -7 • : r. . 7 7 v v s £. ■ ■ ; A superheterodyne with three stages of amplification in '• . ■ : :.. - ; : 7'' . '.7 ’ - . . 7'- .. w ... , the i. f. instead of the r. f . would have only two coil ’ 7 / 7 • • ■ '. 7 ' ' 7 . ' .. 71- , switching assemblies, the mixer-first detector and the h. f. oscillator.

A superhet with one stage of r. f . and two stages of

i. f. would be a better receiver than either of the three

stages of r, f. or the three stages of 1. f.

: ;77, ■' 77 . ~ 77.-,.; • 1^4

c:' ••

' " - ' - ' . f -... . " .- --, ' , ' CHAPTER XXII

SELECTING A RADIO RECEIVER

How to Select a Radio Receiver

- - - . . - . : ■ ■ ' . ■ ; . . ■ , ' ■, : - . . Purpose of the Chapter; The purpose of this chapter is to point out a few of the things you should look for in

■ ■ > ,. ' : . - selecting a radio receiver. Friends and relatives who know you have been studying radio are likely to ask your advice when buying a radio. It will increase your prestige with them if you can be of some help. You may want to buy a short wave communications receiver yourself some day if you eontlnue to be interested in radio. The following discussion applies to communications receivers as well as broadcast receivers. There are other features which must be considered in selecting a communications receiver.

Select a Superheterodyne: In the preceding chapter we saw the advantages of the superheterodyne circuit over other types. Nearly all except the very cheapest sets on the mar­ ket at the present time are superheterodyne s, and indications are that this will be true for many years unless some revo­ lutionary discovery is made. Such a discovery is not likely as radio engineers know enough about radio now to be fairly

sure that the superheterodyne is the best type of circuit.

Your first rule then for buying a radio receiver is to be sure it is a superheterodyne.

Select a Big Chassis: The old saying that "good things 184

come in small packages" certainly does not apply to radio receiving sets. In general, the set with the most tubes will be the better set if it is of modern design. If we select a set with too few tubes we can be sure that some­ thing has been left out. The same is true of a very small

chassis; Some compromises have been made in order to cut down size and cost.

Speakers and Cabinets: Again size is important. Too •*

small a speaker will not reproduce the low notes nearly as

well as the high notes, so that we get a "tinny" sound, it

is difficult to say just what size is best because of the

great variety of conditions under which speakers are com­

pelled to work. In general, the speaker should be at least

six inches in diameter. For people who are especially fond

of bass notes a ten or twelve Inch speaker would be even f

better. - .- ^ "

The cabinet has as much to do with ton# as the speaker!

% e strength of sound waves depend# upon the amodnt of move­

ment of the air. High notes are caused by rapid vibration of the speaker cone, while the low notes are set up by compara­

tively slow movement of the cone. -v ^ r

if we have a speaker which is not mounted in a cabinet,

the sound waves from low notes have time to go around to the

back of the speaker before it can return from its forward

movement. As the speaker cone moves back this wave goes

around to the front so that'instead of having all of the air 165

in the room set in motion at low frequencies we have only a small section of the air moved. The high notes carry very well from our unmounted speaker because the speaker is vi­ brating fast enough that the sound waves cannot get around it before the cone returns. For this reason, the speaker seems to reproduce the high notes very much better than the low notes. ' - ■■■ ’ ■ *-- ■ " •- ■ ■. Vn; ■ ,, ^ -

Fastening the speaker to a "baffle board" brings out the low notes because it increases the distanoh which the sound wave must travel in order to get- around the speaker. The length of the path which the sound wave must travel deter­ mines the ability of the speaker to "bring out" the low notes. The longer this, path is the lower the note which can be reproduced properly.

Baffles may be either flat or box type. Radio cabinets are the box type baffles* The effective distance can be found by measuring from the front edge of the speaker around the cabinet to the nearest point of the cone. The best baffle would be"a flat baffle with a distance of about nine feet for the sound to. travel.- : -

The Ideal baffle could be had:by mounting the speaker

in the wall between two rooms so that its effective distance would be very great.

Most people would rather compromise on tone and keep the speaker in the radio cabinet. In selecting a cabinet which has at least two feet of effective length we can get &*#

very, good tone..: This condition is impossible in the midget

zadios on the market today. : sometimes these sets appear to have good quality on the. low notes, but this is because the

high notes have been intentionally suppressed to match the

•oppressed.low notes. Another good rule, then, is to select a set with a good,

sized speaker and a large enough cabinet to have at least two feet of effective distance" from front to back of the

speaker.;- v -.v .

ControOLs: The set selected should have a tone control

so that, the listener may select the tone which he prefers to

hear./

An automatic volume control usually is and should be

' bmilt into the set. % '

A "magic eye" is a valuable tuning aid, but we must

remember that this is counted as a tube in stating the num­ ber of tubes in the set.

Push button tuning may also be desired and costs very

little more than sets without it.

Desirable Circuit Features; We here seen how selec­

tivity can be controlled in a superheterodyne by coaetrseting

the i. f. transformers to have either good selectivity or broad tuning for high fidelity.

.The Federal Communications Commission allows ten kilo­ cycles on.the band.for each;station, and requires that they

do not use any more than ten kilocycles. This means that if 167

we select a set which has "ten kilocycle selectivity" that we can completely tune out one station and tune.in another.

There is no advantage of. having more selectivity on the broad­ cast band. There is the disadvantage of reducing the quality of the tone. ■ ■ v;' f- : ■ ■ ■ - -

Preselector: : The set selected should have at leas^one stage of amplification ahead of the detector. This will help to give a high degree of sensitivity and a good aignal-to- noise ratio. It will also help to keep out images and other unwanted signals.

Listen carefully to the set to see that the noise level is very low when a station is tuned in.

All-Wave Receivers: If it is desired to purchase an all-wave receiver with the idea of listening often on the short wave bands, several other characteristics must be oon- sidered. Most important of these is "band spread". Band spread means the ability of a set to tune very slowly in frequencies, or to change a comparatively few kilocycles with each revolution of the dial or tuning know. This is accomplished in various ways. Some sets have separate knobs for band setting and band spread, others change gear ratios by pulling out or pushing in on the knob. Sometimes the band spread is accomplished by use of.separate very small tuning condensers in parallel with the main tuning condensers, and in other sets by gears to slow down the tuning of the main condensers. Each has its advantages, but some form of band 168

spread is essential on the short waves.

Another valuable, but expensive feature is variable selectivity of the i. f.’s. Noise silencing or noise limiting circuits are also desirable but expensive. The technical phases of variable selectivity and noise silencing will not be considered in this discussion, . -■ - ' . ■ . ■ " . : . : ..

Locating the Radio in the Home Proper Placing in the Room; For best results, the radio

should be placed at one end of the room so that the opposite wall will be as far away as possible. Room enough should be

left back of the radio so that it will have plenty of venti­

lation for cooling the tubes, and so that the air baek of She speaker will be free to vibrate.

Bare rooms are not well suited for radio reception.

Bare rooms reflect the sound, causing some frequencies to be

reinforced and others cancelled. This gives an unnatural

ring to the sound. ;

The best room for. radio reception is one in which a

carpet nearly covers the floor, there should be a few drapes

and curtains. Openings and furnishings should be grouped

irregularly to break the reflections which cause reinforce­

ment and cancellation of the various frequencies. Loose pic­

tures, vases, and furnishings should not be allowed to

vibrate. The radio should face the center of the room.

*. CHAPTER XXIII

CLASSIFICATION OF AMPLIFIERS Three Glasses of Amplifiers

Class A Amplifiers: Amplifiers are classed as A, B, or

C depending upon how much grid bias voltage is used. Each type of amplifier has its own characteristics which make it beet suited for particular circuits. Class A amplification is the type used in radio frequency, intermediate frequency, and some audio frequency amplifiers in receiving sets. The chief characteristics of Class A amplifiers are high voltage amplification of the entire wave form, and comparatively low

"power output".

Fig. 29A shows the characteristic curve of a Class A amplifier. The bias voltage "W* is placed far enou&i above cut-off to have some current flowing at all times. Note that all of the amplification Is on the straight part of the curve so that all parts of the signal are amplified by the same amount. This gives us an exact copy of the input sig­ nal except in an amplified form. This type of amplification is used wherever practical, because it is more nearly free from distortion than either Class B or Class C amplification.

The efficiency of a Class A amplifier is low, between

20 and 35 per cent. Efficiency of a vacuum tube is found by dividing the output power of the tube by the power input to S'-

170

She plate. The difference in plate input and output is lost

as heat. The eomnon name for this loss is "plate dissipa­

tion". Plate dissipation is calculated by the formula

Hw = I2R. since current is flowing at all times in a Class A

amplifier the plate dissipation is high, making thi efficien­

cy low. Plate input may be measured by measuring plate

voltage, and plate current,- The formula \l = El gives us the

power input in watts. Output is harder to measure since it

is at; either radio or audio frequencies. This output power

* - ' ' ; / jL#_%##lly measured indirectly by the. amount of heat or

. \ ; * - light that it will produce. r ; - ' '• Class B Amplifiers: Fig. 29B shows the characteristic curve of a Class B amplifier. The grid bias is set at a

point ”S" which is at or very niar cut-off. Plate current can flow during only one-half of the input cycle. As we

have seen, this condition is desirable in a detector.

A tube operating in Class B operates at an efficiency of

from 50 to 60 per cent. This higher efficiency is due to

having little or no current flowing for half of the cycle.

In the Class A system power had to be dissipated at all times

whether or not there was a signal on the grid. With.Class B

operation when there is no signal on the grid little or no

current flows, and the tube if not called upon to dissipate

Power.. _ - .. . • The high efficiency of Class B amplification makes it

more suitable where large amounts of power are needed. An 171

example of this is in driving a loudspeaker.

One disadvantage of a Class.B amplifier is the rather

•: . high degree of distortion since it is operating only half of

W T S i a e . This distortion can be overcome by using two tubes booked up so that each is working while the other is resting...... ' ■" $ ». Fig. 46A shows‘the wave form as it comes from one tube . i . .

■ , . . . • • •- . « w * •' •. ^ ^ ** -- - -v '• - ' ^22/ ’ ' v ■ - ^ - •* --* * f 4 <• • - .- ' ••

%b/ \ y y 7 j

t-.. h ^ e j * o

Fig. 46 * . V

vmrking as a Class B amplifier. .Only one-half of the sine

wave is present. When two tubes are used in "push-pull" the

resulting wave form is- almost a perfect sine wave. (Fig. 46B).

Each tube makes up for the time in which the other is not

working. . . . . - / c : - . The circuit of a push-pull Class B audio amplifier is

shown in Fig. 47. Class B tubes work out of and in to center tffped transformers. This tap establishes the zero.point for

the audio.frequency so that when one end of the transformer

i# positive the other end is negative. : 3 «, L'

Fl-S» 47

.■ ■ When the top half of T is positive, plate ourrent can ■ . -1. . flow in tube a . At the same time tube B is "resting". The

negative voltage on the grid of tube B does not allow any

current to flow. When the cycle changes, the lower half of

becomes positive and allows plate current to flow in tube

B. At this instant the grid of tub# A is negative, allowing

tube A to rest.

These resting periods of a half cycle allow the plate to

eool off after dissipating power during the half cycle it is

working. Since we do have rest periods during which the

plates have a chance to cool, we can work them harder when they do work. This means that we can run comparatively heavy

currents to Class B tubes, and thus get high power outputs. Notice also in Fig. 47 that we do not have a grid leak

resistor. In order to get high power outputs for half a

cycle in each tube, we usually use tubes whose cut-off bias Ji#: Just about *@3:0. Any positive swing of the grid will then make it swing positive as compared with the filament.

This causes large currents to flow in the plate circuit.

(Mote; Voltages of all elements of the tube except filament voltage are measured with the connection of the B minus with the filament or cathode as the zero point.) . $

Swinging the grid positive causes it to attract elec­ trons which in turn causes current to flow in the grid cir­ cuit. Transformers are nearly always used to couple the grids of Class B tubes to the preceding stage to avoid distortion caused by heavy grid currents flowing through a resistor or choke when using resistance or impedance , coupling. A center tapped transformer is again necessary to prevent distortion in the output circuit, and to couple a push-pull circuit to a single circuit output such as a l > . -L * f": ' speaker. z. -C; V " ""5 \ - / ; ; M : - : > * ' . ' , 4 : Class AB Amplifiers; Class a B: amplifiers are a,compro- i: ' : •. ... ' ■ . . • ' ' - " ' mise between the high fidelity of Class A and the high power output of Class B. The two tubes are always used in pueh- -f ■■ .- ' ; . :- pull just as in Class B to ocjbeel out the distortion.

Class AB is sometimes subdivided into Class AB^ and

Class ABg. Class .is. mor*-. nwrly'like Class A,1 while Class ABg is more like Class ;B.

Class AB amplification•is widely used whore considerable power is needed with high fiedlity.

Class C Amplifiers: A Class C amplifier is one in " w

which the negative grid bias is at least twice cut-off. The

Class C amplifier is limited to use in radio frequency trans­ mission where very high efficiency is essential. Plate cur­ rent of a Class C amplifier flows during only about one-third of the input cycle.. This gives twice as much resting time as working time so that extremely large currents are allowed to flow during the working period.. . ,. : :

Class C amplifiers require considerable driving power and high voltages. Under favorable conditions efficiencies of the order of 75 to 85 per cent can be obtained. Efficien­ cy of 90 per cent can be obtained by increasing plate volt­ age, grid bias, and grid drive to extremes, but such oper­ ation is not practical. Fig. 48 shows a typical Class C

L"!"— r.'..1— j-r.;—s — -v- — T-rr"", ...... •' ' - ■.", 1 „ "T

'C+>’4 6+rrm*t~ f/#

Fig. 48 amplifier circuit and ite characteristic curve. Note on the curve the comparatively large grid swing re<|titf@4 for Glass ' ...... C as compared with Class A operation as shorn in Fig. 29A..

Class C r. f. amplifiers work into sharply•tuned cir­ cuits which supply the other half wave by the flywheel effect : : . L'_ ' . .'r . X . .( .I ' of the coil and condenser. Glass B audio outputs cannot do . . y ' - ' . . ; . i- / , ' -_yi_ 'i -iry this because sharp tuning would not allow then to pass all of the audio frequencies.

-re, : : ! .. - ^ rV. ^ CH/iPTER XXIV ' - - - . ' tvtifrv. ' m SERIES Aim PARALLEL CIRCUITS Batteries "... - . ; . . - . _^ T" / ^ . - L _ Dry Cells: All batteries and cells depend upon chomi-

: ' _ _ . - . . . - ^ oal action to produce or to store energy. The common dry cell such as is used in flashlights, aoiae telephones, and as

nAM batteries, depend upon the breaking down of the line case for their electrical energy. Dry cells, incidentally, are not dry. The chemical action depends upon zinc ions dissolving in the solution. Zinc atoms do not disolve read- ily in water solution, but the zinc ions are soluble. Zinc

: -: 0 '-~ ' ■■ : -V:'.: '/ e ; ;\i:''t i . t# ions are atoms which have lost one or more electrons, usual- ly two. as these zinc ions go into the solution, the zinc

case of the cell is left with an excess of electrons. The

excess of electrons gives us a difference in voltage between

the case and the carbon pole.

Dry cells are usually made by placing a carbon pole in

.the center of a zinc can and filling the space between the

carbon pole and the zinc can with an electrolyte. This

electrolyte usually contains a mixture of powdered carbon

and another block powder known as manganese dioxide. Enough

water is added to the mixture to dampen it thoroughly, and

then the top of the cell Is sealed with sealing wax to pre-

vent evaporation. Often vsnon a cell ceases to supply IV?

current it can be brought back to life for a short time by pitching a bole in tne wax and adding water.

C; Correctly speaking, a cell is not a battery. Two or more cells connected together become a battery.

Lead-Acid Storage Cells; The other type of battery commonly used.in radio work is made up of lead-acid storage cells. Car batteries are of the lead-acid type. The.lead-, held cell is made by taking several lead plates for the nega­ tive plates and several lead oxide plates for the positive plates. All of the lead plates are connected together and , the lead oxide plates are connected -t#g#th@r, but are insu­ lated from the lead plates. Usually grooved sheets of wood or of hard rubber are placed between the alternate positive lead oxide plates and negative lead plates* sulphuric acid is used as the electrolyte, and hard rubber for the con­ tainer .

In discharging the sulphuric acid resets with the lead and lead oxide, changing both plates to lead sulphate. The electrolyte becomes weaker.as;the sulphuric acid is used up in the reaction. ; f ; ‘

Charging is accomplished by passing electric current through the cell in the opposite direction. This causes the lead sulphate to break down leaving lead plates on the nega­ tive side and lead oxide plates on the positive. Sulphuric acid is put back In solution again strengthening the1elec­ trolyte. . - V'- '• . 178

Lead-aeld cells produce: two volts of electrical pres­ sure. Where more than two volts is needed, several cells are eonneoted in series. Most car batteries consist of three i? cells in series to get six volts. i- * -

i ‘ Size of Cells: The size of a cell makes no difference in the voltage. Dry cells produce about one and one-half volts, while lead-acid cells produce two volts* The size of the cell doss make a difference in the amount of current which a cell will produce, or in the length of time the cell will last. A larger dry cell has more zinc to form ions and release electrons. A larger lead-acid cell has more lead plate and lead oxide plate to form lead sulphate. dells In Series and Parallel Circuits: Dry cells in series are shown in Fig. 49A. The plus .pole, the carbon, of each cell is connected to the-negative pole of the next.

A B C ______=___ :______;___ i___ — ______- - : „ Fig. 40 ' ' - ' -

The result is that we multiply the voltage of one cell by . ■ v' ' : ' - ' . ' .. ■■■ : ; : " ' . i- : the number of cells. In this case we have three times as much voltage but the current remains the same as for one cell. - ■ ■' ■ ' ' ■ ■ - Dry cells are connected in parallel as shown in. Fig. 49B. ijttl bf the carbon poles connect together and all of the neg­ ative poles connect together* The voltage of any number of

‘bills'in parallel is the same as the voltage of a single

"##11, but the current is multiplied by the .number of cells.

In this case we have one and one-half volts at three times the current of one cell. ‘ ■ ^ T .r ^ .. .. . : . i Fig. 49C shows six dry colls connected in series par­

allel. Here we:would have four and one-half volts at twice

the current of one cell. Iheae.Mx.dry cells could also be connected to give three volts at three times the current of

one cell. How would you connect them in this case? TY/et" ceils may"be connected in series,or parallel just : - r-. -.-T ■ : - • v " as are dry cells. ;' i ’ ; . • - '

Condensers

Condensers in Series and Parallel: Sometimes it is

desirable to use condensers in series to withstand higher

voltages, or to use them in parallel to increase the total

capacity of the circuit. Fig. 50A shows two condensers

'Connected in scries. Connecting condensers in series has -

the effect of increasing the thickness of the dielectric and

thus raising the breakdown voltage. Two exactly similar

8 iafd. 450 volt condensers £h series would be equivalent to 180

a single 4 affl. 600 volt ocmdenaef.- The leakage current varies greatly in condensers so that the chance of getting two exactly alike are small. The insulating material is not perfect. Some current does pass through even the best insulators. This is known as leakage current. In order to make' sure the leakage is about the same and thus make d. c. voltage divide evenly across two

similar condensers.in series, we usually connect a half- megohm 1-watt resistor across each one. .The leakage current

is very small in comparison with the current across the

resistors, so may be neglected in voltage division.

C, cJmU C[ fw. e?itS«Z A oL1 9 * tf I I c**»^*nfcr I 4-fP*

A. B

Fig. 50

Alternating current is not effected by the leakage cur­

rent. A. c. voltage divides across two condensers in inverse

proportion to the capacities. If v/e had a 1-mfd. condenser

in series with a 4-mfd. condenser, and put 600 volts of a. o.

across them, the effective voltage on the 1-mfd. condenser 181

would be 400 volts. The effective voltage on the 4-nfd. condenser would be only 100 volts.

The capacity of two similar condensers in series is equal to one-half the capacity of either condenser. Three similar condensers in series would have an effective capacity of one-third that of any one condenser, but would stand three times the voltage.

Effective capacity of any number of unequal condensers in series can be found from the formula:

"=r where C is the effective capacity* is the capacity of the

first condenser, C2 the capacity of the second condenser,

and Cg the capacity of the third condenser. The effective capacity of condensers In parallel.

Fig. SOB, is equal to the sum of the capacities of all the

condensers. The breakdown voltage of the circuit is equal

to the lowest breakdown voltage of any of the condensers.

Resistors

Resistors in Series and Parallel: The resistance of a

group of resistors in series is equal to the sum of the individual resistances.

Resistance of similar resistors in parallel can be found by dividing the value of one of the resistors by the

number of resistors in parallel. 188

Effective resistance of unlike resistors in parallel may be found from the formula:

R = ______1______A_ _JL_ 4. -JL. 4. • • • • Rl H2 H3 Hn where R is the effective resistance, R, is the resistance of the first resistor, Rg tb# resl#t*nee of the second resistor, and so on. . ; _ •; • . . , - • .

Wattage Ratings: Resistors are obtainable in various wattage ratings. In order to determine the wattage rating needed we must know either (a) voltage drop across the resis­ tor and current flowing through it; (bj current flowing and the value of the resistance; or (c) the voltage drop and the value of the resistance. The formulas which may be used are:

(a) W = El; (b) W = I2R; and (c) W = E2/R.:

Once the wattage rating has been obtained, a resistor of slightly higher rating should be used in order to allow it to run cool. The wattage rating is the amount of power which the resistor is designed to dissipate as heat. . CHAPTER XXV

TRANSMITTING 'Government Licenses

A license Is required for Transmitting; Before you can operate a radio transmitter of any class you must have an operator’s license. These licenses are issued by the Federal

Communications Commission (F.C.C.) after the applicant has satisfactorily passed an examination.

The licenses issued by the F.C.C. are divided in various grades or classes. Each class of license has its own require meats and privileges. For example, police officers operat­ ing two-way radio from patrol ears are required to have a

"Radio Telephone Third Class License". Broadcast station

technicians must have a "Radio Telephone First Class License"

The "Radio Telephone Third Class License" limits the holder

to operation of only a few types of transmitters. The third

class operator is not allowed to service his own equipments

but must have his transmitter repairs and adjustments made

by a holder of a "Radiophone Second", or "Radio Telephone

First Class License".

Classes of Licenses: Licenses are divided into three

general classifications: Radio Telephone, Radio Telegraph,

and Amateur. Radio Telephone Licenses are divided into

three classes, designated as first, second, and third class. The first class carries more privileges, and requires more knowledge to pass the teit. - i

Radio Telegraph Licenses are divided into three classes, designated as first, s e c b M ,"aHS thl^d class. Radio Tele­ graph First Class License is the highest classification, and

Radio Telegraph Third Class the lowest. Holders of the Radio

Telegraph First Class License may operate any radio telegraph

station except amateur, second and third class license holders are limited to certain specified types of stations.

The highest class of commercial license in known as

"Radio Extra-First Class License”. This license requires several years* experience and ability to use rapidly both

the" American Morse and International Morse Codes. This

license entitles the holder to operate any radio station,

phone or telegraph, except amateur stations.

The third classification, amateur, is divided into

Class A, Class B, and Class C. Holders of Class A licenses

may use some frequencies for radio telephone which are pro­

hibited to Class B and Class C License holders.

Radio Telegraph and Amateur Licenses require that the

applicant pass a code test as well as a written examination

on theory, and on the laws, rules, and regulations con­

cerning operation of radio stations. Radio Telephone

Licenses do not require passing a code test. 185

Preparing for Communications Work Where to Start: The remaining few chapters are designed to give you some idea of communications work. This section will give you a broader background in case you prefer radio servicing, selling, public address, or other jobs in the public service type of radio work, in case you decide that you prefer.communications type of radio work, including broadcast technicians, these chapters will give you an idea of how to start. . ■- v - : ' ■

Radio Extra-First Class License, Radio Telegraph First -

Class, and Class A Amateur licenses are limited to applicants who have had definite periods of experience under some of the other" classifications.' " - - " „ , . ;

Our goal should be Radio Telephone First Class , Radio

Telegraph Second Class, and Amateur Class A licenses so that we will be prepared to operate any 1phone station, most telegraph stations, and any amateur station.' with-these qualifications the chances of getting a job are good. Only one year is required between. Amateur Class B or C and Class A.

The logical;way to go about attaining the goal which"we have set is to start with the easier examinations, and aa our experience and knowledge increases keep moving progressively higher. Probably the easiest examination is Radio Telephone Third Class. However, this:is merely an examination on the

laws, rules, and.regulations concerning radio, and does not

test your teehnloal knowledge of radio. It is not necessary 186

to take the third class radio telephone examination before taking second class. The Radio Telephone Third Class License does not quali­ fy you to hold many well paid jobs. The only real advantage in taking this examination is to cheek up on your knowledge of the laws, rules, and regulations governing the operation . of radio stations.

The first real test of technical knowledge is the

Amateur Class B or Class C examination. The operating privi­ leges are the same for Class B and Class C. Class B examina­ tions can be given only by a "Radio Inspector" of the Federal

Communications Commission at certain designated places.

If you live 185 miles or less from one of the designated cities you will be required to travel to the examining point and take the Amateur Class B examination from a Radio Inspec­ tor. Class G Licenses are not granted to people who live

185 miles or less from one of these examining points.

If you live more than 185 miles airline from one of these designated cities you may take the Amateur Class C examination. For Class C the Federal Communications Commis­ sion sends the applicant a sealed envelope containing ques­ tions to be answered. The envelope may be opened and the examination given to the applicant by a Class A or Class B .

License holder, or by certain classes of commercial operators.

Class C operators must take the Class B examination before taking Class A. Both Class B and Class A examinations may tie taken the same day if desired by a Class G operator. An applicant for a Class A license must have held a Class C or Class B License tor one year before taking the examination for Class A License. '''

' Advantages of an Amateur License: There are several' advantages in taking the examination for an amateur license. The first of these has already been mentioned, it gives you your first real check on your technical knowledge, it also gives you a cheek on your ability to send and receive "Inter­ national Morse Code". A code speed of thirteen words per minute is required for an Amateur Class B or Class C License.

Another important advantage is' the experience gained in building and operating your own transmitting and receiving equipment. Amateur experience is considered valuable by employers'In the commercial "field. " . "

Possession of an Amateur License and station gives you a chance to build up your code speed by actual use "on the air". It gives you a chance to practice handling messages, using "Q," signals, abbreviations, and other short cuts used by radio operators. "Q" signals are groups of letters all beginning with Q. *Q" signals enable the operator to send

certain ideas without spelling but long sentences. They also form an international language making it possible for you to converse with a foreigner regardless of the language he speaks. ; ' '' ' v '

Amateur radio gives you a chance to check your theories 188

and Ideas In actual use. It forms a valuable background of worthwhile experiences. It is your first step toward pro­ ficiency in the oosamnioations field. Bequlrements for Class B and C Licenses: In order to

secure an Amateur Class B or C License, it is necessary to pass an examination on International Morse Code, and to pass •

a written examination on the technical side of radio, and on

the laws, rules, and regulations dealing with Amateur Radio.

The applicant must prove his ability to send and receive

thirteen words per minute, five letters to the word, before taking the written examination.

The written examination consists of five questions on

theory and practice in radio, and five questions on laws,

rules and regulations. These questions are selected at ran­

dom by the F.C.C. from a large group of questions used for

this purpose. The questions with their correct answers are

given in "The Radio Amateur’s License Manual", published by

the American Radio Relay League, best Hartford, Connecticut.

If you are seriously interested in taking the Class B or

Class C examination we suggest that you secure one of these

booklets. The price is twenty-five cents postpaid. Another

helpful booklet published by the American Radio Relay League is "How to Become a Radio Amateur". Its price is also

twenty-five cents postpaid.

It is not the purpose of these chapters to prepare you

for any of the government Licenses. The purpose is to show 189

you how to start. Begin learning the code, and at the same time build a short-wave receiver or change a receiver already built to a short-wave receiver so that you can become famil­ iar with the short waves.

Either the "Radio Amateurf s Handbook"* published by the

American Radio Relay League, West Hartford, Connecticut, or

"The ’Radio* Handbook" by the Editors of "Radio" magazine, and published by Radio, Ltd., 1300 Kenwood Road, Santa Bar­ bara, California, will prove valuable as a reference book in your study of radio. CHAPTER XXVI

THE INTERNATIONAL MORSE CODS How the Code Is Used

The Two Codes* There ere two codes in common use in the United States. The American Horse is used exclusively

■ . ' for wire telegraphy. The code used for radio telegraph is known as International Morse Code. Both codes are made up of combinations of dots and dashes representing letters.

Only a few letters differ in the two codes but the sounds heard differ enough that each must be learned separately by any one who expects to qualify for "Radio Extra-First Class

License". However, since it takes several years to become expert enough to qualify for this license, we shall study only the International Morse Code at"this time.

Sound of the Continental Code: if you have had the opportunity of visiting a telegraph office recently you prob­ ably noticed the clicking of the telegraph instrument, the sounder. This sounder clicks when the key*is pressed and clicks again when the key is released. The telegraph oper­ ator can tell from the length of time between clicks whether a dot or dash has been sent. In sending a dash, the key is held down three times as long as for a dot. The letter nAn is represented by a dot followed immediately by a dash. In

Continental Code this is heard as two clicks in rapid 191

succession followed by two more clicks with more time between them. 'Next there is a small time during which no sound is heard before beginning the next letter. Longer spaces are left between words.

Sound of the International Morse Code: The sound of

International Morse, as used in radio work, differs from the sounds in the wire telegraph. In radio work instead of hearing a click when the key is pressed and another when the key is released we hear a continuous whistle during the entire time the key is down. A dot is heard as a short whis­ tle. A dash is a whistle three times as long as the dot.

The letter "A" would be heard as a short whistle followed immediately by a longer whistle. The sound of ;,A" can be

closely approached by saying "did-dah*. «B" is represented

by a dash and three dots. The sound would be "dah-did-did-

did". "Did" should be spoken rapidly and "dab" more slowly.

Learning the Code

The International Morse Code: Fig. 51 shows the code

with the sound first, and then the letter represented by

the sound.

Learning by Sound: Notice that in Fig. 51 the sound of the letter comes first, then the letter, and finally the

combination of dots and dashes. In learning the code it is ■ ' : ■ . ' - - - ■ ; . - . - • ■''' ■■ : . ■ . ' ■ . ■■ better to forget that you ever heard of the words "dot" and

"dash". Think "dld-dph", a";, "dah-did-did-dld* b"; and %»2

"dah-dld-dah-aid, cM. This may seem a little awkward at first but w h w you start receiving code you will find it easier to build up speed if you have learned the sound first, followed by the letter. ' ’ ;

did—dah - A •- N -.

d&h-Sld—did—did B -... dah^dg^-dadi - - 0 — — '

dah—did—dah—did C — did-dah—dah—did P .— .

dah-did-dld ' b ' dah-dah-did-dah Q ——.— r • did E . did—dah-did R .- .

did—did—dah—did F . ♦ did-did-did S ... • -

dah-dah-did " G dah T - did-did-did-did H : did-dAd—dah ‘ D v ■

dld-dld ' I .. ' did-did-did-doh ■ V .i.- did-dah-dedi-dah J =. - did-dah-dah W .—

d#i-dld-dah • K ' - dah—did—did—dah X —.»— did-dah-did-did L . dah-did—dah—dah Y —. — dah—dah ' M — • dah—dah-did—did Z — ••

:: . did—dah—dah—dah™dah 1 Aid—dld-dld 6 — eeee did—did—dah—dah—dah 2 •.——— dah-dah—did—did—did 7 —— # # # did—did—did—dah—dah 3 •••—— d&h-dEdi—d&h—did—did 8 —— *#

did-did-did-did—dah 4 dah—dah—dah—dah-did 9 did-did-did-did-did 5 ..... dah-dah-dah-dah-dah O — -- PeriodSld—dBli—did—dah—didi”4eli dah-dah-did-dld-dah-dah Comma dld-dld-dah-dah-dld-dld Interrogation *«. — ** dah.-did—d Id—dab—d id Fraction Bar dah-did-did-did-dah Double Dash (Break) did-did-did-did-did-dld-did did Error (Erase) Sign did—dob—did—dah—did End of Message did—did-did-dab—did—dab End of Transmission

Fig. 51

Your first job is to memorize the entire code including numbers and punctuation marks. Notice that there is a

definite system to the numbers so probably you can learn them

very quickly. Some people like to take the alphabet five

letters at a time and memorize each group of five letters before going to the next group, still others like to group

the letters into groups which are either similar or opposites.

These groups are shown in Fig. 52.

did E - . dab T

did-did I .. dah-dah II

did-did-did S ... dah-dsh-dah 0

did-did-did-did H e • ♦ e

did-did-did-did-did 5 # # * # * dah—dab—dah-dah—dah 0 (zero) 194

did-dah A # * did-dah A *"» did-dah-dah Y/ . did—did—dah U #*- did-dah-dah-dah J dld-did—did—dah V #*»* did-dah-dah-dah-dah 1 e— — did-did-did-did-dah 4 did-dah-did a » —* dah—did N dsh-dld-dah K dah-did-did D

dah-dld-dld-did B mm m m m

dah-did-did-did-did 6 ***** dah-did-dah K , dah-did-did-dab X dld-dah-did-did L * ** dah-did-did-did-dah Bk did-dld-dah-did F *****

did-dah-dah-did P dsh-did-dah-dah Y dah-did-did-dah X dah-dah-diu-dah <4

Fig. 52

After the entire ootie is learned ycsi should begin prac­

ticing with an audio oscillator or busier. The circuit

diagram of the audio oscillator is shown in Fig. 42. If a

buzzer is used it should be adjusted to vibrate rapidly.

Notes of about 1000 cycles are most pleasant to listen to

for long periods. In buying a key, select a fairly good

one so that you can use it to key your transmitter when you get on the air. . Correct Sending: The best way to learn to send and to receive Is to get some one to practice with you. You and your partner take turns sending and receiving. Sending is easier than receiving.

If possible get some good operator to help you get started. Most operators will be glad to show you the best and easiest way to send. Get him to send some for you at slow speeds so that you can hear what good sending Sounds like. In case you do not have an experienced operator to help you, it will be necessary to depend upon your receiver for correct sounding.

In sending each letter, the space between dots and dashes should be equal to the length of one dot. At the end of each letter the spacing should be equal to three dots. At the end of a word a space of five dots should be left before beginning the next word. This applies only to speeds of ten words per minute or more. At speeds of less than ten words per minute the spacing in the letters should be the same as for ten words per minute. The spacing between letters and between words is lengthened for slow sending. All dots and

dashes forming a single letter should be sent as a group.

Learning to Receive Code: Practice slowly until you can receive every letter that your partner sends. As he .

sends each letter try to think only the letter, not the

sound, and then write it down. When you hear "dld-dab",

write "a"; try to keep from thinking "aid-dab, a". The sooner you can write the letters without thinking the sounds the sooner you will be able to receive rapidly. Write the letter as quickly as possible when you recognize the aouM.

As. soon as you can recognize all the letters you are ready to start working for speed. In practicing for speed, the person sending should wateh you and send always just a little faster than you cam copy "solid". This keeps you straining to increase your speed to get a perfect copy. As your speed increases the sender should keep increasing his speed until you are copying two or three letters behind his sending* ' *

In attempting to copy code which is too fast for you# write down every-letter you recognize. Do not take much time.trying to think of a letter which you fall to recognize

Skip it and go on to the next letter. If you try to think of the letter missed you are likely to miss the next three or four letters* m e n you get on the air this habit will help.you greatly* If only one or two letters are missing usually you can recognize the word. If several letters are missing the meaning of the entire sentence may be lost.

It is well to practice until you can copy fifteen or sixteen,words per ninuto solid before taking the test, nervousness will usually slow you down some when you take the test . : ' ' " ■ :i*- ' ' *

When you got so you can receive about ten words per minute start practicing by copying code on your short wave 197

receiver. You can usually find amateurs on the 3.5 megacycle band, and on the 7 megacycle band who are going slow enough for you to copy. You may not be able to get solid copy but keep trying and write down every letter you recognize. Soon you will be copying most of the slower stations.

In case you cannot find any one to practice with, prac­ tice on the oscillator or buzzer until you are familiar with the sound of the letters and then use your short wave receiver to practice receiving. This method is slower and harder but has been used by a large number of other fellows.

Automatic Code Machines: Machines which use tapes or phonograph records to send code automatically are very good

for building up speed, but are rather expensive. The speed

can be set so that you are always straining to keep up.

Each time that you get so you can copy solid for four or five minutes increase the:tape speed..

ihen you start practicing for your "Radio Telegraph

Second Class” license you may want to use one of these ma­

chines. They can be rented with the condition that the rent­

al may be applied to the purchase price in case you decide to buy.

The Code May be Hard for You: It is harder for some

people to learn the code than for others. Slow reaction time

and poor coordination may make learning the code more diffi­ cult for you. In this case you will have to work just that

much harder. Any one can master the code, so do not give up. CHAPTER XXVII

THE AMATEUR1 S RECEIVER

Changing the ’30 Tube Receiver for Short Waves Importance of the Receiver: ’’You can’t work DX If you can’t hear it." We often hear this expression in Amateur

Radio. "Work" as used by radio men means two-way contact with another station. Working DX means contacts with for­ eign stations. This often used statement shows the impor­ tance of a good receiver in an amateur station. The receiver should be your first consideration in building up a station.

If the receiver is important to the amateur who uses it only for pleasure, think how much more important a good receiver must be to the Commercial operator who uses it to receive important messages. If forced to compromise on either the receiver or transmitter, the experienced radio man usually will select a better receiver and compromise on the transmitting equipment.

In preparing for communications work you will find your short wave receiver of value to you. You will find it very useful in building up your code speed. The short wave receiver will help you to become familiar with the many types of stations using short waves. You will got many hours of pleasure listening to airplanes in flight; to the fishing fleet; to police stations; to amateurs; and to short wave its

broadcast stations. As your code speed increases you will enjoy practicing by copying "press". You hear the news several hours before the newspapers are printed.

The Communications Act of 1934 provides that the con­ tents or meaning of an addressed message must not be divulged to other than the addressee or his agent. Even though the messages you copy may be very interesting, you must not tell others the contents of messages copied. There is no law against copying anything you hear as long as you keep it to yourself.

Short Wave Coils for the *30 Tube Receiver: The *30 tube receiver described in Chapters IX and XVII may be con­ verted into a short wave receiver by winding short wave coils. ?/e constructed the coil on a "plug-in" form so that once we get some short wave coils wound it will be an easy matter to change wave band®. We simply pull out one coil . and insert another. The coll winding data are shown in Fig.- 53. ..

All coils are wound as shown in Fig. 23, and connected to the prongs as shown in Fig. 22B. Windings are spread to take up the space allowed on the form. This spacing is about

1/16" on the secondary of the 00-160 meter coil, and about

1/8" on the secondaries of each of the other®. It is

essential that the prong connections are the same on all

coils so that no changes will be necessary in the circuit

when changing coils. • Amateur Band A : Secondary B Covered (Meters) : Turns Wire

80-160 h” : 36 20 P.8. i" 40 1 " : eh 16 P.E.

20 1 " : 3& 16 P.E. i"

Primary Tickler Turns Wire Turns Wire

5! 30 D.S.C. 111 30 D.S.C.

3 30 D.S.C. 6 30 D.S.C.

3 30 D.S.C. 8 30 D.S.C. : .

Fig. 53

Band-Swaading the *30 Tube Receiver: Band-spread is not good enough on the original receiver for use on the short waves. In order to add band-spread all we need to do is

connect a .014 mmfd. variable condenser in parallel with the tuning condenser in Fig. BIB. Tuning is accomplished by

setting to the approximate frequency and then doing the

fine tuning with the band-spread condenser.

Adjustments: Since every receiver differs slightly

from every other receiver you may find it necessary to make

some adjustments in the coils. If the set will not oscillate with any particular coil, try reversing the tickler connec­

tions. If this does not make it work you may need a few #01

more turns on the tickler coil. If the set will not stop oscillating when you turn the regeneration control you

Should remove a few turns from the tickler coil. If the frequency tuned by the set is too high, add turns to the secondary. If the frequency tuned is too low, take a

few turns off the secondary. These coils cover wide bands

of frequencies in addition to the amateur bands so that you will have a chance to hear many kinds of stations.

A Popular Amateur Receiver

Modern Two-Tube Receiver: Fig. 54 shows the schematic diagram of a popular two-tube receiver.

C. — 15 fd. midget variable Cg — 100 fd. midget variable Cg — 100 fd. smallest size mica condenser C4 — 0.25 fd. tubular, 400 v. Rg — 0.25 meg., & watt C g — .0005 fd. midget mica R4 — 0.5 meg., &watt Cg — .01 fd. tubular, 400 v. BC — 1$ volt bias cell — 3 meg., | watt CHj — 300 or more by., Rg -- 50,000 ohm pot. — See coil table

Fig. 54 This receiver is designed to operate with only 45 volts on the plates of the tubes. This may be supplied by a sin­ gle "B" battery or by a power supply such as described in

Chapter XVI. If this type power supply is used the filament voltage may be taken directly from the 6.3 volt winding of the transformer.

In case no power supply is available, a midget 6.3. volt filament transformer may be used to supply filament voltage. A single "B" battery will last about a year on this set, so it is hardly worth while to construct a power supply unless it is to be used for other purposes as well.

If you plan to build a simple transmitter as described in the next chapter, you may use the same power supply for both receiver and transmitter.

Where no 110 volt a. c. is available, an automobile battery may be used for the filaments. This feature makes this set very desirable for portable use.

Fig. 55 shows the coil data for the four most popular amateur bands.

The chassis is a piece of Masonite Spreadwood" fastened to two blocks of irood 1 3/4" x 3/4" x 6", as shown in • r’ - ' - ' Fig. 56A. The arrangement of parts is shown in Fig. 56B.

Make all leads as short as possible and keep most of the wiring under the chassis. 209

COIL TABLE For Two-Tube Autodyne

All coils wound with no. 23. d.c.c. on standard 1| inch form#

80 M. 29 turns close wound; cathode tap 1& turns from ground

40 M. 16 turns spaced 1$ Inches; cathode tap 1& turns from ground

20 K. 7 turns spaced 1& inches; cathode tap Ijt turns from ground 10 M. 4 turns spaced l£ inches; cathode tap 1 turn from ground.

Fig. 55

^ t C “rC'

PresJwo.4 W * CA- k« B

Fig. 56

The front panel is a 7 x 11 inch metal sheet fastened to the wooden blocks with wood screws. The metal panel is 204

used to shield the receiver from "hand capacity". If a presdwood panel had been used, tuning would be difficult because moving of your hand would change the tuning capacity.

Socket connections for the 6J7 and CC5 are shown in

Fig. 57.

Bottom Views

Fig. 57

This set was taken from "The •Radio* Handbook", Sixth

Edition. For further information see "Radio" Handbook,

pages 153-136 inclusive.

You may connect the coils to any prongs as long as all

coils are alike. We suggest that you use five prong forms

as they can be used in other type sets as well as this one. CHAPTER XXVIII

SHORT WAVE RECEPTION

How the Short Waves Act

DX: One of the first things you notice on the short waves is the greater distances of the stations heard. At times, European broadcast stations can be heard on the short waves as loud as broadcast stations a few hundred miles away.

The thrill of listening to distant stations is one of the chief attractions of the short waves.

Also you notice that at certain times reception is good on one frequency while on another frequency no stations can be heard. Because of the large numbers of amateur stations crowded into narrow bands, these frequency bands are com­ paratively easy to find and identify. This chapter gives

you some idea of what to expect on the various amateur bands.

The commercial frequencies between amateur bands change

gradually in character. Any given frequency will resemble

the amateur band nearest to it in frequency.

The 1715-ke. Band: Conditions on the 1715-ke. band do

not vary greatly from those of the broadcast band. The broad­ cast band is from 550 to 1600 kilocycles. Both phone and

code stations can be heard on the 1715-ke. band. Distances

at which you can hear stations are about like those of the

broadcast band. Daytime reception is very poor, but reception at night is usually good for several hundred miles. This band is used by most beginners using "phone"

stations. It is also known as the 160 meter band.

The 3500-kc. Band; Reception over much greater dis­

tances is possible on this band during the winter season.

Contacts are common at about 1000 miles with an occasional

coast-to-eoast contact. This band Is used for code by all

classes of Amateurs, and for phone by Glass A License

holders♦ The entire band is often called the 80-meter band,

while the phone section of the band is called the 75-meter

phone band.

The 7000-ko. Band: This band is strictly a code band. At night it is good for consistent ooaet-to-ooast contacts.

Foreign stations can be heard and contacted at certain times

of the day. This band is the favorite of many, "old timers"

and beginners alike. Almost any time of the day or night it

is possible to hear stations on. this band. Many of them are

beginners who send slowly enough for you to copy while prac­

ticing code in preparation for your examination, others

send so rapidly that to most of us it seems to be just a

musical rush of dots and dashes. This 7000-ko. band is the

lowest frequency used by amateurs for DX work. The band is

often called the 40-meter band.

The 14,000-tec, or 14 Tic. Bands The 14 megacycle band

is used for daylight DX. Coast-to-coast and foreign trans­

mission and reception are good only during daylight and 20V

early evening hours* Daylight contacts of several thousand miles are easy, "but contacts with stations within three or

four hundred miles, and more than ten or twenty miles away,

are impossible except on rare occasions. Sudden changes nay

be expected at any moment on this band. Tho 14 Me. or

20-meter band is used by all classes of amateurs for code,

and by Class A amateurs for phone.

The 28 He. Band: The 28 Me. or 10-meter band is used

chiefly for phone stations by all classes of amateurs. This

band is only "open" daylight and very early evening hours

during the winter months. During the summer months you sel­

dom hear a station on "ten". DX possibilities with low

power are very great when conditions arc good. Some amateurs have "worked all continents" within a few hours when con­

ditions were favorable. Others have tried for years without accomplishing this feat of working all continents.

At the time this is being written foreign contacts are

prohibited by order of The Federal Communications Commission because of the unsettled conditions in Europe.

The 56 Me. Band: The 56 Me. or 5-meter band is good

for short distances up to ten or twenty miles. Once or twice

a year this band may open up for DX work of several thousand

miles, but this is so seldom that the band is considered a

"local band". Because of its local character, there is very

little interference from other stations. Usually there are

comparatively few stations in a community using the same band at the same time.

The Ultra-High Frequencies: All frequencies above SO megacycles are considered as ultra-high frequencies. These are experimental bands not suited to long distance communi­

cation, and are as yet comparatively unexplored.

The Kennelly-Heaviside Layer

Fading and Skip Distance: One of the things which we

notice on the short waves is that signals fade out only to

come back strong a few minutes later without any change in

the receiver. Sometimes the fade is rapid, while at other

times it may take several minutes to go through the cycle. Another peculiarity of short wave reception is that

often you can hear stations several thousand miles away but

you cannot hear stations in your own state even though they

may be on the air. This is known as skip, and the distance

across the circle in which the station cannot be heard is called "skip distance*1. Skip and fading are best explained

by the Kennelly-Heavislde Layer Theory.

According to this theory, a layer of ionized air com­

pletely surrounds the earth at a region about one hundred

miles above the surface of the earth. The ionization is

probably caused by ultra-violet rays from the sun knocking

electrons out of the atoms which make up the atmosphere.

During daylight hours when ultra-violet rays are striking

the atmosphere directly many electrons are freed. This causes the lower side of the ionized layer to he much nearer the surface of the earth. At night when few rays reach the air the electrons combine with the positive atoms. This decreases the thickness of the layer and raises the lower

surface. This Kennelly-Heaviside Layer, more commonly known as

the Heaviside layer, was named for the originators of the

theory, Kennelly and Heaviside.

Ionized air is a conductor of electricity. Air under

ordinary conditions is not a conductor. This layer of ion­

ized air acts as a reflector to reflect radio waves back to

the surface of the earth.

Waves are sent out in all directions from the transmit­

ting station. Waves which go directly to the receiving an­ tenna from the transmitting antenna are known as ground waves

The distance at which the ground waves can be heard is

limited by the curvature of the earth and the heights of the

receiving and transmitting antennas. See Fig. 58. .

The ground wave W may be heard at receiver A since th* wave can go in a straight line from the transmitting antenna

to the antenna of receiver A. Receiver B cannot hear the

ground wave because waves which would go in a straight line

to the antenna of receiver B strike the earth and are ab­

sorbed or reflected by the earth.

Waves which go up and strike the Heaviside Layer before

returning to earth are known as sky waves. Sky waves which *10

strike the Heaviside Layer at low angles are reflected back to the earth. Waves and in Fig. 58 represent these reflected sky waves. Other waves as and which strike the Heaviside Layer at a high angle nay not be reflected back to earth. These waves are merely bent slightly and go on out into space. In general, the higher the frequency of the wave the less it is bent, and the smaller its chance of being reflected back. For example, the high frequencies such as

56 mo., five meters, is a local band because only the ground wave can be used. The sky wave is seldom reflected back.

Of J,

jfcC.tXlfer'C jCe/Uri

Fig. 58

On wave lengths in the neighborhood of 40 meters the

sky wave is usually reflected. Suppose that %% in Fig. 58

is the highest angle at which the 40 meter, 7 me., wave will

be reflected back to earth. This wave strikes the antenna

of receiver C so that we can hear the station at this point. 211

Wave strikes even further away so that we could hear the station at a distance greater than from the transmitter to receiver C.

Now let us consider receiver B. The ground wave does not reach B so we cannot get a signal from the ground wave. The first returning sky wave reaches the earth far beyond receiver B. At the point B we do not receive any signal from either the ground or sky wave and so cannot hear the station. This distance between the end of the ground wave and the first returning sky wave is known as skip distance.

The lower surface of the Heaviside Layer is apparently in continuous motion much like the surface of the ocean. This continual shifting and changing of the reflector changes the angle of reflection. Suppose that receiver C is at the nearest point where the sky wave comes back. If the angle at which Wg Is reflected back should decrease slightly, receiver C would be within the skip distance. This change caused by the continual shifting of the Heaviside Layer may cause the station to fade out completely and a few seconds later come back as strong or stronger than before. Here we have one explanation of fading.

Another possibility is that we may be located at a

point where both the ground wave and sky wave reach the receiver. This happens often on the broadcast band, in

traveling from the transmitter to the receiver the ground

wave goes directly, the shortest distance. The sky wave ait

must travel a greater distance to reach the receiver. Since the Heaviside Layer is continually shifting, the distance which the sky wave must travel varies from moment to moment.

This causes the sky v/ave to arrive at the receiver in phase with the ground wave.at one moment, reinforcing it and giving us a strong signal. Perhaps at the next moment the Heaviside Layer shifts enough to make the sky wave arrive exactly out of phase with the ground wave. The out*of-phase waves can­ cel each other and the signal fades out. Here is another cause of fading. Both explanations depend upon the theory that there is a layer of ionized air, and that it is contin­ ually in motion.

At night when the Heaviside Layer rises, waves go much higher before being reflected. This causes them to strike

the earth at greater distances from the station. During the

daytime when the Heaviside Layer is low, sky waves do not

get far before being reflected back. Distances good for

reception are shorter in summer than in winter because of

the greater intensity of the rays from the sun ionizes the

air to a lower level in summer.

Length of the Ground Wave: At wave lengths below ten

meters we consider the ground wave to be the same as the

"llne-of-sight. This means that the transmitter can be heard from any point high enough to see the antenna of the transmitter.

Under ordinary conditions we can say that the length of 213

the ground wave in miles is just about equal to the wave length in meters. A 20 meter transmitter should have a ground wave which would cover an area whose radius is twenty miles. This is not strictly true, but does give us some idea of the length of the ground wave.

Time of Day: Another consideration in short-wave trans­ mission and reception is the time of day at the other end of the contact-. Heaviside Layer conditions at your end of the contact may be excellent, and at the same time at the other end they may be very bad because of the time of day. For example, when the 80 meter band is opening up here in the

United States it is still night in Japan. Tne 20 meter band is a daylight band so we could not axpeot to be heard in

Japan or to hear a Japanese station on 20 meters when it is night either here or there.

As a general rule to follow, listen below 30 meters for

daytime DX and above 30 meters for nighttime DX. CHAPTER XXIX

TRANSMITTING EQUIPMENT Planning the Transmitter

Should You Use Phone or Code? Suppose that you have constructed or converted a receiver for use on short waves. Suppose also that you have finally managed to get your code speed up to thirteen words per minute, and have just com­ pleted taking your examination for the Class B or C License.

The questions were not nearly as hard as you were afraid they would be, and you are reasonably sure that you made a passing grade. It will probably be five or six weeks before you receive your license direct from the F.C.C. offices in

Washington D.C. In the meantime you can start making plans

for your transmitting equipment.

In studying "question and answer" books and "hand books"

in preparation for your examination, you found that you

could use either a phone transmitter or code transmitter.

Code stations send out a continuous wave rather than a modu- - I • lated wave. The dots and dashes are made by placing this continuous wave on the air only when the key is down. A

code signal is really the same as the carrier wave of a phone

station without modulation. "Continuous wave" is usually

abbreviated Hc. w.Before you proceed with the plans for

a transmitter, you must decide which type of transmission you want to use.

Advantages of the Phone Transmitter: It would be nice to have a phone station so that your friends could hear and understand both sides of the conversation. Also it is easier and faster for the beginner to use a phone instead of sending and receiving code signals. A recent survey by the

American Radio Relay League shown that about forty-five per cent of the amateurs prefer phone to e. w. However, this is only half of the story. The survey does not show how many of these fellows started out using c. w. It also does not show how many of the c . w. men have gone into the commercial field.

Learn the Code Thoroughly: There are several good rea­

sons for selecting a c. w. in preference to phone for your

first rig (transmitter). You have learned to send and

receive code at thirteen words per minute, but have not used

it enough to "fix" the code in your mind. If you start out

using a phone transmitter it will not be long until you lose

your ability to receive the code. This means that if you

want to get a commercial radio telegraph license you will

have to learn the code all over again! The chances are

pretty good that even if you only follow radio as a hobby

you will some day went to know the code again. So why not

start right in using the code end plant it so firmly in your

mind that it will never get away from you? You will save

yourself the trouble of learning the code again, and will be surprised at how rapidly your code speed increases ’.Then you start using the code on the uir.

Phone Transmitters Cost More: A phone transmitter costs about two and one-half tines as much to build as a c. w. transmitter of the same power. The same is true of the operating costs of phone and c. x?. This moans that we can have two and one-half times as much power for the same money if we use c. w. .

Not only is a c. w. station cheaper to build but can be copied (understood) at a distance of about one and one-half times that of a phone station using the same power.

0. W. Transmitters are Easier to Construct; The c. w. station is easier to construct than a phone station. Only the radio frequency section of the transmitter and its power supplies are required. This r. f. section may be limited to on® tube if desired.

For a beginner, the c. w. transmitter seems to be the

logical choice. This c. w. transmitter may be converted to a phone transmitter at any time by adding the speech ampli­

fiers and modulators.

Should you Build or Buy a Transmitter: Transmitters are

not made in large quantities like receivers, and for that

reason do not compare favorably in price with receivers.

There is little doubt that manufactured transmitters still

cost more than a similar home-made transmitter.

If you really want to learn how a transmitter works Elf

build it yourself. Try to underetand Just what each pert is used for as you proofed with the construction. You cen find the trouble when something goes wrong much'mere easily in a transmitter you have built yourself. The best way to learn radio is to build as much of your equipment as is practical.

How Much Power; The question of how much power depends

chiefly upon the amount of money available for the purpose. .

The legal limit is 1,000 watts input, but only a small per­

centage of the amateurs have transmitters of such high power. The other extreme is represented by a single receiving type

tube as a crystal oscillator. (See Fig. 44). This is the type of transmitter recommended by several books for

beginners. This low powered oscillator has the advantage of

being easy to construct and can be added to later if desired.

However, most amateurs find that this simple one-tube trans­

mitter does not "work out'* well enough to suit them, and

they add another stage within a very short time after the transmitter is completed.

By designing your transmitter with one stage of ampli­

fication following the oscillator, you can save by not having

to duplicate some parts and replace others. For example, the

oscillator in Fig. 44 calls for a power supply delivering

350 volts to the plate of the tube. In order to increase the

power very much it will be necessary to increase the plate .

voltage. Power in watts is equal to volts tines amperes.

This means that most of the parts in the power supply will 218

have to be replaced or duplicated when increasing the power.

Another advantage of using an amplifier after the oscillator is that more of the power output con be actually used. Taking large quantities of power from a crystal oscillator causes the r. f. current to increase and may destroy the crystal.

A good compromise between power output and cost would be in the neighborhood of 50 to 100 watts. This much power can be obtained without using expensive parts designed es­ pecially for transmitting equipment. Selecting the Type of Tubes to Use: As a crystal oscil­ lator the type 6L6 tube has proved its superiority over most receiving type tubes, and is equal to many of the more expen­ sive tubes designed for transmitting, since the majority of amateurs are using 6L6s for oscillators you will probably be well satisfied with a 6L6., £

Selection of an amplifier is a little more difficult because of the wide variety bf tubes which could bo used.

Since the average beginner must keep the cost as low as pos­

sible we should select a tube, which.will give a fair power output using the same power supply used by the oscillator.

In studying the characteristics of receiving type tubes and

small transmitting tubes, we find that the type 809 meets

the requirements very well. The 809 is a low priced trans­ mitting tube of the triode type. One of these tubes will

give us a power output of forty or forty-five watts at 600 219

volts, . Suppose we decide to use a 6L6 crystal oscillator

followed by a single 809 amplifier/ If we decide to increase

the power output after using the transmitter we can double

it by merely adding another 809 in parallel with the one already in the transmitter, xvhen tubes are connected in

parallel, each element of one tube is connected to the cor­

responding element of the other.

Selecting the Circuit: The circuit of your first trans­

mitter should be simple, straight forward, and conventional. Leave the "trick circuits’* alone until you have had more ex­

perience. Fig. 59 shows such a circuit. The 6L6 is used as

Fig. 59 2*0

a straight oscillator using a 40 meter crystal and using the

809 as an amplifier on 40 meters. By changing the coil Lg» w@ can make the 809 act as a frequency doubler as well as an

amplifier. The transmitter should be capable of about 55

watts output on either twenty or forty meters. Adding

another 809 in parallel as shown by the dotted lines should give us about seventy watts output. We would like Just a

little more voltage but the transformer T is the highest

voltage transformer available with filament windings. A 750 volt transformer would require two extra filament trans­

formers and transmitting type rectifiers. We save at least four dollars by compromising on a 600 volt transformer.

Fig. 60 shows the values of the parts used.

R 50.000 ohm, 1-watt, carbon 300 ohms, 10 watts 15.000 ohms, 10 watts 10.000 ohms, 10 watts I 5.000 ohms, 10 watts 500.000 ohms, £ watt ik°1?2C3 0° .006 mfd. 50 mmfd. double spaced midget variable 100 mmfd. mica 50 mmfd. double spaced dual section variable I .5 to 18.5 mmfd. neutralizing condenser, James Lillian Product ?8C9 8 mfd. 450 v. working voltage 20 turns, l£n dia., no. 18 enam. spaced 2" same as Li for 40 meter operation 9 t . Ig” dia., no. 18 enam. spaced 1|H for 20 meters

Fig. 60

This discussion is not designed to tell you how to build the transmitter but to aid you in deciding what type to build. However, with the information given and a little help from a handbook or an experienced radio man, you could build this very practical and economical transmitter. CHAPTER XXX

ANTENNAS Selecting A Transmitting Antenna

Importance of the Antenna: The transmitting antenna is a very important part of the station. With a poor antenna even high powered stations have trouble "getting out". A good antenna enables even very low powered stations to be heard at great distances.

Types of Antennas: Unlike receiving antennas, trans­ mitting antennas must be cut to exact lengths. The antenna

acts as a tuned circuit and must be cut so that it will

resonate with the tank circuit, LgCg, of the final amplifier.

Usually the antenna is cut to one-half wave length or to a

quarter wave length or to some multiple of a quarter wave.

Quarter- wave antennas must be tuned against ground to supply

the other quarter wave length. These antennas may be either

horizontal or vertical, and may be almost any height. Ver­

tical antennas work well with the lower end within a few feet of the ground.

On the lower frequencies a half wave vertical requires

too high a pole to be economical. Herisontal antennas for transmitting should be as high as practical up to half wave.

Some amateurs claim that adding ten feet to the height of the

antenna gives the same effect as doubling the power of the transmitter. This Is true only up to about a half wave

length In height.

Long, wire Antennas: Directional antennas are gome times used to concentrate most of the available power in a desired

direction. Directional antennas aro of two general types,

long wire antennas, and close spaced half-wavo antennas.

Long wire antennas are directional off the ends. The longer

the wire, in terms of wave length, the more directional is

the antenna. Sometimes two 'or more long wires are used to

form a Vee Beam or a Rhombic antenna. These are shown in

Fig. 61. The arrows indicate the directions in which tr&ns-

Fig. 61

mission and reception are favored with these antennas. Rhom­

bic antennas are often made directional in one direction by

putting a heavy carbon resistor across the end opposite the

transmitter. This makes the antenna best for transmission in

the direction away from the transmitter. Reception is best from that direction

There are two objections to long wire antennas. First, they take up more room than is available to the average per­ son, and, seeondt the favored direction cannot be easily changed. Directors and Reflectors: If another wire is cut slightly longer than the half-wave antenna and placed .15 of a wave length from the antenna, this longer wire will act as a reflector. If a wire is cut slightly shorter than a half­ wave and placed at .1 wave length from the half-wave antenna, this shorter wire will act as a director. Fig. 62 shows a half-wave antenna with a director and reflector. It is best

i X fh\ .i>'A - R *fU '.hr

Fig. 62

for transmission in the direction away from the reflector

and toward the director as indicated by the arrow. This type

of antenna for 14-M. C. and higher is often mounted on a pole

or tower so that the entire array can be rotated from inside

the station. This concentrates the energy in any desired

direction. The chief disadvantage is that the array becomes too large to use oa the lower frequencies.

Other Arrays: There are many other types of directional antennas, most of them being folded long wires or groups of half-wave antennas fed either in phase or out of phase. En­

tire books have been written concerning directional arrays.

The Half->Iave Doublet: One of the simplest antennas

for the beginner to construct and to make operate is the half-wave doublet. The diagram of this antenna is shown in

Fig. 63. This antenna consists of a half-wave antenna

h-ooo-

ON ^ *r- E0- I

Fig. 63

broken by an 18" insulator in the middle. Energy is picked

up from the final tank coil of the transmitter, Lg in

Fig. 59, by two or three turns of insulated wire wrapped

around the coil at its center. Twisted lamp cord is used to

carry the energy to the antenna and is connected to the

antenna on each side of the center Insulator.

The actual length of the antenna must be about five per

cent less than a half-wave because of the resistance of the wire. The length in feet, not including the eighteen inches

of insulation in the center, can be found from the formula:

Length (feet) = 468____ Freq. (Me.}

The length in feet is found by dividing 468 by the frequency

of the transmitter in megacycles.

Advantages of the Doublet: There are several advantages

of the doublet over other types of antennae for the beginner.

The first advantage is that the length does not have to be

exact. The difference of even a few feet will have little effect upon its operation.

The fact that the antenna is practically non-direotional

is an advantage to the beginner who wants to contact stations in all directions.

' Another advantage of the doublet is that no tuning is

required on the feed line. Two or three turns of insulated wire around the tank coil picks up the power effectively.

Some antennas require tuned feed lines using one or more coil

and condenser combinations which must be tuned correctly.

A disadvantage of the doublet is that it can be used for

only one band. Two such antennas would be necessary for

twenty and forty meter operation. This can be done by using

two forty foot poles about seventy-five feet apart. The

forty meter antenna can be horizontal between the poles, while

the twenty meter antenna may be vertical on one of the poles.

A double pole double throw switch at the transmitter could be 22?

used to change antennas.

This type of feed line may be run close to the house or other objects without losing much power. It may also make sharp bends without destroying its efficiency. This cannot be done with tuned feeder systems.

Height Above Ground: The forty rue ter doublet should be at least thirty feet above the ground, and higher If possible.

A half-wave at forty meters Is about sixty-six feet. Low powered transmitters have been heard for several thousand miles on forty meters when using antennas only a few feet off the ground, but these are exceptions rather than the rule.

In general, the antenna should be as high as possible up to a half-wave high. It should be above any trees and build­ ings in the neighborhood if convenient. Sometimes these objects seem to absorb power,.but often seem to make little difference. Put It up as high as is conveniently possible and forget about the surrounding objects. m&Frm uu CCKCLDDim S T A T m m T S The problem of this study was to combine modern theories and practices into an elementary textbook which would be suitable for use in radio classes of the tenth grade level, or in radio clubs sponsored by the high school. This has been done in the preceding chapters. No attempt has been made to completely train individuals for work in any of the various fields of radio, nor is an attempt made to qualify the student for an amateur radio operator's license.

The material has been gathered from a large number of technical books, periodicals, experiments, notes taken in radio class, and experiences of the author in teaching of radio and in five years of operating an amateur radio station. The general plan has been to divide the available time about evenly between class and shop work. Shop work and theory have been coordinated in such manner that the student may learn the purpose of each part as he uses it in the con­ struction of actual receiving sets.

Designs of a short-wave receiver and. a short-wave trans­ mitter have been included which the more advanced student or the student who has progressed rapidly may build.

In writing this thesis the author discovered another problem which may be of Interest to the reader. In Chap­ ter XXVI the suggestion was made that the student learn the

International Morse Code by learning the sound first and followed instantly by thinking- the letter. For example, the letter "a" should be learned "did-dah, a" rather than as

”a, did-dah”. This suggestion is based upon the experience* of the author with a limited number 'of individuals. It would be interesting to take a large group of students with no knowledge of the code, teach half of them the latter followed by the sound, the conventional way, and teach the other half the sound followed by the letter. The efficiency of the method suggested could be determined by the length of time required for the average person of the experimental group to reach a definite code speed, such as twenty words per minute, as compared with the average of the control group. BIBLIOGRAPHY A. Courses

1. Ghirardl, Alfred 2. Radio Physios Course. Radio & Technical Publishing Co,, Nevi York; 1933.

2. Shuart, George W. Radio Amateur Courae. Short v/avo & Television, New York; 1939.

3. Smith, J. 2. "National Radio institute Correspondence Course" ('complete J National Radio Institute, Washington D. G.; 1937.

B. Handbooks

4. - Allied*s. Radio Builder's Handbook. Allied kadio Corporation, Chicago; 1940.

5. American Radio Relay League. How to Become a Radio Amateur. American Radio Relay League,West Hartford, Conn.; 1939.

6. American Radio Relay League* The Radio Amateur's License Manual. American Radio RelayLeague, WestHartford, Conn.; 1939. 7. American Radio Relay League. The Radio Amateur's Handbook. Editions of 1935, 1936, 19i58, and 1940. American Radio Relay League, West Hartford, Conn..

8. Denton, Clifford E. Short Wave Radio Handbook. Radio & Technical Publishing Co., New York; 01934.

9. Hawkins, J. N. A. & Engineering Staff of "Radio". The Radio Antenna Handbook. Radio Ltd., Los Angeles; 1936. 10. Henney, Keith. Principles of Radio. John Wiley & Sons, "inc., Hew York; 1938.

11. Institute of Radio Service Ken, copyrighted by "Questions and Answers Handbook". Allied Radio Corporation, Chicago; 1936. 12. Manley, Harold ?. Drake's Cyclopedia of Radio and Electronics. Frederick J. Drake & Co., Chicago; 1936.

13. Radio, Editors of The "Radio" Handbook. Publishers of Radio, San Franeisoo, 1935.> 14. Radio, Editors of The "Radio" Handbook. Pacific Radio Publishing Co., San Francisco, 1936.

15. Radio, Editors of The "Radio" Handbook. Editions of 1938 and 1940. Radio Ltd., Santa Barbara, Calif.

C. Tube Manuals 16. RCA Bulletin 464. RCA Air-Cooled Transmitting Tubes. ICA Radiotron Co., Inc., Camden, II. J.; 1934. 17. RCA Supplement to Bulletin 464. fCA Radiotron Co., Inc., Camden, N. J.; 1935.

18. RCA The RCA Radiotron Cunningham Radio Tubes Manual. RCA Radiotron Co.,Inc., Camden, M. J.; 1933

D. Other Books 19. Anderson, J. E. and Bernard, Herman. Foothold on Radio. Hennessy Radio Publications Corp., New York; 1930. 20. Codel, Martin. Radio And Its Future. Harper & Brothers, New York; 1930. 21. Culver, Charles A. Electricity and Magnetism. fhe MacMillan Conpany, New York; 1930.

22. Gernshack, Hugo. Radio for Beginners. *8ernsback's Educational Library" No. 8. Radio Publications, New YorMi #1938.

23. Mills, John. Letters of a Radio-Engineer to His Son. Harcourt, brace & 0o.7^ew York; 1922.

24. Moreeroft, John H. . Electron Tubes and Their Application. John Wiley ic Sons, Inc., New York; 1933. 25. Moreeroft, J. H., Assisted by Pinto, A. and Currey, W.A. Principles of Radio Communication. John Wiley & Sons,Inc., New York; 1933. 26. Teraan, Frederick Emmons Radio Engineering. McGraw-Hill Book Co., New York; 1937.

: ^ . 1. Periodicals. 27. American Radio'Relay League.

$11es complete from April, 1935, to August, 1940. American Radio Relay League, West Wesl Hartford, Conn. 28. Radio,, Editors of Radio. Files Incomplete 1936-1938 inc., complete Jan. 1939- July, 1940. Radio, Ltd., Santa Barbara, California.

'.•v ■ . F. Other Soursea

29. Raskin, Frederick J., Director Arizona Republican Information Bureau. Washington, D. C. APPENDIX I

ABBREVIATIONS

amp. ampere A.G.y a.c. alternating current A.F., a.f. audio frequency C. capacity, capacitance . C. W., c.w. continuous wave D. C., d.c. direct current D.C.C. double cotton covered D. S.C. double silk covered DX distant E. effective voltage frequency heating effective in watts h or by henry (inductance) h . f. — — high frequency I effective current (amperesj l. f. ^ — intermediate frequency k c —— kilocycle L self-Inductance Me —— megacycle m. —— meter (cf length), also milli- mr” —— mi or of ar ad mmfd. or mmfd. micro-microfarad------P.E. y.-^^-plane enameled QSL •^'^'varlfication card R.F., r.f. -- radio frequency RFC — - radio frequency choke Sw switch TRF — tuned radio frequency v. — volt • i w . watt Xtal crystal APPENDIX H INDEX

(The figures refer to page numbers)

A

Absorption on energy, 150 Amplifier, 98, 148, 157-160 Airway radio, 13, 70 audio, 131, 136 All-wave receivers, 16 classes of, 169-175 Alternating current,.41-42, 66, Antenna, 5, 17, 47, 104, 67, 72, 88-92, 110-113, 117, 105, 208, 222-227 119, 122, 123-128, 136, 137, directional, 223-225 138 doublet, 125-227 Amateurs, 1, 16-18, 70 length, 33 Amateur Licenses, 183-189, 214 receiving, construction Ampere, 77-81 of, 32-34 Amplification, 8, 53, 93-96, 99, Atoms, 176 104, 105, 145, 146, 157-162 structure of, 56-67 audio, 131-136 factor, 95-96, 139, 141 radio frequency 45

B

"B" batteries, 103, 114, 135, Bias, grid, 93-98 137 Bleeder, purpose of, 123 Bands, amateur, 201, 205 Blocking condenser, 111, 112 Band spread, 167, 200 BreacUcoen voltage, 112, 113, Batteries, 103, 104, 107, 135, , 119, 181 137, 158, 176, 202 Broadcast, 1 , 9, 11, 44, 45, dry cell, 176-179 69, 84, 93, 103, 154 lead acid, 177 By-pass condenser, 57, 112, Beam power tubes, 141, 142 117

C

Cabinet, 164, 165 Chassis , 33, 37, 72, 128 Call letters, 9, 10 130, 163, 164 Capacity or Capacitance, 106- Chokes, 67, 68, 123-126, 113 129, 154, 135 . Carborundum detector, 6 Circuit Carrier wave, 46 oscillator, 155 Qat whisker, 36-37 receiver, 32 Cathode, 138, 173 transmitter, 219-221 Cells, dry, 176 Classes of amplifiers, 169- Characteristic curves, 96 175 Code classification of, 114- Continental, 190 117 International, 190-197, 219- construction of, 117-119 221 coupling. 111, 112, 117, Coils, 47-48, 51, 147, 149 120 inductance, 65-66 in parallel, 167, 179-18] edupling, 47, 49, 66, 67 in series, 178-181 receiver, 74, 199-201, 203, Conductors, 59 204 Continuous wave, 214, 217 Commercial licence, 183-186, Cosmic rays. 43 190, 214, 215 Coupling, 131-135 Commercial operator, 70, 98 impedance, 134 Communications, 11, 12, 13, resistance, 135 17, 69, 163, 198 transformer, 131-184 Condenser, 36, 71, 106-113, Crystal detector set, 6, 7, 123, 124-126, 129, 147, 9, 20, 29, 30, 31, 33, 149, 175 36-37, 39, 46, 50-51, 53 blocking, 111, 112, 120 69, 73, 87, 102 by-pass, 37, 102, 117, 120 Current flow, 59-60 Cut-off, 98,. 101 Cycle, 42, 43, 4'1, 89, 90-98, 101, 103,

Deforest, Dr. Lee, 8 Dielectrics, 109, 115, 116, Detector, 88, 97-98, 99, 146, 118 147, 159 Distortion, 173 Diode (See vacuum tube) Distribution, 14 Direct current, 5, 9, 67, 88, Dmnwtiody, General, 6 89-92, 110-113, 120, 122, DX, 16, 198, 199, 205, 207, 123-128, 137 213 - Direction of current flow, 60-61 of electron flow, 60-61

Earphone, operation of, 51 emission, 85-87 Edison effect, 7, 85 flow, 58-60 Electrolytic condenser, 116, theory, 51-55 118, 120 Electronics, 15 dry, 119, 120 Engineer, Radio, 28 Electromotive force, 79 Electrons, 56-60, 82, 83, 90- 92, 100, 101, 137, 138, F

Facsimile, 15, 19, 99 Fixed condenser, 118 Fading, 208 Flemming, A. J., 7 Federal Communications Com­ Flemming valve, 7 mission, 10, 11, 166, 207 Flywheel effect, 65, 150, Feedback, 148 175 Fidelity, 164 Frequency, 41, 42, 43, 50, Field, about a coil, 62-68, 70, 153-156, 171, 175, 106, 133 222 Filament, 7, 85-87, 137, 138, Full-wave rectifiers, 91, 92 173, 202 Future of radio, 18 Filter, power supply, 124-126

Gang condenser, 121 Grid, 8, 139, 140-142, 178 Gas filled tubes, 143-144 bias, 93-98 Government lioanees, 183-184 leak, 100-102 • swing, 97

Half-wave rectifier, 89-92 Hertzian waves, 4 Hand-capacity effect, 204 Heterodyne, 154 Hand brace and bits, 25, 27 High frequency oscillator, Heat loss, 81, 170 158 Heaviside layer, 208-213 Hobby, radio as a, 16 Hertz, 4, 5 Horse power, 80

Ignition noise, 5 Insulators, 33, 34, 59, 130, Image interference, 160 138 Impedance, 67-68 Intermediate.frequency, 157 coupling, 134-35 International Korso, 187, 190 Inductance, 65-66 Ionization, 208-209 Inductive coupling, 66-67 Ion, 176

K Kennelly-Heaviside Layer, 208- Kilocycle (See cycle j 213 237

L Lead-acid cell, 177, 178 Lines of force, 62-68 Lead-in, 34 Loud speaker, 19, 64, 78, Leakage in condenser, 180 152, 164, 171 Licences, 10, 183-189 baffles, 165

Magnetism, 62-64, 82, 93 Milliampere, 78 Marconi, 5, 6 Modulation, 44-46 Megohm, 100, 101 Molecules, 55-56, 85-87 Mercury vapor rectifier, 143, Multi-purpose tubes, 142, 144 143 Microfarad, 100, 116, 117

a Boise, 160, 161 Noise level,.145

Ohm, 77, 78, 79 audio, 151-152 Ohm's Lay, 79, 127 high frequency, 153 Oscillation, 149, 150-156, transmitter, 154-155 217 Overseas Communication, 12 Oscillators, 149-156

P

Packard, G. Y/., 6 Police radio, 13 Parallel circuits, 178-181 Power supplies, 122-130, 222 Pentodes, 141 Pre-selection, 148, 153, 158 Phones, ear, head, 32, 33, 37, 160,,100: 51, 53, 62, 102, 103, 132, Presdwood, 38, 72 152 Protons, 56-57, 82, 83 Photo-electric cell, 15 Pulsating direct current Pl,ate, of tube, 7, 87, 88, 93, Push pull, 170, 173 94 Pliers, 24, 25, 27

Q

"Q" signals, 187 quality, 164 qSL, 17 233

B

Radiation, 150 Rectifier, 51 Radio chokes, 102 full-wave, 91-92 Rag Chasers Club, 16 half-wave, 89 Rate of current flow, 58 mercury vapor, 143, 144 Rate of electron flow, 58 Regeneration, 104, 105, 145, Receivers, 28, 169 148, 151, 154 automobile, 18 Resistance, 60, 67, 78-82, battery operated, 18, 69, 76, 127, 135, 136, 150 135 parallel, 181 regenerative, 74r76 series, 182 short-wave, 71, 198-204 Resonance, 156 superheterodyne, 154, 157- Right hand rule, 64, 65 162, 163, 166

S

Schematic circuit diagrams, 29, Servicing, radio, 10, 14 73 Short-wave, 1 Screen grid, 139-141 Reception, 208-213 Screwdrivers, 25-27 Skip distance, 208-213 Secondary emission, 141 Signal-to-noise ratio, 145 Selecting a receiver, 163-168 Spark gap transmitter, 4, 5 Selectivity, 146, 147, 161, Soldering iron, 82-24, 27 167, 168 Superheterodyne receivers, Sensitivity, 145, 146, 161, 167 157-162, 163, 166 Series circuits, 178-179 Supressor grid, 141

T

Technicians, 11 Transformer, 44, 49, 51, 52, Telephone, 12 . 66, 109, 123, 133, 134, Television, 15, 19, 99 171, 173, 202 Tetrodes, 139-140 coupling, audio, 133, 134 ' Therapy, 15 Transmitter, 69, 154-155, Thermal agitation, 160 814-221 Three element vacuum tube, 95- Triodea, 93-98, 137, 138 98, 137, 138 Tubes (See vacuum tubes) Tone control, 166 Tuned circuits, 48, 156, 175 Tools, 21-27 Tuned radio frequency, 147, 148

D

Ultra-high frequency, 19 Ultra-violet rays, 43, 208 239

f V

Vacuum tube, 6, 7, 9, 29, 63, Volts and Voltage, 77-80, 61, 74, 138, 139, 218, 219 87, 93-98, 102, 103, 111, detectors, 88 122, 123-127, 139, 140, diode, 84-92, 97 173, 176-182 three element, 93, 137 Volume, 71 Variable condenser, 114 Video, 99

W

Watt, 80, 81 Waves, Wattage rating of resistors, electrical, 42-44 182 mechanical, 40 Wave length, 20, 41 radio, 40, 42, 44, 48, 50-61, 66 sound, 43, 51 water, 40

X \ X-rays, 43 Xtal, 32-36

Z Zino ion, 176 E 9791 94 0 ASHE J LKWADIO FOR HIGH SCHOOL STUDENTS

UNIVERSITY OF ARIZONA LIBRARY

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