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View of the High Transmission Losses Incurred in R-F Cables at Microwave Frequencies, the Preferred Detector Location Is at the Antenna

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View of the High Transmission Losses Incurred in R-F Cables at Microwave Frequencies, the Preferred Detector Location Is at the Antenna INSTRUMENTATION FOR MICROWAVE FIELD MEASUREMENTS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University by Jack Bacon, B .E .E ., M.Sc. The Ohio State University I960 Approved by d . ___ Adviser Department of Electrical Engineering ACKNOWLEDGMENT Because the research reported here has extended over such a long interval of time, it is difficult to acknowledge all the individuals who have contributed to its success. In particular, the most valu­ able technical suggestions have been given by Professors R. L. Cos griff, T. E. Tice, and F. C. Weimer. Although the advice of R. A. Fouty was of a different kind, it has been no less valuable. In the beginning he pointed out several areas where contributions could be made to advantage. His suggestions, which came when the writer was unfamiliar with the field of endeavor, proved to be in­ valuable. No acknowledgment could be complete without recognizing the contributions made by the supporting staff of the Antenna Laboratory. This includes particularly the drafting and editorial personnel. Specifically, without the willingness of Miss Dorothy McGinty and Mrs. Barbara Kerwood to work long hours of overtime beyond the call of duty the preparation of this manuscript in completed form would have been impossible in the allotted time. The encourage­ ment of my collegues and the forbearance of my wife has been no less a factor in the completion of this dissertation. The research reported herein was sponsored in part by the Air Research and Development Command, Wright-Patterson Air Force Base, Ohio, under contracts with the Ohio State University Research Foundation. TABLE OF CONTENTS Page SECTION I - INTRODUCTION 1 SECTION II - IMPROVEMENT OF EQUIPMENT STABILITY 4 A. SELECTIVE AUDIO AMPLIFIER 4 1» Introduction 2. The Selective Audio Amplifier 6 3. Performance of the Amplifier 9 B. MECHANICAL RECTIFICATION 10 1. Introduction 10 2. Linear Rectifiers 13 3. Phase Discriminator 17 SECTION in - AUTOMATIC RECORDING 19 A. GENERAL CONSIDERATIONS 19 B. SQUARE ROOT RECORDERS 36 C. LOGARITHMIC RECORDERS 41 1. Commentary 41 2. Dual Detection Type 42 3. Audio Type 47 D. PHASE RECORDERS 51 iv Page SECTION IV - ECHO AREA INSTRUMENTATION 63 A. GENERAL CONSIDERATIONS 63 B„ RADAR INSTRUMENTATION 68 1. Commentary 68 2. Innovations 7 0 3. Open-Loop Design 80 SECTION V - HIGH SENSITIVITY SYSTEMS 91 SECTION VI - RATE OF RECORDING 102 A. INTRODUCTION 102 B. CALCULATIONS 103 1. Null Depth 103 2. Beam Shifting 112 SECTION VII - CONCLUSIONS 115 APPENDICES 119 BIBLIOGRAPHY 189 AUTOBIOGRAPHY 199 v SECTION I INSTRUMENTATION FOR MICROWAVE FIELD MEASUREMENTS I. INTRODUCTION The need for precision instrumentation has long been recognized as essential to a well-rounded program of research in antennas, radar echo, radomes, wave propagation, and other related topics which re­ quire microwave field measurements. The instrumentation research described in the following pages has been concerned with either im ­ proving the accuracy and reliability of existing instrumentation or with devising techniques for new types of measurements. Since automatic plotting of data is usually the only practical way to record information of this type, heavy reliance on servomechanisms is inevitable. One illustration of the need for such instrumentation is the measure­ ment of aircraft antenna patterns by the method of electromagnetic 1 2 3 4 5 6 7 modeling. ’ ’ ’ ' * ’ This technique is based on maintaining the same ratio of aircraft size to wavelength in the model as in the full-sized system. Figure 1 shows a simplified block diagram of a typical model range used to measure antenna radiation patterns. In order to improve the stability, accuracy, reliability, and dynamic range of the automatic antenna pattern-measuring systems, the principles described in the following pages may be applied. They 1 Aircraft Antenna Detector Support Tower Turntable Transmitter Re cording Instrumen­ Modulator tation Azimuth Information Fig. 1. Instrumentation for model antenna measurements'. have been tested sufficiently in actual applications to guarantee improved performance. In addition to recorders which plot a parameter proportional to the voltage at the receiving antenna terminals, for certain types of interpretation and analysis it is often desirable to record patterns proportional to the logarithm of the input, i.e. , to record in decibels.8’9 The development of this type of recorder to a highly practical state is described in the following pages. 2 A key unit in both recording systems is a selective amplifier of improved design. Other specialized instruments which will be dis­ cussed include a practical automatic microwave phase plotter, a super sensitive synchronous detection system, and numerous im­ provements in both continuous-wave and pulse radar echo measuring systems. The discussion which follows is not intended to be either a complete or a tutorial presentation of the subject matter. Rather it is intended to present the author* s personal contributions to the advancement of human knowledge of the field. 3 SECTION II IMPROVEMENT OF EQUIPMENT STABILITY A. SELECTIVE AUDIO AMPLIFIER 1. Introduction In a conventional pattern range ( see Fig. 1) the transm itter is amplitude-modulated by an audio-frequency voltage. The received signal is detected to recover an a-c voltage having the same frequency as the modulator and an intensity proportional to the incident power. Receiving equipment used in early measurements of this type is indicated in Fig. 2. It consists of a square-law detector (bolometer Narrow Input Square Root R dss Band ______^ Signal Amplifier Amplifier Polar r Recorder Azimuth Information Fig. 2. Component parts of an early receiving installation. or crystal) , a narrow-band selective amplifier, and a linear recorder. By introducing a square-root amplifier to compensate for the inherent squaring action of the detector, a plot proportional to the voltage at the terminals of the receiving antenna results. The linear or circular 4 motion of the recorder chart is synchronized with the azimuthal position of the turntable by means of a servo link. In order to improve discrimination between the weak received signal and the spurious noise, it is desirable to decrease the band­ width of the selective audio amplifier. One method of doing this is shown in Fig. 3. This employs a General Radio Model 736-A wave Wide Square Wave Band Wave Root ’ Signal Bolometer Analyzer Recorder Amplifier Amplifier Fig. 3. Wave analyzer application in receiving equipment. analyzer as a narrow band filter. The instrument is a superheterodyne receiver with a 4 cps pass-band i. f. amplifier centered at 50 kc«1(* The 4 cps bandwidth is a good compromise between noise suppression on the one hand and-limitations associated with the reduction of the rate of recording antenna patterns on the other. In order to improve the stability, accuracy, and dynamic range of the system, the various techniques described in the following pages may be introduced. First, a narrow band audio amplifier may be used to supercede both the wave analyzer and the input amplifier, while providing a comparable degree of selectivity and gain. For simplicity, the superheterodyne principle is eliminated and selectivity is obtained directly at the audio frequency output of the r-f detector. An addition­ al advantage here is that the filtering begins at a low enough signal level that additional noise components are not generated because of nonlinearities in the preceding amplifier. 2. The Selective Audio Amplifier System analysis and practical measuring experience define a practical standard of performance, e.g. , a gain of 93 db is usually sufficient. In the design now in common use, 0. 5 volts output results for an 11. 5 microvolts input with noise sufficiently low to provide a usable dynamic range below this level. Earlier results of other investigators11 indicated that the desired selectivity could be achieved by using some of the better toroidal inductors in resonant circuits. The tube complement consists of a high gain pentode amplifier followed by four low-gain stages using dual triodes. RCA Red Seal tubes are selected for longevity and low noise. By virtue of small coupling capacitors in the triode stages, gain is suppressed while the driving impedance of the resonant circuits is increased to realize the full Q of the coils. I-Ialf-henry inductors are used throughout. Using these, the amplifier has an output which deviates negligibly from linearity over an 80 db range extending from 6 volts downward. The slight deviation from linearity which is observed with large signal levels is due to a reduction in Q and hence a decrease in the resonant impedance QX. High Q toroidal inductors are characterized by a Q which increases with frequency over the normal range of 400 to 1000 cps. Tests of several selective amplifiers tuned to spot frequencies throughout this band indicate that the bandwidth is uniform at 4 cps. The clear im pli­ cation here is that the Q is proportional to frequency over this range. A sample frequency characteristic curve, centered at 1000 cps, is shown in Fig. 4. Since the gain is nearly proportional to the magnitude of the load impedance in each stage, one would expect the response curve to have the form given by Eq. ( 1) . For a half-power bandwidth of 4 cps, T = 0. 0308, K (1) G =-----------------_ |l+jwT| From the well known relationship-of Eq. ( 2) , the corresponding Q is found to be about 97. Actually this is a bit low in comparison to quoted figures. The decrease is doubtless due to loading in the circuit, Calculation of the response curve, based on Eq. ( 1) , shows good agreement over a wide range of values. 7 rtx mi 0 L - . 00 i ; ' 'j-~~ SELECTIVE AMPLIFIER - ■ FREQUENT RESPONSE CHARACTERISTIC j SOOO 9 to t9 CP* Fig.
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