I 7956-1-T Air Force Avionics Laboratory Research and Technology Division Air Force Systems Command Wright-Patterson Air Force Base, Ohio TRANSMITTER IMPEDANCE CHARACTERISTICS FOR AIRBORNE SPECTRUM SIGNATURE Interim Technical Report No. 1 1 April - 30 June 1966 J. E. Ferris, W. DeHart, R. L. Wolford and W. B. Henry 15 July 1966 Contract AF-33(615)-3454 Contract Monitor: K. W. Tomlinson AVWE 7956-1 -T = RL-2166 THE UNIVERSITY OF MICHIGAN COLLEGE OF ENGINEERING DEPARTMENT OF ELECTRICAL ENGINEERING Radiation Laboratory Administered through: OFFICE OF RESEARCH ADMINISTRATION * ANN ARBOR

THE UNIVERSITY OF MICHIGAN 7956-1-T TABLE OF CONTENTS Page ABSTRACT ii INTRODUCTION 1 II COMPLEX LOAD IMPEDANCE MEASUREMENTS 3 2.1 300 MHz Anechoic Chamber 3 2.1. 1 300 MHz Chamber Construction 4 2.1.2 300 MHz Chamber Evaluation 6 m SOURCE IMPEDANCE MEASUREMENTS 9 3.1 Need For a Frequency Selective Receiver 9 3.1. 1 Modification of APR-4 Radar Receiver ~ 10 3.2 Source Non-linearity at The Fundamental Frequency 13 3.2. 1 Theoretical Rieke Diagrams 13 3.2.2 Experimental Rieke Diagrams 20 3.2.3 Conclusions 30 3. 3 Source Non-linearity at Harmonic and Spurious Frequencies 33 i

THE UNIVER SITY OF MICHIGAN 7956-1-T ABSTRACT Under a previous contract, AF-33(615)-2606, an investigation was made of measurement techniques applicable to the accurate determination of spectrum signatures of airborne transmitters. It was shown that in principle, only three measurements are required for the prediction of power radiated from a transmitterantenna combination. These are: (1) the antenna VSWR, (2) the transmitter VSWR, and (3), the maximum power available from the transmitter. In this report, a method is described for obtaining the antenna VSWR under circumstances that would otherwise result in interference to other systems. A small anechoic chamber used to isolate the antenna is described and the chamber's performance is evaluated. When determining the impedance (or VSWR) of a transmitter at the fundamental, spurious and harmonic frequencies, it is necessary to employ a frequency selective detector. The modification of an APR-4 radar receiver for incorporation in a frequency selective detection system is described in this report. The techniques for measuring the impedance of a transmitter, developed under the above contract, are based on the assumption of transmitter linearity. This report describes an investigation of the linearity of sample transmitters. Determination of source linearity is best accomplished through the use of the Rieke diagram. Rieke diagrams, at the fundamental frequency, for three types of sources are presented and the results are discussed. Also included is a discussion of source non-linearity at harmonic and spurious frequencies. ii

THE UNIVERSITY OF MICHIGAN 7956-1-T I. INTRODUCTION The statement of problem as set forth in the contract which provides for the present investigation is as follows. A determination of the power delivered to the antenna for "spectrum signature" purposes will require a measurement of the antenna impedance, transmission line characteristics, transmitter maximum power output, and transmitter output impedance at the fundamental, spurious and harmonic frequencies. The transmitter output impedance at the spurious and harmonic frequencies is not well Wunderstood and, therefore, requires further study. The prine payoff in this study will be better "spectrum signatures" f6r more accurate predictions of interference between systems. There is a need to verify the results of the earlier successful program, Contract AF 33(615)2606 "Simplified Modeling Techniques for Avionic Antenna Pattern Signatures", with a mock-up of an aircraft transmitter system. The stated objective of the contrat is: To conclude the development of "simplified" techniques for determining the RF spectrum signatures of flight vehicle electronics systems. To establish the validity of the techniques by comparing the results of data obtained by the "simplified" techniques with data obtained from tests employing a typical transmitter system in a mock-up. 1

THE UNIVERSITY OF MICHIGAN 7956-1-T A previous report (Ferris et al, 1966) discussed the theoretical aspects of power transfer under unmatched conditions: a subject of great importance in the prediction of spurious radiation from a transmitter - antenna combination. It was shown how, in principle, the power radiated can be predicted from measurements of the transmitter, transmission line, and antenna. Although two of these measurements (i. e., transmitter and antenna) Involve simply the determination of standing wave ratios, the present study is being conducted under more stringent requirements, i. e., the determination of transmitter and antenna impedances. It is felt that this approach provides a more sound foundation for the verification of the simplified measurement techniques being developed. 2

THE UNIVER SITY OF MICHIGAN 7956-1-T II. COMPLEX LOAD IMPEDANCE MEASUREMENTS In the final report (Ferris et al, 18S) en the predecessor contract, it was recommended that the technique developed by Michigan for measuring the impedance of a source be further evaluated by employing typical transmitters used by the military services. The source impedance measurement technique discussed in the above report utilizes as the terminating Impedance eithqr a shortcircuit or a complex load. The use of either of these terminations has been experimentally verified by measurements of the impedance of a laboratory type signal generator and a tuning stub. When measuring the medane of a typical transmitter used by the service, it may be impractical to use a short-circuit termination since the transmitter may be designed to work into a well matched load at the fundamental frequency. Thus, it will be necessary to use a complex load when making the impedance measurement and it appears logical to use the typical system antenna U the complex load to best satisfy all conditions. It has be6q shown that the only requirement pertaining to the use of the Complex load is that it's VSWR be accurately known at the frequency of interest. The VSWR of the antenna must be known also for power transfer considerations. Further, the system antenna is the most convenient device to use in the field when making system measurements. 2.1 300 MHz Agechoc Cham r In view of the above considerations to use the system antenna as the complex load when making source Impedance measurements on a typical transmitter, a 3

THE UNIVERSITY Or- MICHIGAN 7956-1-T 300 MHz anechoic chamber was constructed. This enclosure will house the antenna for measurement purposes. The chamber will provide a free-space environment for the antenna since it is not always practical to make laboratory type investigations in the field. Stray reflections are undesirable when the VSWR of the antenna is being determined or the impedance (or VSWR) of the transmitter is being meas ured. If a relatively lew power source is not available for determining the VSWR of the antenna, the chamber will allow the measurement to be made with the typical service transmitter which produces relatively high power (10 to 30 watts). In the event the transmitter is of the communications type as is being used in this investigation, the antenna must be electrically isolated from the surroundings so as not to cause interference to other systems in the locality. The 300 MHz chamber constructed by Michigan meets these requirements as will be shown below. 2. 1.1 300 MHz Chamber Costructia The chamber consists of a 64 cubic foot cube as shown in Figure 2-1. Aluminum sheets are suported on the outside by a wood frame. All interior joints have been sealed to prevent rf leakage. The inside of the cube is lined on all six sides will broadband absoeerbing material covered with vinyl. An eight cubic foot volume of space is left in the Interior of the chamber for the antenna installation. The absorber is described by the famiufacturer as having moderate performance for frequencies of 300 MHz and up. Our ekaluation has 4

THE UNIVERSITY OF MICHIGAN 7956-1-T FIG. 2-1: 300 MHz ANECHOIC CHAMBER (With Blade ntenna AT-256A/ARC-27) 5

THE UNIVERSITY OF MICHIGAN 7956-1-T shown the absorber to perform effectively at 200 MHz and higher. Electronic ' gasketing has been installed to prevent rf leakage around the door which forms one side of the chamber. 2. 1. 2 300 MHz C r EvaluaI The chamber was tested for its effectiveness as a free-apace environmeit by comparing impedance data taken for two typical military antennas, inside and outside the chamber. The two antennas measured were the blade antenna AT-256A/ARC-27, and the modified monopole antenna, both of which are designed to operate over the frequency range of 200 - 400 MHz. An impedance plot for the blade antenna from 200 - 1500 MHz is shown in Fig. 2-2. The impedance data for the antenna inside the chamber, shown in Fig. 2-1, is superimposed on the data taken in a free-space environment in the field, shown in Fig. 2-3. As can be seen from Fig. 2-2, the two sets of data compare favorably. Agreement is typically within 10 percent. Measurement inaccuracies are felt to be responsible for the few data points not demonstrating the typical agreement. Data for the modified monopoe is not shown due to a delay in data reduction. 6

7956-1-T no GC 1950*o NAETi LE 0 -G. NO0 _________ARFia 73 CN RDI WEST CO CORD, Mk!,SACHUSETT-S P LWITH ORQwARTTA ronu)OMIN3cy 0) Oktdoor Envirounmn Cfhamber Environmen 1. A j7 - r - I.1..101 A1% I..ill t.2e2: NIPNDANCE (UASAOTEjWIWOF BLADE ANTENNA AT-2564ARG-27 RACIALLY SCAkfL naAuErEPSas n kmA -A at(FArur*M)w +MS~~~~~~~~~~4+AAA +trJLAJ* 9J.44m&LJ CENTE -

THE UNIVERSITY OF MICHIGAN 7956-1-T FIG. 2-3: BLADE ANTENNA AT-256A/ARC-27 ON OUTDOOR IMPEDANCE PLATFORM 8

THE UNIVERSITY OF MICHIGAN 7956-1-T III. SOURCE IMPEDANCE MEASUREMENTS Techniques for measuring the impedance of a source have been developed and were presented in the final report (Ferris et al, 1966) on the predecessor contract. The application of the techniques to date has been very successful. The accuracies attained are sufficient for the predictio of power transfer to the antenna at the fundamental frequency. The above report contains the results of laboratory measurements of source Impedance of a signal generator. The present study involves the application of these measurement techniques to a typical service transmitter at the fudamental, spurious and harmonic frequencies. This type of source presents the investigator with complications not previously encountered with the laboratory type signal generator. Therefore, before applying the measurement techniques to the transmitter, consideration has been given to the need for a frequency selective detection system, and an investigation of the nonlinear characteristics of the source. 3.1 Need For A Frequency Selective Receiver In order to determine the impedance of a transmitter at the fundamental, spurious and harmonic frequencies, one must be able to detect and measure only the frequency of interest. The impedance measurement technique requires measurement of the standing wave produced on the lineat the frequency of interest. Since the typical transmitter produces several spurious and harmonic frequencies in addition to the fundamental, several standing waves appear on the line simultaneously. 9

THE UNIVER SITY OF MICHIGAN 7956-1-T The apparatus used to detect the desired frequency must then obviously be frequency selective. Therefore, consideration has been given to the modification of an APR-4 radar receiver to be incorporated in a frequency selective detection system for the source impedance measurements. 3.1.1 Modification of the APR-4 Radar Receiver An APR-4 radar receiver IF chassis and appropriate tuning heads.have been obtained. The receiver is shown in Fig. 3-1. After checking and replacing defective components, the receiver IF was realigned with a sweep generator and oscilloscope until the response curve in the wide mode was similar to that in Fig. 3-2. Next, the circuit was modified as shown in Fig. 3-3, (the 6H6 detector was removed) and the IF output was connected to a type N connector, which was installed in place of the VHF "Pan" output connector. Finally, the appropriate receiver tuning units were tuned and installed. Following the above preparations, the sensitivity and linearity of the unit were measured. The sensitivity of the unit is approWmately -65 dbm (minimum discernable signal). However, it should be noted that the sensitivity varies considerably with frequency. The receiver is linear (square law) within - 1 db over a 35 - 40 db dynamic range. The performance of the unit is felt to be satisfactory for use in the frequency selective detection system. 10

THE UNIVE TASITY OF MICHIGAN 7956-1-T la t FIG. 3-1: APR-4 RADAR RECEIVER 11 I

\, —W C, W-4 A:1 -4 C) n -i C)

THE UNIVERSITY OF MICHIGAN 7956-1-T 3.2 Source Non-Linearity at the Fundamental Frequency Expressions for power transfer on a transmission line are usually derived from a model similar to Fig. 3-4. It is usually thought reasonable to assume that the generator impedance Z is not a function of the load impedance, e. g., the g model is a linear model. This assumption may or may not be accurate depending upon the particular generator used and the variations in load impedance. A convenient method by which the accuracy of the assumption may be determined for a particular generator is to compare Rieke diagrams of the linear model and the real generator. A Rieke diagram is a plot on the complex impedance (Smith) chart of constant power contours. A constant power contour for a constant source impedance can be shown to be a circle. A comparison of the actual constant power contour obtained by experiment and the theoretical contour of a linear model shows the variation of the source impedance as a function of the load impedance. 3.2.1 Thoretical Rieke Diaams Figure 3-4 is the model from which the diagrams will be plotted. The average steady-state power delivered to the load is the real part of the rms voltage appearing across the load multiplied by the complex conjugate of the rms current through the load. PI Re E1 I1] (3.1) 13

THE UNIVSE i-SITY OF MIOHIGAN 7956-1-T g * % * I UI I I 4 S.,-, - - II I I I I I I I:- I I i i I i I I i I I i II I i I. — J - - - - - - - - - - - - k-.71-1 40 i i i 0 I I I i i:1 I -.., 1 I I I 1. - % I i I I I I I I I I I Pz %Wf aE;L 7. 40,.:"4 Lx b-" b-4:r*0 0 ii I I jI I' h\I I /: I - - - - - - - - - - I 14

TIE1 UNIVEirSITY OF MICHIGAN 7956-1-T However, the load will not in general be located directly at the generator terminals. Let the impedance seen by looking into the line in the direction of the load at the plane A A' be Z. The average power delivered to Z is a a P Re [E 1I (3.2) a a a where E I and. =I Z (3.3) a Z +Z a a a g a thus E E =., z (3.4) a Z +Z a g a It is now evident that E E a a z Z + Z a T R o g a g a E 2 E ]2 t Eg a " g a g a; The result (3. 6b) may be reduced to a more convenient form. Since it is of interest to plot colstant.ower contours as a function of Za define a constant K.a n such that 2 PK * (3 7) n a n + Where the oomplex I _edao Z has been expressed as Z (R + JXa) a 15

THE NvF7Iy OF MICHIGAN 7956-1-T Coinhning (3. 6, t)and ( 3.7), K 'R = I + 2R R + R7+ X +2X X +x2 n a a a a g a n g g (3.8) Rearragn (3*f8) and completirg the square yields 2R K2 K2 -4K R LHa + +[Xa + xg2 (3..)- A Equation (3.9)1La of the form U + V 3 A, implying that Rieke diagrama Plotted on a rectangular chart (iH. abscissa; iX, ordinate) and assumn a constant generator internal impedance, wi'll be circles with center at 4KR 1/2 and radlui Z. Furthenmore, 9 er 1 for the radius to be real K2>4K R (.0 which implies that 2 i. e., the maximum- power available to the load can never exceed the isquare of the generator voltage divide'Z by four times the real part of the generator impedance. The load Impedance corresponding to maximum generator power output for a given lead impedance can be found directly from d Pa(RSE+ RX2 +(X Og+ X~ (3a) ft 16

THE UNITVE- SITY OF MICHIGAN 7956-1-T Obviously P will be a maximum when (X + X; is a minimum, spcifying a xg a/ that X= - X. P (ma, (R is determined by setting dP a. a 2 22R 2 dP (R + E 2 - E2 R (2Ra + 2R } S a/ I | a a.S 0 -(3. 12) a Rg + R 9 a 0 R +2R R + R2 - 2R - 2R R R - R (3.13) g g a a a g a g a Since the values of R and R are restricted to positive real numbers, (3.13 rea g duces to R =R (3. 4) g a Thus we have the familiar result that maximum power transfer results when the load impedance at the generator terminals s the complex conjugate of the generator impedance. Substituting this result into ( 3 6b) 2 -ainax. 4R (3.15) P(max)= 4a g ve rifing (3.11). The result (3.9) may also be expressed in terms of the load impedance Z1 and length of lossless transmission line by substituting an expression for Za in terms f a terms of Z1. 17 era i

TTIE UNIVrERSITY OF MICh TIGAN 7956-1-T 7a ' Z cos B +'7 s in l j1 (3.16) 0 where i = and I is the electrical transmission line length from the generator terminals to the load. Equation (3.9) lends itself readily to mapping on a rectangular impedance chart or it may be point plotted on a Smith Chart. One method for mapping (3.9) on a Smith Chart is to recognize that circles on a rectangular chart transform to circles on the Smith Chart. It is now necessary only to plot three points from (3.9) on the Smith Chart and draw a circle whiose circumference includes the points. The center of the circle is located by the intersection of the perpendicular bisectors of straight lines joining the points. A less tedious method involves a transformation of variables from the Z plane to the p plane. It has been shown that a bilinear transformation of a circle in one complex plane yields a circle In the second (Guillemin, 1949). Thus the circle defined in the Z-plane by equation (3.9) transforms to a circle in the p plane, the transformation equation being - +p (-P 3. 17) Ieich, Ordang, Krauss, and Skolnik (1953) have demonstrated that the circle =r + +j0 ie (3.1 ) 18

TI1-IL UI ST 01 MIi1U' 7956-1-T with ecnrt.ci at (r jx -%) nd rudiul-t {, in the 7-plane transformi-s to thie circle. 0 0 I I j~.~ (3I 19) in the i — plane In terms of the paranietALcrs of (3 -18); -j, -, I -, L %. I-) p -. I - f-) - + X -.. 11 0 0 + 'V U L F I 9) I.+i\1 +x - I 0 ra + 0 2 1/2 (3. 20) (3. 21) (3.229) =-t2rx i - 'rJ\ rtt, -) 0 1 - -I +X - -/:-~ubsli, in Jng 1 "') K - 2. Lk 0 x=-X andiR= 0 (~ 2 -3) fron- (3.*u) into (3. 1V). (3. 17), anti (3- 10) LA~ I~U 1)2 +K+ ( a 234) 19

iw TiH UNivJY-ITY OF MIC}IIAN 7956-1-T R 2+X 2 1 2+4IX 2 (3.25) [ K J n S -2X tn -I A (3. 26) 0 R 2 +X -2 1 o0 the parameters of (3 9) have be determinid, (S. 0) can be mapped dtroctly oto a Smith Chart as the circle (3.19) with center at IP l ) and radus R. Figures 3-5, 3-6, 3-7, and 3-8 are plots of (3.9) on rectangular and Smith charts. Figures 3-5 and 3-6 are the special case R - R ' the transmission line impedance. In Figs. 3-7 and 3-8, Z - 1..5 - J0.5 (normalized). Notice that the loci of the centers of the circles form a straight line on each chart corresponding to a constant reactance on the rectangular chart and a constant angle of reflection coefficient on the Smith chart. 3.2.2 4 per mental Rike Diagrams To investigate the internal Impedance variati of a real generator, one must plot an experimental Rieke diagram of the generator. A convenient approach to the equipment setup is shown in Fig. 3-9. This is basically the arrangement which was used to collect the experimental data presented in this report. However. the presence of strong harmonic and spurious outputs would neoessitate a more elaborate setup. Figures 3-10 through 3-13 are experimental Rieke diagrams obtained with the equipment shown in Fig. 3-9. The figures include the ideal model contours superimpoed on the experimental data points. 20

956- 1 -T J 4 — 4 -i- f- T-4-1 -4 - T1 -, ---4 T, -7 -4- - IT, _44 4- t T 4- 4 F t 7 -I -4.+ L+ + r t4l T-4-i 4- -4 - -T000 ole 4 —t __Tt t - i_ - I I -T I F +.4 A T -Tbum".-.Po oplgeo 4- 4 -T.......... T rT 4.- 4p — t- Lb C4 r-T - + 7 T —i AlTi:4-t -T 7. _ ~ i —V -H ---- 4 JL4W _ - _ I I-rTh ry j"77TtT t7hLity 21.A i

No 4 z A ip - FIG. I I y la 306: 'fL0 0 THEORETICAL R1ESE DIAGRAM: Z JR 1 + JO~' (Special Case) RA^D IAL.LV SCALED PARA M E I RS 50 Ly'4 AtL~r ' — '+444+A' - -f Y T' P? - 44A~-ioo ~ L- 0 CENTER a. I A.5 9

7956-1-T 4i~~b-4- 444 - 4 r:Itt>4 L4- - -t ~ f 4 -4- F4-4-A- ~ t 4 J 44ISM - -1- - 4-p-4lI -I T 41.4 1.1 - - A -- F- - ' I —.II- - IL ii4~ K ~t-I ' XI 1 -7iJi 0 - 0 z u U ~ w w I * I- a Ld 0 b IAJ 9 I - L I I I I I I I N i i i i i 1 ri I -1 1 IL, 1 j i 1 1 - 1 1 1! 1 1; N if 1 1 1 1 i 1 1 1 1 -.t' I i I I I w I I I I I I i N - 4- 1 1 1 -- 4a A i i 1 4 —l- 4 1 4- i 1 1 i I i i i i -+ i i i i -% f i i I I-I -1 I I I I x I I I t - - — I - -t-H-k IT11IV11I7 I I I K.LJI 'r T I 1 1! -fI - I I I I I - I I - i-4 - I --,-I! T. 6 1 4 1 -I-.-I, [ .-.. -. It -4....+.i f - i -4-1 I I --- - ", i 1 4 -L-' 4. 1: FILT --- L I 41- i i i.h' i -1 —I --- t -~- 1 t'-t-t-t 4 —t- i I -- +!IF i i i i i i + 10- 1 - I L! "L 'I 1 I 4 5! 4 1 of 4 t1i I 't &ft t-t I 11 - -2- IHILkIfI-I I 2 —ici io I i -4- i 4 i i Ak-4 i - - I Pt i i q i i. I i t:j -e - t — I -f -- - I IL I, I I I I I I I-Ht —. — F-Ir i I I I; I I f i II- -t - 1__z - I I I! I I I -II. -I- I I I ---- 4- -4 — r— f i; I - 1 I I I 1 -4- -.1 if 1 1 1 1 1 IF j 1!-+ -4 - VTAT q-L i -i i i i i.tlI I i 1 -1 -1 - f +4+ I -I-F{I-i i: I IF i i igi i 1: 1 1 1 1 1 - 4 I I i I I i-: i -L, -7? i 1 1 f+ - I'l-t - I k i i i i i - I i i I I-Irtrr! 1 1 1 -. I I -Ir-. I I " + I.. - i i i 6F I I.. - I I I - — +- I i - --- I --- + ttit. -4-I I - -4-4-i-4 —4-4 —4- 4 --- —4 ---I&T TX -4-4F —+I4- +-+-*-h-t-i —it-tt-t - I A 1-1 I I - I d+ i I I I I I i I I I I I I I 4 I r%,L I -— 4. Ivr I ii I, I 4 - - — 4 ill:4~ .:W. I 1 1 p4kj 4 j 36 1 - 1 1 1 1 1 1 I i I P, I I I I I I.: -, I I -t Or - - r 1 iL -; — 4-i-- 1! 1 i 1 1 i -+ -+- - i i 1 11 1 1 2'4 1 1 14d 1 'Rt f I! i -t I i I 1, F i -T- I- - -; I; I L —I- I 1 -ir I I 1, I I I - -!-+ —4-, L 1 - - 1, - I I I, - I i v -. I- I. - 4 1! 1 - - —4- i m i. -I — I --- i I t i i - Illt:- k,-, —~ I +- I. E=l i i i i i I. LLL.d W:4 —=60409 I I I i II I I I I, I - I i I I I I I Ti — II t'- --- I,. -1 - -i i I I i - i i I'. I. 7 —r,, Ji1 I I 1 1 i 1 t I - - b I i i II J6 1 1 1 1 11 I 7 —+ I --. I. -- -.. i 'T IkIN i i i. I I I.. I I - I II i I I -- i I I I I I I I f I I..... - la is - -1 — 11 -t ---t - L i - il I I I I -1 I IL I I I M-1- - I i 1-4 [4 --- I I I I, I I I I t- I [-,H- j.-I f I I i-I -I I! A -t 4 4- I I I F i — +- 1 i 1 1 -4 —f —+ I I I I I lI I, I I j -4-4 ---- 1 1! 1 -+ i + ----L, i f -t - - 1 -4 i i i; i i L —I- i i i i i i - i i i f i I i p 1~ J= i i 4 - -4 —4 i i i i - i i i i! ---+ — 4 1 1 1 1 1 -+ IL.1-t 4 4 1 I i f t 4 I 114 41 1 4 __j 4 — i i i i i- i i i i '.'.FF.FFF — 1 —I I I I I i i I I -1 I -i I I-A- L-L -Li I I I I I I L 1 1 i 1 4 4 1 1 -+ i i i I I I 1 I I ' llI-ldi i i I i.. I I I I I i 4 1! t 4 -i I I I I - 1 it -1 - I I I I I t 1 1 1 II I I- I- I I I I-t,II -1 I - i- I 11 i I II 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 - I 23

7956-1-T o GC 19so9.....- - - DWG NO - D. — _I --- - — 1t C) CMPa CtST 'NJ.Q MkSSAGHuSETTS ______ IMPEOANCE OR AWITTArK E COORDN ATES......- - _ ~ - ~_..~ /, - - - \ -=i. - __<- -* ---..A //, \/ %.-' -- _-',._ _-. ' Si. 7.. A. II: ~. '. /-.~_- ~ ~.',;,. -. if ""'L G| FIG. 34: THEORETICAL RIEKE DIAGRAM; Z R * 1.5 J O. 5 RADIALLY SCALED PARAMETEM 0 'I *-AO:-E -y rT *-r -t DB 1~ ~ fc g g Ie~ ~t! 8- is 8 2 - a f n-4 | t 0 - t i t - - ~ o -- - 5 s - di 0~

T' _1 UI...'IT~7 OP ICHIGN 7956-1 -T za I.., - sU L~ ~ ei _ _ __... _ '.._ {3i. a 25

7956-1-T - GCc 195i.P A- r, D WG NC I___A__jA j7 L~' JNA `-W)MASSA( >JLSEZ IMPWANCE OR ADM'V"6 INCE COORN NATES QP=0.95 P ___ max 0P=0.60 P max N. lif If' Ir * c A-,r, 4. - V, I-X FIG. 3410: RIEKE DIAGRAM OF H. P. 612-A SIGNAL GENERATOR *~ I RADIALLY SCALED PARAMETERS co 00o 0 Q - r 0 - 0 & s-C E N T ft FK

7956-i-T c GC 19509 j C* H AHAPT - iCl 5~'S '4- -,ENNAL RADI-LhOMPkNY WI -ONCfIp4~MASC &CHUSET IS 1 MPEDANCE OR ADM!TTAN(O-C C&CRDIuNATES a I.- Q p= 0.9 gop" I 4-A 7I a* "" 4, 4 Iiiv 4 — A 04 — FIG. 3-11: RIEKE DIAGRAM OF TRANSMITTERRT18AC2 1~DO~4rATm~RADIALLY CLU!MFER o, esIV V a all r F% 0% a a

7956-1-T7'' GC 19509,.I-'Gr~~M3yCENE'..i RA. - u P, MA S:j A IMPEDANCE OP A~e, TT-Y *FOiNATES AP= 080 P max i II LOTI N A - - I -A I -Y f I -1 A I- - k m _ — \ Ir I I N -1 /r f / r4 /1l 'Ir I >AlA. ILL b - -. "KI IA -- - I I - I I.. - I.. -.. - -:1 -1 i.. T-I,- -. I I J, ~,,. '1 I. I I I v I N NN -. - -T I.-, k - i - r - -V I - 7 - I Al. V - 7 L-z.11 -L- L1 I- A -.r 4. ' - r '0u C P & a.. - 1*11- L4 L..LLL4L..,. *T~A~ T'TL. N-T H

7936-1-TOW.N _____ - - - ~ - - - - - - - -MATE r 'a$J IEEI~ VCOMPANY__WEST Z-ONCRU ASCHJr IMPEIWNCL OR ADMITTANCE COORDINATES QPsO. 74 P -11 4p ifc f tft V1.T 4 >6 V. / s t -ItI N ~ ~ O 0,A N.A '= ~m — t - FIG. 34wl3: HESDIAGRAM 0?FTRANSMITTER RT4178/ARC-27 r 'AU" - r+rr. aA~ eTOWAND LOAD '~j &a a F a 41 1U 1aa S - -w

TI T - U N [ V: -- R S I -r Y OF MICHIGAN 795C-1-T Figure 3-10 ib tlit lieke diagram of a standard laboratory signal generator. The experimental points conform well with the predicted result. In fact, previous impedance meas urements of this generator by another method suggest that the deviations from the theoretical contoturs are due primarily to experimental error. The signal generator, a Hewlett-Packard 612-A, is of the type shown in the center of Fig. 3-14. Figures 3-11 to 3-13 are Rieke diagrams of a military type RT-178/ARC-27 transmitter. This experimental data does not conform exactly to the contours drawn from the linear model indicating that the transmitter internal impedance does not remain constant as the load is varied. The transmitter is seen at the right in Fig. 3-14. Some experimental data was gathered on a third type of source, a laboratory power oscillator. However, a Rieke diagram was not plotted because of the irregular nature of the data obtained. The oscillator, an AIL type 124-A, shown at the left in Fig. 3-14, is of the resonate cavity type. The internal impedance of this oscillator varies drastically with variations of the load impedance. 3.2.3 Conclusions The method of comparing the Rieke diagrams of a real transmitter and its theoretical linear model provides a graphic, qualitative description of the linearity of the real transmitter. The Rileke diagram of the standard laboratory signal generator exhibits a high degree of linearity. As this result was anticipated, this type of source was chosen to illustrate linear behavior. However, the 30

THE UN lVT-PSlTYT OF MICH IGAN 7956-1-T 4- t~ it 4 al~h C.-. I -,,., 1 v.. — '.4 9.314LEFT: AL OWER OSCILLATOR TYPE 124-A; CENTER: STANDARD LABORATORYIGAL GENERATOR; RIGHT: TRANSMITTER RT-17B/RC27 31

T H Ui N1 V -i [Ii ~ OF MIC I C N I - 7956-1-T generator is not of the type used in the field by the military. The AIL power oscillator is of the resonant cavity type and may be found in the field, but experimentation has shown this type of source to be highly non-linear. This oscillator is an example of a source which does not lend itself to measurement of internal impedance by the Michigan developed technique. The Rieke diagrams of the typical service transmitter RT-17 8/ARvC-27 indicate the internal impedance of the transmitter varies slightly with variations of the load impedance, i. e., nonlinearity is present, but is relatively small. It is felt that the degree of nonlinearity exhibited is not sufficient to cause inaccuracy in the prediction of power transferred to the antenna at the fundamental frequency. All data presented above is for the fundamental frequency; data for harmonic frequencies has not as yet been collected. It is anticipated that the general character of the data for harmonic frequencies will not differ greatly fromn that at the fundamental frequency if the transmitter is properly terminated at the fundamental. Under these conditions, predicition of the power coupled to the antenna promises to be simple and sufficiently accurate. Other variables, such as tuning of the transmitter and variations fromn transmitter to transmitter of the samea type, will need to be investigated to determine their effect on prediction. The variations of source impedance and available power output at each frequency are the important quantities. 32

TiI - UNIi vvsI -Y OF TICIIIG AN 795t-1-T 3. 3 Source Non-Linearity at Iarmonic and Spurious Frequencies Source imlpedance measurements at harnmonic t'Vetllclljit' may be complicated by the non-linearity of the source impedance at the fundamt ntal. Let us denote the source impedance as: S = source impedance at the fundamental S. = source impedance at the second harmonic S = source impedance at the nth spurious n and the terminating impedance as: T = terminating impedance at the fundamental T = terminating impedance at the second harmonic T = terminating impedance at the nth spurious n At present only the variation of S1 as a function of T1 has been determined. The variation is present but is relatively small as was seen in Figs. 3-11 to 3-13. Future measurements will involve S2 versus T2, S3 versus T3 and so on. Inasmuch as the bulk of the transmitter output is dissipated in S and T1, it is anticipated that the termination T2, T and so on will not affect S1 appreciably. In 2_ ' 1 fact, even though no particular attention was paid to the values of T2, T3, etc., in the measuring apparatus, no adverse effects were noted. It is likewise anticipated that if T1 is maintained at or near the design termination, the impedances S,, S3, etc. will be relatively constant with wide variations in T2, T3, etc. If this last assumption does not hold, then this approach to the prediction of spurious radiation must be grossly modified or, perhaps, abandoned. 33

TI I; U iV:- Is TY O- F MI I C IGAN 795-1- Fr Assume for the purpose of the present discussion that S.)...., - n will be relatively constant. It is apparent from measurements nmadc so far that S1 must be nearly matched to T1 in order that good measurements can be made. The immediate problem becomes one of providing T1 to match S1 while measuring S, utilizing a highly reactive load at the second harmonic frequency, and likewise for S3, S, etc. Some thought has been given to this problem. A cursory examination reveals many ways, using shorted tuning stubs and other apparatus, to produce a nearly pure reactance at any given frequency while maintaining a matched termination at the fundamental frequency. The best method will therefore be that combination of coaxial apparatus which is most conveniently adjusted and calibrated. Work on this problem will proceed as the situation warrants. 34

THE UNIVERSITY OF MICHIGAN 7956-1-T REFIRENCES Ferris, J. E. et al (1966), "Investigation of Measurement Techniques For Obtaihing Airborne Antenna Spectrum Signatures", Final Report AFAL-TR-66-101, July, 1966, University of Michigan, Radiation Laboratory Report 07274-1-F, 137 pages. Guillemin, E. A., The Mathematics of Circuit Analysis, pp. 360-373, John Wiley and Sons, Inc., 1949. iteich, H. J., Philip Ctdung, Herbert Krauss, and John G. Skolnik, Microwave Theory and Techniques, D. VanNostrand Co., Inc., Princeton, N. J., 1953, pp. 856-862. 35