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Alatroct* flS**t I lhptrrf s Sh*sLr &lria $. fiafo[], THE ATTENUATION OF RADIO -FREQUENCY DISTURBANCE Oil HIGH-VOLTAGE T.RANffi~SSION LINES by MELVIN JULIUS KOFOID

A THESIS submitted to the

OREGON STATE AGRICULTUP~ COLLEGE

/

in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE June 1935 AIPBOTED: Redacted for Privacy r* d Researoh Professor of &Lectrleal Englneerln6 A. ,\ In Charge of MaJor l Redacted for Privacy n n {- .\ Eead of Deperfuent of Slectrlca1 Englueering sb

q a Redacted for Privacy ,l t- rl f Cbalrmau of $obool Graduate Connlttee a\ b- Redacted for Privacy

Chalrmao of Colle Oraduat e Counoll ACKNO V~EDGEMENT

The author wishes to give full credit to ~rofessor F. o. McMillan for the original suggestion of the possible use of.a filter circuit, utilizing the insulators as capacitors, on a high-voltage transmission line to attenuate the transmission of radio frequency interference. The encouragement and continued assistance and suggestions given by ~rofessors F. o. McMillan, A. L. Albert, and E. C. Starr of the Electrical Engineering Department of Oregon State College is very gratefully acknowledged. The laboratory investigation was made possible by the financial support and laboratory facilit~s furnished by the Oregon State College Engineering Experiment Station and Electrical Engineering Department. TABL ·

List of Illustrations. I. INTRODUCTION. The Problem of Radio-FreQ:uency Interference from High-Voltage Transmission Lines 1 Some Previous Contributions to the Elimination of Radio-Frequency Interference 3 Limitations of Present Methods of Radio­ ~requency Inter·ference Attenuation 3

II. RADI0-~3~ UENCY FIL~~~ S~CTIONS FOR HIGH­ VOLT G • T£U.l S ...I SION LIHES. Field of Application 5 Frequency Range of Attenuation 5 Plan of Filter Section Construction 6

III. FILTER S~CTION D.clSIGN D THEO ETICAL CHA.F.ACTERISTICS.

Desired Impedance Characteristics 8 Theoretical Impedance Characteristics of 8 Tenative Section Number of Filter Circuits in Section 10 Distribution of Resonant Frequencies of ~ilter Circuits in a ~ilter ection 10 IV . FILTER CIRCUIT CAPACITO,S. Dielectrics of High-Voltage Insulators ll Capacitanceu of High-Voltage Insulators ll V. FILTER CIRCUIT I JDUC TANCES. Inductance equireuents 14 Practical and Economical Considerations 14 TABLE 0~, CONTENTS (Continued)

Electrical Considerations 14 Inductance Calculations and Coil Design 16

VI. LABO TORY DETEIDviii~ATION OF FILTER CIRCUIT AND FILTER SECTION CHARACTERISTICS. Test Inductances and Insulators 18 Test Circuits, Apparatus, and Armangements 18 Testing Procedure 20 VII. EXPERIMENTAL FILTER CIRCUIT AND FILTER SECTION CHAR CTERI STI CS. Impedance Char acteristics of Insulators Alone 22 Impedance Char ac te:ristics of Individual Filter Cir·cu its 22 Impedance Char acteristics of l'art of a Comple te Filter Section 23

VIII. FILTER SECTION INSTALLATION ON A THRE~l'HASE TRANSMISSION LINE. Arrangement of Coils Throughout Section 26

Installation of Coils .26

Adjustment of Inducta nce of Coils 27 IX. CONCLUSIONS. 28

:X:. BIBLIOGRAPHY. 30 LIST OF ILLUS TRATIONS

Fig. 1 Installation of Inductances on a High-Volt~ge Transmission Line for Attenuating Radio-~requency Disturbance. Fig. 2 A fheoretical Admittance Diagram of a Radio­ Frequency Filter Circuit on a High-Voltage Transmission Line. Fig. 3 Photographs of Lab oratory Arrangements for Obtaining Impedance Characteristics of Radio­ Frequency Filter Circuits and Sections. Fig. 4 Circuit Diagram for Obta ingng Impeda nce Characteristics of Radio-Frequency Filter Circuits.

Fig . 5 Head to Pin Impe dance of 66 Kv. Reffrey-Dewitt iJO. 94 Insulators. Fig . 6 Line to Ground Impeda nce of Radio-Frequency Series Fil te:r Circuits .

Fig. 7 Line to Gr-ound I mpedance of Radio-Frequency Series Filter Circuits. Fig. 8 Line to Ground I mpedance of Three Radio­ :IPrequency Series Filter Circuits in :Parallel. Fig. 9 Line to Ground Impedance of Three Radio­ Frequency Series Filter Circ~uts in Parallel. Fig. 10 Typical Installation of Attenuation Coils oh a Wood :Pole of a Three-Phase Transmission Line.­ ~ ATTENUATION Q! RADIO FREQUENCY DISTURBAl~CE .....ON. ~ VOLTAGE !:RANSMISSION LINES

INTRODUCTION

The Problem of Radio Frequency Interference from High Voltage Transmission Lines:

A high-voltage transmission line causes interference in radio receivers because of the electromagnetic radiation resulting from distur~ances on the conductors or associated equipment. The magnetic and dielectric fields of this radiation are between conductors and between conductors and ground, the major portion of the interference caused in radio receivers being due to the fields between conductors and ground. A common source of electrical disturvances of radio frequencies from high-voltage transmission lines is the ionization and break-down of over-stressed air in the pin hole of the insulator, and between the conductor, tie wire, and insulator head. In some cases interference is caused by leakage currents flowing through conducting deposits of various kinds on the insulator surface. Other sources of interference are poor connections, loose tie wires, loose hardware, and defective equipment. 2 The most evident problem that presents itself is that of the elimination of the source of the disturvances as completely as possible. High class line construction and maintenance is necessarily rather expensive, and while essential for the complete elimination of sources of inter­ ference in urban areas, its use throughout, on long power lines traversing sparcely inhabited districts between the plant and the load center in many oases is not economically • justified. The additional expense of constructing a new line so that it would be free from radio frequency inter­ ferenoe causing disturbances may not be great since insulators quite free from radio interference are now available, but the riddance of disturvanoe from a complete line already installed generally would involve an expense that would be prohibitive. While high frequency disturbances on a line in lightly populated areas is not of serious consequence in such areas, such disturbances will travel along the transmission line for lo~g distances with remarkably small attenuation, into what may be othercise disturbance-free sections. Satisfac­ tory means of isolating interference-producing and inter­ ference-free sections of transmission lines at radio frequencies are therefore necessary. 3 Some Frevious Contribu.tions to the Elimination of Radio

Freguency Interf~rence:

Radio interference due to corona on the head and in the pin holes of insulators can be satisfactorily eliminated by aPPlying an asphalt emulsion treatment to replace the regions of otherwise over-stressed air with solid insulati~ mater~al.. ' Insulator manufacturers have, and are, doing much work in designing radio interference-free insulators. The other sources of interference can obviously be quite completely eliminated by proper maintenance. Choke coils have been designed and manufactured for isolating interference-free and interference-producing sections of lines. These devices are air-core coils of such inductance as to offer exceedingly high impedance to currents of radio frequencies and practically zero impedance to currents of power frequencies. A choke coil is inserted in series with each of the conductors at the point of the desired isolation.

Limitations of !'resent Methods of Radio Freguency Interference Attenuation:

Series choke coils must conduct the total line current and must be mechanically and electrically capable of withstanding line surges. Choke coils capable of conducting comparitively small currents, such as might be required to 4 prevent interference from being conducted into the vicinity of a zadio transmitter over the power line supplying the transmitter present a lesser problem and have been used satisfactorily . But choke coils designed for major distribution lines carrying large amounts of power at high voltages must be rugged, are massive, and are very expensive. 5 RADIO- FREQUENCY FILTER SECTIONS FOR HIGH-VOLTAGE TRANSMISSION LINES

Field o£ A~plication:

This paper is a report on an investigation of the possibility of constructing a radio-frequency filter section to afford a low impedance path for radio frequen­ cies between conductors and between conductors and ground, utilizing the high-voltage pin type insulators as capaci­ tors. Such a filter section, if found highly efficient, should be a convenient and relatively inexpensive means of preventing disturbances from heing conducted on into a disturbance-free section from the section of its origin. This study is limited to po wer lines using pin type insulators. A filter of the same type could be construct­ ed for use on a line employing suspension type insulators; pin type insulators have higher capacitances than suspen­ sion type insulators and therefore much larger induct~nces would be required in the filter circuits when suspension type units are used . Steel tower construction with the suspension insulators does not c onstitute nearly as serious a radio frequency interference problem a s does wood pole construction using pin type insulators.

FreRuency Range of Attenuation:

The experimental filter section wa s designed for 6 effective operation between the frequencies of 500 and 1500 kilocycles. In a commercial installation the range should perhaps be extended to the slightly higher frequencies now being used for brEladcast programs. The attenuation, due to the normal characteristics of the line, of disturbances being transmitted on a power line is much greater at very high frequencies than at the regular broadcast frequencies. A single high-voltage insulator of 20 micro-micro-farads capacitance would have approximately a head to pin impedance of 8000 ohms at 1000 kilocycles, or 300 meters, and an impedance of only 800 ohms at 10,000 kilocycles, or 30 meters. Therefore, it is expe cted that the interference conducted into a high class section of a line of such frequencies as to interfere with short wave braodcast reception should be comparitively small.

?lan of Filter ~ect ion Construction:

The filter section is to be composed of a number of filter circuits in parallel, each composed of an insulator, acting as the capacitor , and a coil of such inductance as w cause series resonance ~ ith the capacitor at some desired frequency in the radio braodcasting band. t the resonant frequency the inductive reactance, 2~L, of this circuit is numerically equal to the capacitive reactance, l/2fffC, and the imped ance is the resistance only. At frequencies other 7 than the resonant frequency the total impedance of the particular circuit is equal to the square root of the sum of the square of the res istance plus the s quare of the difference of the capacitive and inductive reactances. Therefore, a series filter circuit makes the greatest reduction in the impedance through it at its resonant frequency and reduces the impedance to various lesser degrees in the band of frequencies on either side. The impedance : characteristics of several individual experi­ mental filter circuits are shown in Figs. 6 and ~. By utilizing a number of filter circuits in parallel between a conductor and ground, and having their resonant frequen­ cies differ by a suitable amount, an over-lapping of their band-pass characteristics can be obtained tha t will reduce the line to ground impedance to a relatively small value for an entire band of frequencies. See Figs. 2, 8, and 9. Thus, a filter section was designed to lower the conductor to ground impedance, and the impedance between conductors for the entire radio broadcast band o£ 500 to 1500 kilo­ cycles. 8 FILTER SECTION DESIGN AND THEORETICAL CHARACTERISTICS

Desired IffiPedanoe Characteristics:

A well designed filter section should offer as nearly as possible the same admittance to all frequencies within the band, with the admittance in no case deviating very greatly from the average admittance. The average admittance should be made as high as is economically feasible.

Theoretical Impedance Characteristics:

The diagram of Fig. 2 indicates roughly the desirable admittance characteristics of ten individual filter cir­ cuits, a filter section composed of ten individual filter circuits in parallel, and the ten insulators of the filter circuits in parallel without the inductances. The diagram represents no particular data but is presented merely to indicate desirable theoretical characteristics. At 1,000 kilocycles the reactance of a 66 kv. insula­ tor may be 8,000 or 10,000 ohms. At this ssme frequency, the impedance through this same inallator and an inductance to complete the filter circuit, may be as low as 150 or 200 ohms. It appears that with an infinite numher of filter circuits the impedance at no frequency in the band consid­ ered should be greater than the impedance at resonance of INSTALLATION Of INDUCTANCES ON A HIGH- VOLTAGE TRANSMISSION LINE FOR ATTENUATING RADIQ-FREQUENCY DISTURBANCE FIG. I 1 RANSM!SSION LINE DISTURBANCE PRODUCING DISTURBANCE FREE -sECTION DISTURBANCE ATTENUA11NG SECTION SECTION---'._

POLE I

I i I I I I I I I I I I I I I I I I I I I I I I -!- + ~ ~ ~ -!- + -!- -!- + ~ ~ ::0 ::0 ::0,..... ::0 ::0 ::0 :;o ::0...., ::0 ::0 IT' IT' fT1 fT1 ...., f"' f"' rn (Jl (Jl (Jl (Jl (Jl (Jl (Jl (Jl (Jl (Jl 0 0 0 0 0 0 0 0 z z z z 0 0z z z z z )> )> )> )> z )> )> )> )> z z )> )> z z z z z z z z -I -1 -1 ...., -1 -1 -1 -1 -1 -1 )> )> )> )> )> )> )> )> -1 ~ ~ -1 -I -1 --, -1 -1 -1 1\) t1o 0) ..... ()) 0 ­ w ~ 10 (}I (}I (}I t1o t1o t1o (.)1 (.)1 (}I (}I 0 0 0 0 0 0 0 0 0 0

:,;: :,;; :,;: :,;: :,;; ~ :,;: :,;; ;x r A 0 () 0 0 () r () () _::c:---- . =c l __ :; :; :i :; :; }: ;: lLce ·;: l~o ~-_t ~ SCHEMATIC ELECTRICAL CIRCUIT

NOTE: THE 3· PHASE LINE IS REPRESENTED AS A SINGLE CONDUCTOR . AT EACH POLE IDENTICAL COI L S ARE PLACED BETWE:.EN EACH INSULATOR PIN AND GROUND

.;~, OF'"O'D A THEORETICAL ADMITTANCE DIAGRAM OF AN Rf. FILTER SECTION ON A HIGH-VOLTAGE TRANSMISSION LINE FIG. 2

I 10 PARALLEL ,, ! FILTER CIRCUITS ,, 1\ ,,,~ IIl ,,'~ ,,'~ II'-, I I / J I\ 1\ 1' 1\ 1\ t\ 1\ \ I\/\ J\ I\ I 1 1\ t\ 1\ \ f\ I\ 1\ I\ 1\ I\ I\ I INDIVIDUAL I \ I \v/ \,/ 'v1 \,./ \,./ \ ../ \,./ FILTER CIRCUITS w I \ I I u ' z I ' ...... ;I '\_> ~ POLE 1/ \ POLE 10 ..... \ ~ 0 I \ <( I ,___ , 10 PARALLEL I INSULATORS I \ I \ I \

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400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 FREQUENCY IN KC. 9 an individual circuit. In fact, it appe,ars t.hat it may be much less tban this va Jn.e. The total admittance between the line and ground at a particular frequency is the vector sum of the admittances at this :frequency of all of the filter circuits in paralle 1 between the line and ground. In Fig. 2, it is indicated

(by t..he broken c. ur: ve) that Vlhen all of the citcuits are in parallel, the admittance at 1050 kilocycles is greater than the admi ttaoo.e of circuit 6, which is resonant at 1050 kilocycles, by the admittances of circuits 5 and 7, 4 and 3, and all of the remaining circuits by respectively lesser amounts. At 1000 kilocycles circuits 5, 6, 4, 7, and all other circuits contribute jointly to the admittance. The comp osite curve of Fig. 2 is actually erroneous in that it is simply a representation of the numerical sum of the admittances. The actual characteristic is much dif£erent, and very much more complex, as the admittances actually add vectoriall y. Later in this paper, in the discussion on the experimental characteristics of three filter circuits in parallel , the results of the vector addition of the seYeral admittances is clearly shown and discussed. The admittances of the ten insu.lators, alone, in parallel add ?frectly. But, it is evident that the average, and even the minimum, admit!tance of the filter section to the band of 10 frequencies for which it is designed may be greater than tie total admittance of the insulators in the section alone.

Number of Filter Circ~ in Section:

The admittance characteristics of the filter section over a band of frequencies is determined by the combined admittances of the component circuits. The number of circuits used will be determined by the resultant over-all characteristics and the economics of the problem. The use of ten filter circuits, necessitating the construction of thirty inductances for an installation on a three-phase line, seemed reasonable from the point of the expense involved. Therefore, the experimental section was designed with ten different filter circuits arranged as shown in Figs. 1 and 2; the success of an arrangement of this degree of complexity to be used as a criterion of the practical value of such a filter section.

Distribution of Resonant Frequencies of Filter Circuits In a Filter Section:

Best results should be obtained with uniform spacing of the resonant frequencies, as then the maximum admittance will be the least and the minimum admittance will be the greatest, each being nearer the average admittance. It is not desirable to have a circuit resonant at a boundary frequency as then half of its filtering value is unused. 11 FILTER CIRCUIT CAPACITORS

Dielectrics of High-Voltage Insulators:

Insulators are made of high quality porcelain having a dielectric constant of about six and of pyrex glass having a dielectric constant of about five. The leakage resist­ ances are so high that the impedance may be treated as entirely capacitive reactance for practical puxposes. The dielectric constant at very high frequencies may be consmer­ ably lower than the value at low frequencies. Severe tenper­ ature and humidity conditions should not be detrimenial to the operation of a filter section utilizing insulators as capacitors because only very damp conditions could change the capacitance and the leakage resistance appreciably, and under such conditions the dielectric flux distr-ibution of the insulator would be improved so that the interference caused by corona on the insulator would be eliminated. Any changes in the capacitances or inductances would merely shift the band of frequencies passed by the section.

Capacitance of High=!£ltage Insulat~

The capacitance of standard 60 to 70 kv. pin type insulators is small. Measurements were previously made at the Oregon State College Electrical Engineering Laboratories o:f the capacitance of pin and pedestal type 4­ insulators. The average capacitance of fourteen represen­ 12 tative standard 60 to 70 kv. insulators was 19.• 5 micro­ micro-farads. Special types of insulators may have a quite different value of capacitance; three 66 kv. insulators with pressed metal thimbles instead of porcelain insulata thread in the pin hole had an average capacitance of 29.2 micro-micro-farads. Treating insulators with some form of conductive coating for the elimination of radio interfer­ ence also increases their capacitance. The values of capacitance quoted above were measured with a Western Electric 4-A Capacity Unbalance Bridge at a frequency of 1200 cycles. In the laboratory filter 6ircuit test :effrey-Dewitt No. 94 porcelain insulators of a nominal :rating of 66 kv. were used. The heads of these insulators were made of metal and were secuxed to the porcelain body of the insulators by means of projections secured in recesses in the porcelain by a specaal alloy designed to match the thermal expansion characteristics of the porcelain. Their capacitance when measured on the above mentioned capacity unbalance bridge at a frequency of 1000 cycles was found to be uniformly 20.0 micro-micro-farads. From the reactance data of later experimental tests their capacitance varied but slightly for frequencies from 530 to 1500 kilo­ cycles; this is clearly shown by the curves of Fig. 5. Their capacitance was calculated to be 17.5 micro-micro­ farads at 530 kilocycles.

• 13 When using insula tors as capacitors in;_,a filter section, it is very deSt rabl e to hav:·e th,e insulator capacit ances fairly uniform. It is obvious that the use of insulators with higher ca:pacitances allow the use of smaller inductances in the filter e:D:cuit. 14

~TER CIRCUIT INDUCTANCES

~Fductance Requirements:

For the experimental filter sedtion ten sets of three identical inductances were required; each coil of a set in series with an insulator of very closely 17.5 micro-micro-farads capacitance to resonate at some particular frequency in the braadcast band. The ten selected frequencies were 550, 650, 750 , 850 , 950, 1050, 1150, 1250, 1350, and 1450 kilocycles.

Fractical and Economical Considerations:

For uniformity and ease of production it is desirable that the coil forms be of the same diameter, and as compact as possible consistent with meeting the electrical reqUirements. They should be of a design that is simple and economical to construct. The form must have sufficient strength to prevent its collapse if a large current surge should pass through the winding.

Electrical Considerations:

In case of a flash-over of one of the insula tors in the filter section, the filter circuit inductance would have to c arry the short circuit current until the protective gap across the coil broke down or the oil switoh supplying the line opened. The winding was designed for use on a 66 kv. system with a 20,000 kva. load, 15 assuming the short-circuit current to be ten times the normal current and that the oil switch would open in one- tenth of a second, or less. Under these conditions the short-circuit. current at the maximum load of 20,000 kva. would be 1~50 amperes. The wire size select.ed for winding tbe inductance coils was No. 14 A. w. G. with double cotton insulation. With 1750 amperes flowing through the wire the temperature will increase from 20 to 475 degrees centigrade in 0.07 second and will increase to

1083 degrees centigrade, or the melting poi~ of copper, 5 in 0 .12 second. The charring temperature for the cotton insulation is approximately 475 dggrees centigrade, for ve~y short time transient heating and cooling of the conductor. Therefore , this size of conductor was considered adequate for the conditions assumed.

The distributed capacitance of the rad~-frequency filter coils should be low, so as to eliminate the necessity of additional turns to gain the necessary inductance because : the increased res ista nee of additional turns causes the resonance characteristic to be less sharp and the imped a nce at all times to be greater. Of the several ty_fes of coils, the single layer solenoid has the le ast distributed ca:ra c i tance and wqs the type used for the filter circuit inductances. A two layer bank winding does not have a hig h distributed capacitance and 16 since it would allow a considera~le saving in fo~m material and space requi~ements , the characteristics of this type of coil should be investigated in conjunction with this p~oblem.

It is desi~able that the form material hav.e a low dielectric constant and high body ~esistance combined with high mechanical st:r·ength. nMicarta", a bakelite varnish impregnated aper material was therefo~e selected. Before installation, the windings are to be treated with Glyptal lacquer or a similar protecting coating to render them impervious to moisture. A test was made and the lacquer was found to not increase the distributed capacitance appreciably.

Inductance Calculations and, Coil Design~

From the measured value of insulator capacitance ani the relation 2 ~L = l/2 ~fC in a series resonant circuit, the inductance re quired to p1oduce resonance at the selected frequency in each of the filte:r· circuits of the filter sect ion was cc, lculated. The number of illrns and the coil dimensions w.ere c.alculat.ed using Lo:renz's formula, L -= an2~, where~ is a function of 2a/b; a being the radius to the center of the winding, b the total length of the -r,: ind ing, and n the .rlmmber of turns. For a table of values of and for the formula derivation the 17 reader is referred to u. s. Bureau of Standazss Scientific 6 Paper No. 169. The calculated values and the values of inductance of the constructed coils , measured with a western lectric Impedance Bridge at a frequency of 1000 cycles, were comparable with an error of only a fraction of a per cent. However, due to the distributed capacitance of the inductance coil, to other stray c apacitances, or some other phenomena that was not accounted for , the filter circuits were resonant at frequencies less than their designed valuea The filter circuit that experimentally resonated a t 1350 kilocycles, shoQld have resonated at 1720 kilocycles

according to calculations us~g the values of capacitance am i liductance meas -JXed at 1000 cycles. The coil diamter of eight inches was chosen after preliminary calcuL.. tions, as being the most desirable for the ave:r ge coil if aJ.1 coils are to be of the se.me diameter. 18 =----.-...,;;;;;;-LABORATORY DETERl,~I H A.~IOU OF l!1ILTE.R-- CIIWUIT AlJD F ILTE...------.­ SECTION CHARACTER!.§. TICS

Test Indue tances and Insu..la tars:

A filter section consisting of three filter circuits was tested in the laboratory to obtain their frequency­ impeda nce characteristics. Three 66 kv. Jeffrey-Dewitt No. 94 pin type insula tors were used. Inductances of 50 , 46, and 42 turns of No. 14 double cotton covered copper wire were close wound on eight inch Micarta tubes one-eighhh inch in thickness. When placed in a closed circuit consisting of an inductance, an insulator, and a vacuum thermocouple, their resonant frequencies, as determined with a Gene:ral Radio recision iavemeter, were 1150, 1250, and

1350 kilocycles , r e~p ectively.

Test Circuits, A£paratusJ-and Arrangements:

Photographs of the lab oratory arrangements are shown in Figs. 3A and 3B. The electrical circuit is shown diagramatically in Fig. 4. Radio frequency energy was supplied by a 100 watt General Electric oscillator which is the center lower unit of the high-voltage cathode ray oscillograph shown. This energy was fed to a shc:I:Ct transmission line attache d to the filter circuit, or £ilter circuits, and terminated in an impedance of approximately 430 ohms at 1000 kilocycles; Fig. 3A. Laboratory Arr~ngements for Obtaining the Impedance of Radio-Frequency Series Filter Circuits

Fig. 3B. Laboratory Arrangements for Obtaining the Impedance of Three Radio-Frequency Series Filter Circuits in ~arallel V. T. rv OSCILLATOR TERMINATING VOLTMETER IMPEDANCE

S. H.RECEIVER AND BEAT OSCILLATOR

CIRCUIT DIAGRAM FOR OBTAINING IMPEDANCE CHARACTERISTICS 0 F R. F. FILTER CIRCUITS FIG. 4 19 471 ohms was the pure resistance value of the terminating impedance. The return circuit was grounded at one point, as shown in the diagram. The radio- frequency potential between the line and the return conductor was of the order of 100 volts maximum, and was measured with a vacuum tube voltmeter at the terminal impedance. The voltmeter utilized a type 31 triode vacuum tube operating as a half wave rectifier with the grid connect­ ed to the filament at the tube socket. The rectified plate current was read with a 0-10 direct current milliammeter shunted with a two minro-farad paper condenser to by-pass the radio-frequency current. The vacuum tube voltmeter was calibrated on 60 cycles alterna ting current and wa s also checked on direct aurrent potentials. The value of the ter-minal impedance was determined by measuring the current through the voltmeter and impedance with a Robert Paul 0-50 vac~um thermocouple milliammeter, making corrections for the rectified current and the current thr·ough the voltmeter due to inter-electrode capacitances. The current through the filter circuits was naasured with Western Electric vacuum thermocou} les and a 0-150 Weston micro-ammeter. Three thermocouples were used to cover a range of currents from 0.2 to 215 milliamperes.

The frequency was determined with a Western Electric field·strength measuring set by adjusting the superhetrodyne 20 receiver to the power oscillator frequency and _zero beating with this, a signal from the calibzated local oscillator of the field strength measuring set. The power oscillator calibration was not used because its calibration is exact when supplying a sweep wave to the oscillograph but changes a small amount with change in l oad.

In general, the circuit and apparatus were arranged to minimize the losses that would in~roduce error in the measured quantities.

Testing Procedure:

Test·s were first made on each individual filter circuit consisting of a single coil and insulator-. Fig. 3A shows the arrangement for these investigations. Finally, a test of a partial filter sectiion with the three :filter circuits resonant 100 kilocycles apa.r,t was made.

The axran~ement for this test is shown in Fig. 3B and differs from the auove arrangement only in the coil and insulator configuration. It was necessary to make slight adjustments in the inductances in this test due to slightly different stray capacitances, different loeations of the circuits with respect to the field of the oscillator , and a slight difference in insulator capacitances at radio frequencies. In each test the p10oceduxe in obtaining the data was the same. The power oscillator was set at a desired frequency, 21 the f:r·equency was accurately checked with the field strength measuring set, and readings of the vacuum tube voltmeter and the filter circuit micro-ammeter were recorded. From the actual values of voltage am current, the filter circuit, or filter section impedance at that frequency was calculated. It was necessa ry to observe several precautions such as keeping all apparatus in the same exact relation to each other throughout the te s ts, to minimize stray pick-up by the measuring instruments to a negligible amount, and to not

interfere Vvi th the field of the ci rc.uit wh ile taking the readings. 22

EnERIMENTAL FILTER C~CUIT AND FILTER SECTION CHARACTERISTICS

Impedance Characte:r·istics of Insulators Alone:

At 530 kilocycles the impedance of a single insulator was found to be 17,000 ohms and that of three insulators in parallel to be 5,700 ohms, indicating capacitances of 17.6 and 52.4 micro-micro-farads in the Eespective cases. The average capacitance of the three insulators in parallel was 17.5 micro-micro-farads. The impedanc$for one insulator and for three insula­ tors in parallel were calculated over the range of frequen­ cies from 530 to 1500 kilocycles, using the values of experimental capacitances determined at 530 kilocycles. The agreement between these calculated values and the measured values of impedance over the range of frequencies investigate4, as shown in Fig. 5, indicates that there is no appreciable change in capacitance with frequency.

Impedance Characteristics of Individual Filter Circuits:

The impedance characteristics of the individual filter circuits are shown by the curves in Fig. 6. The lower portions of these curves are shown to a la~ger scale in Fig. 7. The ratio of the iii!fledance of the insulators alone to the minimum impedance of the filter circuits was 70 for circuit 7 at 1150 kilocycles, 40 for circuit 8 at 1250 HEAD TO PIN IMPEDANCE OF 66 KV. JEFFREY DEWITT NO. 94 INSULATORS FIG.5

CAPACITANCE FOR CALCULATED VALUES WAS OBTAINED FROM EXPERIMENTAL IMPEDANCE AT 530 KC.

• EXPERIMENTAL VALU£S o CALCULATED VALUES 18000 ...

16000 \ \

14000 1\ \

12000 \

~ (/) 2 ' 5 10000 z ' ~ ONE INSULATOR w ~ 8000 (§ w a.. ' ~ ~ " !'. 6000 ~

4000 ...... ' THREE INSULATORS ' ~ IN PARALLEL " ~ -... ~~ 2000

0 400 600 BOO 1000 1200 1600 FREQUENCY IN KC. LINE TO GROUND IMPEDANGEOF R. F. SERIES FILTER CIRCUITS FIG. 6

CAPACITANCES- 66 KV. JE.FFRE:.Y' DEWITT NO. 94 INSULATORS INDUCTANCES- NO. 14 D. C. C. COPPE.R WIRE ON 61N. MICARTA FORMS CIRCUIT 7- 50 TURNS CIRCUIT 6- 46 TURNS CIRCUIT 9- 42 TURNS

14000

12000

~ -

10000 - - 1

({) ~ I ~ I 0 8000 ~ I z \ w ~\ I u " z <( \ 1\. ~ 0 6000 ~ w a... ) 2 "\~ I

I 4000 - f-­ - ~~ I 7 ~ ~ ~ v 1/ - - \ ~~ ~ j j 2000 J CIRCUITS ~~};IJ J ~~ 1 ~- 0 I ~ ~ ~ . -. ~ r· · r..t:=~ 400 600 800 1000 1200 1400 1600 FREQUENCY IN KG . LINE TO GROUND IMPEDANCE OF R. F. SERIES FILTER CIRCUITS FIG. 7

CAPACITANCES - 66 KV. JUFRE.Y DEWITT INSULATORS NO. 94

i~•D:....c...r ANCES - NO. 14 D. C . C. COPPE.R WI RE ON 8 IN. M IC ARTA FORMS CIRCUIT 7- 50 TURNS CIRCU IT 8-46 TURNS CIRCUIT 9- 42 TURNS

1600 1 ~ _,~ -- - - f- ·- ' -- - - f.-- ---

1400 1--f------1-'1 - f--

- - 1--- -·- --1------1- - - - ~ --

1200 1------1-- - -· .. 1-- ~ -- - --

-- f---·------

(.f) __,_ :::E 1000 - 1------I c 0 _,_ . z - - --· ------1------

~ 800 --f------1------z --·- -<{ 0 ------1-- .- -1---· ·----- u.J ' n.. 2 600 1------" - \ , 1\,. ' - 1--- \ 1\ ) 400 -r 7 \ I J J 200 \ \v \ I - 1' CIRCUIT 8 I ' I 5 ·---· 'V - - - ICIRCUIIT 7 I CIRC~IT 9 .--< c.-/ o m 0 1 1 900 1000 1100 1200 1300 1400 1500 FR EQUENCY IN KC. LINE TO GROUND IMPEDANCE OF THREE R. F. SERIES FILTER CIRCUITS IN PARALLEL FIG. 8

CAPACITANCES- 66 KV. JEFFREY DEWITT NO . 94 INSULATORS INDUCTANCES- NO. 14 D.C.C . COPPI:::R WIRE ON 8 IN . MICARTA FORMS CIRCUIT 7- 50 TURNS ..( CIRCUIT 8- 46 TURNS CIRCUIT 9- 42 TURNS

7000

~------. -- -- 0 - t-- -- I 6000 t------\ \ - 5000 f-- - ' ------i (/) t---- I g 1\ --r- ~ I . 0 !\ ' 4000 t----- ~ 1\. -- - --t- z ~ ! \ w ~ u z ~~ ...... ___j_ _._ ~ 3000 1\ ··- ·---- - w WITH F-ILTER NSULATORS 0... I \ CIRCUIT COILS ~ ALON!:. i ~ 2 I I ...... ~ ! ! po...... _ 2000 ! ·-

i ~ - ! ' " I i 1000 -+-- Ill.. ,-- -· ·- '\ \ j t--- " ~ L -- - ~

v ,.~ r>': I 'J \1 O n '' 0 400 600 800 1000 1200 1400 1600 FREQUENCY IN KC. LINE lO GROUND IMPEDANCE 0 F THREE R. F. SERIES FILTER CIRCUITS IN PARALLEL FIG.9

CAPACITANCES- 66 KY. JU FREY DEWITT NO. 94 INSULATORS INDUCTANCES- NO. 14 D. C. C. COPPER WIRE ON 8 IN. MICARTA FORMS CIRCUIT 7- 50 TURNS CIRCUIT 6- 46 TURNS CIRCUIT 9- 42 TURNS

1600 \

1400 \

' \ - -

1200 \

(f) \ L I 0 1000 \ z uw \ z ).. (§ 800 w ' ' Cl... L \ -­

600 \ ~ '~ - I

400 1\ I I

v \ ~ ~ 200 \I \ I( I I

I r---­ \I f

0 I I ~""~ ·o 900 1100 l200 1300 1400 1500 FREQUENCY IN KC. 23 kilocycles, and 60 for circuit 9 at 1350 kilocycles. The degree of sharpness of resonance and the low minimum

impedance indicates satisfacto~ coil design.

Impedance Characteristics of a Part of a Complete Filter Sect ion: The characteristic of three filter sections in parallel over the range of frequencies between 530 and 1500 kilocycles is shown in Fig . 8. The impedance characteristic of these sections is shown to a larger scale in Fig. 9. In this case the ratio of the impedance of three parallel insulators alone to the combined impedance of the t.hree parallel filter circuits was 22 at 1150 kilocycles, 14 at 1250 kilocycles, and 27 at 1350 kilocycles. However, the maximum impedances in this range were of such values as to limit the reduction in impedance to only 1.5 at 1195 kilocycles and 1.6 at 1300 kilocycles. It should be no·ted that the impedance rises to a maximum of 1600 ohms at 1195 kilocycles. The impedance at this frequency with filter circuits 7, 8, and 9 in parallel is grea ter than the impedance of either circuit 7 or 8. The impedances of circUi. ts 7 and 8 r.ere each 700 ohms, but the reactance of circuit 7 was predOO'line:tingly inductive while that of circuit 8 was capacitive. The vector sum of the lagging curr-ent of circuit 7, the leading current of circuit 8, and the smaller leading current of circuit 9, was small 24 indicating a high impedance for the circuits in parallel. This action is characteristic of inductance and capacitance in parallel. The maximum impedance at 1300 kilocycles is explainable in the same manner. By the method of finding t h e total area under the curve and dividing this by the length of the base to find the average ordinate, the avera ge impedance with three filter circuits, between 1100 and 1400 kilocycles was calculated to be 460 ohms . . The av er·ag e impedance of three insula to r·s in parallel for this !land of frequencies was

2500 ohms. The avera ge reduction of imped~nce between 1100 and 1400 kilocycles was thereby approximately 5.5. A very noticeable reduction in impedance is shown for the entire broadcast band while empilioying filter circuits peaked at only 1150, 1250, and 1350 kilocycles. When the avera ge reduction in impeda nce is not as large as is desired wlJen a uniform res(tl)n!l,nt frequency separation is employed, in a practical field installation the resonance poi~ts for the individual circuits constituting the filter section may be selected to resonate at the frequencies of the important broadcasting stations serving that particUlar area. By this method of installation the number of resonant circuits in a xilter section may be reduced to the number of broadcast frequencies on which reception is particularly desired. Such an arrangement makes 25 it possible to effect a maximum improvement in broadcast reception at a minimum expense. 26 EILTER SECTION INSTALLATION OH A THREE-FRASE LINE

Arrangement of Coils Throu ghout Section:

The design of a complete filter section for the broadcast band of frequencies u milizes thir ty inductance coils resonat:illigt·w-d:th the capacitances of the insulators at ten particular frequencies , as indica ted in Fig. 1. Three coils are · mounted on each of t e n consecutive poles; all coils on a particular pole are to be i d entical. Other coil arrangements were considered but none was discovered that afforded greater economy of coils or a more simplified arrangement.

~tallation of Coils:

The sketch in Fig. 10 shows an -· installation on the simplest type of tra nsmiss ion line construction, but all except the e xact physical arrangement could be identical for an H-frameor other type of construction. Common practice finds insulator pins both ungrounded and bonded and grounded. In the former case tground rods must be installed , and in the latter the §onds and ground wire need but be broken a nd the filter coils inserted between the pins and the ground co nductor.

The coils are to be i nstalled in a saitable weather­ proof housing f a stened on the pol e near the cross-arm. Each coil is placed so that the axis of its magnetic field is at I

I \ I

EACH COIL IS MOUNTED COI L::;, ARE MOUNTED AT RIGHT ANGLES TO IN A Wl:.ATHERPROOF BOTH OTHER COILS PROTECTING CASE

PROTECTIVE ~APS ACROSS _.- -- GROUND ROD COILS NOT INDICATED

TYPICAL INSTALLATION OF ATTENUATION COILS ON A WOOD POLE Of A 3-PHASE LINE FIG. 10

...... _. (.~ .. 27 righ~ angles to those of both other coils in order to reduce the mutual coupling to a minimum. An inexpensive protective gap is to be installed across each coil. A good ground is necessary, or else the ground resis"f:ance .may become an appreciable part of the radio­ frequency conductor-to-earth impedance.

Adjustment of Inductance of Coils:

Small capacity discrepancies in insulators and perhaps capacitance differences due to coil positions are to be expected; small shi:fts from the designed :. resonant frequen­ cies will result. It is pra ct~al, then, to use tapped coils capable of adjustment of several turns more or less than their design c~lls for. Final adjustments are to be made after the coils are in place, by means of a small ~ ick-up coil antenna mutually coupled with the filter circuit inductance and connected to a sensitive compact radio receiver. This receiver must have an accurate frequency calibration, so that it m~ be adjusted to any frequency at which resonance may be desired. The tapped indue tance in a filter circuit would then be adjusted to resonance by setting the receiver at the desired resonant frequency and adjusting the inductance to give maximum receiver output. 28 CONCLUSIONS

1. Laboratory tests on three filter circuits, utiliz­ ing standard 66 kv . ~ in type insulators and resonant at 1150, 1250, and 1350 kilocycles, showed an ave rage reduction in conductor to ground impedance of ap proximately 5.5 between 1100 and 1400 kilocycles . The minimum reduction was 1.6 and the maximum reduetion was 26. 2. Upon completion of the remaining filter ci.rcuits of the section, as designed, the impedance in the frequency :range already investigated in labora tory tests may be furths: materially reduced by the action of the additional circuits. 3. Impeda nce characteristics obtainable with filter circuits peaked at frequency differences of 75 and even 50 kilocycles may be vrotthy of serieus investigation. 4. A radio frequency filter section of this type, if proven successful in field tests, should be an economical means of isola ting sections of transmission lines from radio­ frequency interference foreign to thase sections. 5. The maximum iiDP.rovement in broadcast reception on a limited number of frequencies can be made at a minimum expense by adjusting the circuits of a filter section to resonate at the frequencies of the important broadcasting station s serving a particular area.

6. In an installation1 w.here the ability of the inductances to carr y the short-circuit current in case of 29 an insulator flash-over, is not necessary or is not consider­ ed economically ju stified, much more compact inductances may be constructed by using smaller wire. 7. The direct grounding of ungrounded insulator pins will increa se the atte nuation of radio-frequency interf er­ ence being conducted by a line. 30 BIBLIOGRAl?HY

1. Brooks, M. and Turner, H. M. Inductance of coils. Bulletin 53, University of Illinois Engineering Experiment Station, University of Illinois Press, Urbana, Illinois. 2. Everitt, w. L. Communication engineering. McGraw-Hill Book Company, New York, 1932. 3. McMillan, F. o. The coordination of high-voltage transmission lines vi'i th radio. A paper present­ edpa!t the nig.hh annual general meeting of the engineering section, Northwest Electric Light and Power Association, Portland, Oregon, 1932. 4. McMillan, F . o. Progress report on radio interference from high-voltage transmission lines- pin and pedestal tY.I;? e insulators. A report to the Northwest Electric Light and Power Associ a tio.n. 5. Rice, s. o. The heating of copper conductors by transient electric currents. Senior thesis, Oregon State College, 1929. 6. Rosa, E. B. and Grover, F . w. Formulas and tables for tm calculation of mutual and self-inductance. Scientific paper No . 169 of the U~ . s. Bureau of Standards, \iashington Go vernment Printing Offioo, 1916. 7. Terman, F. E. Radio engineering, McGraw-Hill Book Company, New Yo~k , 1932.