COMMUNICATIONS TELEVISION RESEARCH MAINTENANCE I S
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ELECTRONICS RADAR COMMUNICATIONS TELEVISION RESEARCH MAINTENANCE i s 1 ELECTRONICS COMMUNICATIONS TELEVISION RESEARCH MAINTENANCEJ MAItCll,944 ANTENNA PERFORMANCE John Barron 3 ELECTROPHYSICS OF HEARING John D. Goodell 9 CALCULATOR FOR DISTRIBUTION ENGINEERS V. W. Palen 13 ALPHA RECORDING B. H. Porter 14 X-RAY IN AVIATION MEDICINE John L. Bach 16 A MULTIPLE CONVERTER UNIT Herbert L. Lipson 18 ELECTRONIC PROCESSING C. B. F. Macauley 21 INDUSTRIAL REVIEW Y4 NEWSBRIEFS 26 NEW PRODUCTS 30 TECHNICAL BOOKS 34 PERSONALS 36 RADIO -ELECTRONIC ENGINEERING is published each month as a special edition in a limited number of copies of RADIO NEWS, by the Ziff -Davis Publishing Company, 540 N. Michigan Avenue, Chicago, Ill. VOLUME 2 NUMBER 3 Copyright, 1944, Ziff -Davis Publishing Co. COVER PHOTO-BY HANS GROENHOFF \0. !Mk, Operator "tunes" the large induction heating coil used 4:> in setting glues. Laminations for Fairchild Engine and 5f F á Airplane Company's wooden aircraft, as described in this s ' 4 issue, are treated by this method of heat processing. y'NG co 2 ENGINEERING DEPARTMENT A typical standard -broadcast antenna of the type discussed In this article. AKTEN PEIIFOIIMANC By JOHN BARRON Consulting Engineer Washington, D. C. An outline of the procedure for design- ing antennas to meet FCC requirements. ASYSTEMATIC method of selecting coupling networks and testing an- tenna performance is extremely val- uable in satisfying the strict requirements of the Federal Communications Commis- sion. This article includes a detailed ex- planation of such a method of testing the performance of directional antennas for a 5 kw. station. Theoretical and prac- tical data are given so that the installa- tion considered is indicative of the re- quirements to be met and may therefore be used for such purposes. This station under consideration first obtained a construction permit from the Federal Communications Commission to erect a new 5 kw. station operating on 1280 kc. The present station operated with 1000 watts power with a non -direc- tional antenna. Inasmuch as the present station already operated on 1280 kc. all of the preliminary measurements, tuning and adjustments, had to be made during the period when the present station was not in operation, which was approximately from midnight to 6 a.m. However, un- der the terms of the construction permit, no power must be radiated from the an- tennas of a new station except during the equipment test period, which is from 1:00 a.m. to 6:00 a.m. local standard time, until special permission is obtained from the Federal Communications Commission upon completion of the initial adjustments. The horizontal plane radiation pattern determined for this station, together tennas upon the third must be con- with E3, E,, E3 = voltages applied to antennas the antenna arrangement is sidered in the most direct and prac- I, = currents flowing in antennas shown in Fig. 3. This pattern was tical manner. Z11, Z,,, Z = self impedances of antennas decided upon as desirable Zu, Z13, Z = mutual impedances be- for the dou- When an antenna is driven so that tween antennas ble purpose of avoiding interference it has current flowing in it, an in- with co -channel stations and at the duced voltage will exist When applying equation (4), it is in any other important same time beaming a maximum sig- antenna in the vicinity. This effect to note that if some of the nal having the effective power of 20 resembles two circuits antennas are not being driven, then coupled to- the kilowatts toward the desired service gether and the action may be corresponding applied voltages best are zero, area. explained in terms of mutual im- and if some of the antennas are open -circuited Of the various measurements nec- pedance Z12, which is defined by the so that they carry essary, the transmission line and an- relation: no current, then the currents in these tenna impedance measurements are Es antennas are assigned the value of zero. fundamental. Z1s = - (3) The driving point impedance, which First consider the properties of the Where: must be matched to the transmission line, for the transmission line and then those of I1 = current in first antenna first antenna is: the antenna array. The input im- E, = voltage induced in second antenna I, Z21 I, ZD1 = El = Z11 (5) pedance In defining II + I Zu.... of the transmission line which mutual impedance, it is - 1 Ii has negligible losses is: necessary to use a definite reference ZD1 = R1 + i Xa1 (6) point in each antenna. This may be Zr cos ß+j Z. sin ß and for the second antenna: A - (1) further clarified by realizing that Zo cos ß+jZr sin fi mutual impedance E, II ZIs I, represents the ZD, = = + Zss + Z,2.... (7) Where: coupling effect between the antennas I, I, Is expressed in terms of the current at ZDs = R2 + j Xas (8) Zr = Load impedance and for the third antenna : Z. = characteristic impedance a definite point in the first antenna ß =, electrical -angular and the voltage at a corresponding E3 Z!i length of trans- ZDa = = III + Z23 + Zss.... (9) mission line. point in the second antenna. If these I, points are not ZDa = R3 + j Xa3 (10) To measure the characteristic specified, it is assumed im- that they are at the current antinode. In order to match an antenna to a pedance, Z0, of the transmission line It is possible transmission line using a reactive simply to calculate the mutual measure the input impedance impedance between four -terminal network, consider the when the receiving two antennas but end is open -cir- in practice there are too many transmission line impedance as Z. and cuited and short circuited. exter- Let the nal influences which cannot be the resistive component of the antenna values of these impedances com- be repre- puted accurately. In this discussion, impedance as R. The position of the sented by Zoo and Zac respectively. the value of Z12 shall be taken reactances in the four -terminal net- Then the characteristic equal impedance is to Z21, where the latter is the work are shown in Fig. 2 and their given by: mutual impedance obtained by driving an- values are computed by applying the formulas : Zo = 1/ Zoc Zec (2) tenna number two and inducing a voltage in antenna number one. Z.cosß Z.R - (11) It is this impedance, which of course With a three antenna array, the sin ß is entirely resistive, that must be system equations become: properly matched to the antenna. In Rcosß ÿZoR E1 = I1 ZI1 + I2 Z21 + 1s Z81 X2- - (12) this particular application each an- E2 = I1 Z1,+ I2 Z22 + 13 Z82 (4) sin ß tenna is one of an array of three. E3 = I1 Z13 + 12 Zss + 13 Z33 Therefore, the effect of any two an- Where: X, - siß (13) Fig. 1. -f An r bridge used for measuring antenna and transmission line impedances. Both re- where fi is the phase -angle shift in sistive and reactive components are evaluated at the operating and adjacent band frequencies. the four -terminal network. When nondirectional tests are be- ing made on the antenna system, phase relationships are not important and a reactive "L" network may be used. The phase shift introduced by an "L" section is determined by the ratio of R to Zo as shown in the fol- lowing formulas. The design formu- las for the "L" section are obtained from equations (11), (12), and (13) by assigning X1 equal to zero. The new formulas become: cos ß=+NIR (14) Zo X2= ZoR - Ra (15) Zo R X,=- ZoR-R2 (16) With the aid of the foregoing form- ulas and discussion of basic funda- mentals the test procedure and calculations follow in very logical se- quence. 4 ENGINEERING DEPARTMENT As a preliminary test, antenna and pure resistance usually about 78 ohms. transmission line impedances should Also check the reading of the phase be made at 1280 kc. Measurements monitor by connecting any condenser need not be made over a band of fre- of known value in series with the in- quencies as the station is not intended put to the monitor. The monitor will to operate non -directionally except then read the phase shift which is the for test purposes. The impedance angle whose tangent is X/R, where measurements made on each indi- R is the input resistance of the phase vidual antenna should be repeated to monitor and X the reactance of the ascertain whether the resistance and condenser. For example, a 0.002 mfd. reactance changes appreciably when Fig. 2. Basic "T" impedance matching network. condenser will together with a 78 ohm the lighting circuits are connected resistance shift the phase 38.6 degrees and disconnected. If a change of the given by the square root of the prod- and a 0.001 mfd. condenser will shift order of 10 percent occurs, the cause uct of these two figures. If any line the phase 69 degrees at 1280 kc. must be removed. During the prelim- should be approximately 1/4 wave- It is important to note that the inary measurements, each antenna is length long at 1280 kc. or a multiple bridge readings taken on the phase allowed to "float" free (ungrounded). thereof, an erroneous result will be monitor sampling lines should be the While the r -f bridge is connected to obtained or it will be impossible to same for each line, if they are of each antenna, determine the amount balance the bridge. In such cases, the equal length as they should be. If of reactance necessary to resonate frequency should be changed by about these readings are not nearly the the antennas at 1/4 wavelength.