7956-3-T I Air Force Avionics Laboratory Research and Technology Division Air Force Systems Command Wright-Patterson Air Force Base, Ohio.,........ A.RC.. W^ [ ^tP/eNOS^ TRANSMITTER IMPEDANCE CHARACTERISTICS FOR AIRBORNE SPECTRUM SIGNATURE Interim Technical Report No. 3 1 October - 31 December 1966 J. E. Ferris, W. R. DeHart and W. B. Henry 15 January 1967 Contract AF-33(615)-3454 Contract Monitor: K. W. Tomlinson AVWE 7956-3-T = RL-2168 THE UNIVERSITY OF MICHIGAN COLLEGE OF ENGINEERING DEPARTMENT OF ELECTRICAL ENGINEEIING Radiation Laboraory Administrnd,through: OFFICE OF RESEARCH ADMINISTRATION ~ ANN ARBOR

THE UNIVERSITY OF MICHIGAN 7956-3-T TABLE OF CONTENTS Page ii ABSTIRA CT I INTRODUCTION 1 I SOURCE LINEARITY EVALUATION 2.1 Shortcomings of Rieke Diagram Approach 2. 2 Theoretical Power Output Contours as a Function of Transmission Line Length 2. 3 Experimental Power Contours 2.4 Conclusions 3 3 3 4 10 14 REFERENCES i

THE UNIVERSITY OF MICHIGAN 7956-3-T ABSTRACT A simple linear model of a transmitter-tranusmission line-antenna system has been proposed in previous reports. The work described in this report is concerned with evaluating the correspondence between the behavior of the model and a typical transmitter. To evaluate the applicability of the model to power output predictions, a new analytical tool is introduced. Power output contours as a function of the phase angle of the load are developed for the linear model. Also included are a number of such curves generated from experimental data taken from a typical transmitter for a variety of loads. The experimental and theoretical results are compared and discussed. ii

THE UNIVERSITY OF MICHIGAN 7956-3-T I. INTRODUCTION The statement of problem as set forth in the contract which provides for the present investigation is as follows: (1) 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 understood and, therefore, requires further study. The prime payoff in this study will be better "spectrum signatures" for more accurate predictions of interference between systems. (2) There is a requirement 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. (3) The stated objective of the contract 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 implified" techniques with data obtained from tests employing a typical transmitter system in a mock-up. (4) The present phase of the contract is concerned with developing a technique 1

THE UNIVERSITY OF MICHIGAN 7956-3-T for the accurate prediction of the power output of a typical transmitter. The evaluation of this technique requires an investigation of the linearity of a typical transmitter not only at the fundamental but the harmonic and spurious frequencles as well. 2

THE UNIVERSITY OF MICHIGAN 7956-3-T II. SOURCE LINEARITY EVALUATION The possibility of evaluating the power transfer in a given transmittertransmission line-antenna system by modeling the system with linear idealized circuit elements has been introduced in previous reports. To evaluate the feasibility of the linear model approach, the study for the past several months has centered around the development of a convenient technique for the quantitative evaluation of the linear model parameters, where applicable, and the modification of the existing model in accordance with the deviation of the real transmitter behavior from that of the model. 2. 1 Shortcomings of Rieke Diagram Approach In the past, the major analytical tool employed for active source evaluation has been the Rieke diagram. This approach, while it does afford a graphic illustration of the transmitter's behavior has several serious disadvantages. It is difficult to experimentally obtain Rieke diagrams to the required precision, and the diagrams do not readily lend themselves to a quantitative evaluation of the model parameters. For these reasons the data format has been changed from load impedance contours at a constant power output (Rieke diagrams) to power output contours at a constant magnitude of load impedance. 2. 2 Theoretical Power Output Contours as a Function of Transmission Line Length It has been shown (DeHart, 1966) that the power transfer from an idealized source through a lossless transmission line to a linear passive load can be expressed 3

THE UNIVERSITY OF MICHIGAN 7956-3-T as a function of the phase angle of the load impedance. P=ZP 1L......(2. 1) max 2 2co, 2+ sin2 a cos p f sin ( where P the maximum power transfer for a given load max 2 a = the ratio of the maximum to the minimum power transfer for a given load, Pmax /Pmin 6 ~max7 mmn 0 the phase angle (electrical degrees) of the load 2 Furthermore, a can be expressed solely in terms of the source and load VSWR's. 2 /Vi 1 2 (2. 2) where S = the source VSWR g S = the load VSWR Several plots of P(0) for various values of a appear in Figure 2-1. 2.3 E rimental Power Contours The idealized source can now be compared with a physical transmitter by experimentally determining power output contours as a function of the phase angle of the load. This has been done with an RT-178/ARC-27 military transceiver and 4

7956-3-T cr Odb a = 5db - 5.-u _ I ---- 1 lOdb -20 —: - c 20db -25 4 -- -30... — -t -= 30db -35 - -. ---- - -. -,3 ] -40 -- 40db Transmission Line Length (kd, degrees) FIG. 2-1: POWIgER TIRANSFER VERSUS TRANSMISSION LINE LENGTH A.LENGTH

THE UNIVERSITY OF MICHIGAN 7956-3-T the results appear in Figs. 2-2 and 2-3. The equipment arrangement used to collect these data is diagramed in Fig. 2-4. Care was taken to preserve the match at the fundamental (300 MHz) while the load phase angle (transmission line length) was varied at the second (600 MHz) and third (900 MHz) harmonic frequencies. The lower curves represent relatively large values of load VSWR, however, the measured data is within + 1. 5db of the value predicted by the linear model. Once a has been measured for a known load, it is possible to solve for the source VSWR. acS -1 S = (2.3) g S1 - a Assuming that the source VSWR remains constant for a given frequency, a and thus the power transfer are specified at that frequency for a given load VSWR. Thus the source power output can be predicted for the RT-178/ARC-27 type of transmitter from a linear model provided that the fundamental is well matched. It has been previously reported (7956-2-T) that there was evidence of a nonlinear condition existing at the second harmonic with the fundamental terminated in a matched load. Figures 2-6, 2-7, and 2-8 in report 7956-2-T show an unexplained discrepancy between predicted and measured values of load impedance for a given power output. Later investigation revealed an equipment malfunction. The data 6

--— t __ _ _ __ __ fLoad VSWR 3:1 1.1 1 MeasuredData. I IPredicted Data Load, VSWR=.30:1 - 2-2 -.. —..- - - I. _ AP Aw -4 co,;l m 1 W 1 -3 - ------------ A, I I Ii 0 i 0 60 70 i i I I i 800 i 90 __ __. I.........~ Transmission Line Length degrees - -- (Frequency: 600 MHz) FIG. 2-2: POWER TRANSFER, VEIRStJS TRANSMISSION LINE LENGTH

I~ 1 cl --- 011 I i I I I i I I i i p — - i -t --- I II II i II i ---- I i I i Load VSWIR 40:1 Measu, Predic 200 30 40 500 600 700 Transmission Line Length Wdegrees(Frequency: 900 MHz) FIG. 2-3: POWER TRANSFER VERSUS TRANSMISSION LINE LENGTH I i I I I I i I,red Data,.ted Data 800O -14 co Ul C 1 C4 1 -i I c-25 0 0 10 0

Line Stretcher J -m - r- Di -- - I Slotted Line To Appropriate Load Shielded Enclosure 4 i, - Power o o Meter L - - - __ _ I, -., Tunable j Wavemete VSWR Meter Wavemeter m, — 4 co COl I I O") c^3 00 ti3 l 1 Tuning Stub 50 Load id L'_= 50 S 2 Load J / i-700 MHz Low Pass Filter 600 MHz Load 900 MHz Load FIG. 2-4: EXPERIMENTAL EQUIPMENT ARRANGEMENT

THE UNIVERSITY OF MICHIGAN 7956-3-T presented here in Figures 2-2 and 2-3 demonstrate the extremely linear behavior of the second harmonic for a matched termination at the fundamental. Figures 2-5, 2-6, and 2-7 are plots of the power delivered to an antenna by an ARC-27 transmitter as a function of transmission line length at the fundamental, second and third harmonic outputs. Also included are the values (smooth curves) predicted by a linear model assumption. Although the power output at the fundamental frequency agrees well with the predicted values, there is a large discrepancy at the harmonic frequencies that did not exist when the fundamental component was terminated in a matched load. Although it is not apparent from these data, the deviations from the expected values have a period corresponding with the wavelength at the fundamental frequency. What is apparent is that the discrepancy is sufficiently large to require modification of the proposed linear model. 2. 4 Conclusions The data collected from the ARC-27 transmitter correspond well with the data generated from the linear model provided that the fundamental component of the transmitter's output is properly terminated in a matched load. There are wide discrepancies in the data collected at harmonic frequencies when the fundamental is not well matched. Due to the magnitude of the deviations, and the likelihood of a mismatch at the fundamental for most systems of practical significance, the simple linear model is not adequate for accurate power output prediction without some modification. 10

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w'15C 1 1t -- -4 — 4 ---, —r-. Measured Data,fi Predi-cted Data --- -20 __ i _ t r Load: Pickaxe Antenna Load VSWR: 1. 8:1 -25 — 25. —.... _ _ — -.-......... —{ —.........-...-.-..._...._ I. '_ 0~ 105 20~ 30~ 40 50~ 60~ 70~ 80~ 30. - Transmission Line Length - degrees - (Frequency: 600 MHz) FIG. 2-6: POWER TRANSFER VERSUS TRANSMISSION LINE LENGTH ) 9< CD 0 __ _ _ H _re

f ) I 44P i T- - — I!-. — I - i 0 I t - -, -. V- - - I.- I -1 - - 1. I I I 0 1 I 0 i i i i i I - I;. j I AP, I i 0( I -10. -1) )- -, 0 4) A I - -— 4 -- i i t I -- - __ - ____ - - -ti i I I i I I i I I -I.- -. I I - I I i t i i I. — -.,- -. --- I 'iMeasured!Predicted Data.. Data C-) C, I I I T II Load: Pickaxe Antenna Load VSWR:. 5.0:.1 f i i I I I f,,( ) I... - - L — - I --. I -- 4 -- I i i i I I i I I 30 0.I - -L I I i i I. I -+ i i I i I I 40 0 1 1 -1 -.. — - - - —.L )0 7- 0 A I I4-.- - (;(0 70 0 I 0 80 i I I.0 Trcansmission Line Length T- degrees (Frequency: 900 Mfl7) FIG. 2-7: POWER TRANSFER, VERSUS TRANSMISSION LINE LENGTH

THE UNIVERSITY OF MICHIGAN 7956-3-T REFERENCES DeHart, W. Ei., (1966) "Transmitter-To-Antenna Power Transfer Under Unmatched Conditions", IEEE Trans. EMC-8, No. 2, pp. 74-00, (June). Ferris, J. E., et al (1966), "Transmitter Impedance Characteristics for Airborne Spectrum Signature", Interim Technical Report No. 2, October, 1966, University of Michigan Radiation Laboratory Report 7956-2-T, 26 pages. 14