>21.365 [f 655 te 10.15 *p.3 ENGINEERING LIBRARY ANTENNA LABORATORY Technical Report No. 15 DISTRIBUTED COUPLING TO SURFACE WAVE ANTENNAS by Ralph Richard Hodges, Jr. 5 January 1957 Contract No. AF33(616)-3220 Project No. 6(7-4600) Task 40572 WRIGHT AIR DEVELOPMENT CENTER ELECTRICAL ENGINEERING RESEARCH LABORATORY ENGINEERING EXPERIMENT STATION UNIVERSITY OF ILLINOIS URBANA, ILLINOIS Antenna Laboratory Technical Report No. 15 DISTRIBUTED COUPLING TO SURFACE WAVE ANTENNAS by Ralph Richard Hodges, Jr. 5 January 1957 Contract AF33(616.) -3220 Project No. 6(7-4600) Task 40572 WRIGHT AIR DEVELOPMENT CENTER Electrical Engineering Research Laboratory Engineering Experiment Station University of Illinois Urbana, Illinois Digitized by the Internet Archive in 2013 http://archive.org/details/distributedcoupl15hodg 3*r 5 1 teste CONTENTS Page Acknowledgement vi Abstract vii 1. Introduction 1 2. Experimental Procedure and Presentation of Results 3 2.1 Outline of the Problem 3 2.2 Propagation Properties of the Waveguides 3 2.3 Input to Output Attenuation of the Exciting Waveguide 5 2.4 Aperture Distributions 8 2.5 Radiation Patterns 18 3. Conclusions 25 4'. Bibliography 25 ILLUSTRATIONS Figure Number p age 1. Axially Excited Traveling Wave Slot Antenna 2 2. c/v versus Frequency for the Radiating Guide with and without Coupling (Experimental} and for the Exciting Waveguide (Calculated) 4' 3. Electric Field Amplitude Distribution in an End Fed Radiating Guide With Z Q Termination for 83 00 to 8700 Megacycles 4' 4'. Cross -Section of Coupled Waveguides 5 5. Cut-Away View of Variable Length of Coupling Apparatus 6. 6.. Equipment for Exciting Guide Attenuation Measurement 7 7. a-e Attenuation in the Exciting Waveguide versus Length of Coupling Region for 1/4', 9/32 and 5/16. Inch Diameter Coupling Holes, from 8800 to 10,200 Megacycles 9-13 8. Attenuation in the Exciting Waveguide versus Length of Coupling Region for 10 , 100 Megacycles 14' 9. Aperture Amplitude Distribution for 40 Inches of Coupling at 10,100 Megacycles 14' 10. Aperture Amplitude Distribution for 28 Inches of Coupling at 10,100 Megacycles 15 11. Aperture Amplitude Distribution for 25 Inches of Coupling at 10,100 Megacycles 15 12. Attenuation in the Exciting Waveguide versus Length of Coupling Region for 9,700 Megacycles 16. 13. Aperture Amplitude Distributions for 4' and 8 Inches of Coupling at 9700 Megacycles 16. 14'= Attenuation in the Exciting Waveguide versus Length of Coupling Region for 9100 Megacycles 17 15. Aperture Amplitude Distributions for 40 and 28 Inches of Coupling at 9100 Megacycles 17 16.. Patterns for Selected Lengths of Coupling for 9l00 Megacycles 19 17. Patterns for Selected Lengths of Coupling for 92 00 Megacycles 20 ILLUSTRATIONS (continued) Figure Number Page 18. Patterns for Selected Lengths of Coupling for 9300 Megacycles 21 19. Patterns for Selected Lengths of Coupling for 9400 Megacycles 22 20. Patterns for Selected Lengths of Coupling for 9500 Megacycles 23 ACKNOWLEDGMENT The author is indebted to VH. Rumsey and W L. Weeks for their assistance and guidance, and to J. Kemp, W Hartley, and J. Stafford who made most of the measurements. ABSTRACT This investigation concerns the excitation of a traveling wave in a dielectric filled rectangular channel in an infinite ground plane, by means of distributed coupling to a closed waveguide. The data show some of the effects of the size of the coupling holes and the length of the coupling region upon 1) the attenuation between the input and output of the exciting waveguide, 2) the aperture distribution, and 3) the radiation pattern, for various frequencies. 1. INTRODUCTION The axially excited traveling wave slot antenna described by Royal^ consists of a radiating waveguide coupled to an exciting waveguide. The radiating waveguide is a rectangular, dielectric filled channel in an infinite ground plane. The exciting waveguide is partially dielectric filled to give a phase velocity equal to that of the radiating waveguide. The coupling mechanism is a series of holes along the axis of the guides, in their common wall. (See Figure 1) This investigation concerns the effects of the phase velocities and of the coupling length on the waves excited in the radiating guide. The principal interest is centered around the fact that the radiating guide can propagate many modes. For the case of closed waveguides, Miller has shown that energy can be coupled from a single mode guide to a selected mode of a multi-mode structure by proper choice of the coupling mechanism and of the phase velocities. The extent of this effect for an open waveguide is one of the main points of this investigation. 2. EXPERIMENTAL PROCEDURE AND PRESENTATION OF RESULTS 2.1 Outline of the Problem A previous investigation of the antenna in Figure 1 had shown that energy was transferred periodically from guide to guide as the length of the coupling region was increased. This investigation was limited to a relatively short antenna and was hampered by the presence of reflected waves in the radiating waveguide due to its abrupt ends These findings suggested that similar measurements be made on an antenna which would have a maximim coupling length sufficient to show several cycles of power transfer. These measurements include the attentuation of the exciting guide, the aperture distribution, and the radiation pattern for various lengths of coupling. 2.2 Propagation Properties of the Waveguides The cross sectional dimensions of waveguides used in this experiment are shown in Figure 4'. They are identical in cross section to their counterparts in Figure 1 . Figure 2 shows the relative phase velocity, c/v, as a function of frequency for the radiating guide in a region of 5/16' inch diameter coupling holes and in a region of no coupling. A calculated curve of the relative phase velocity of the exciting guide is also presented. It can be seen that the rate of change of phase velocity of the exciting guide is considerably different from that of the radiating guide. This allows the effects of the difference in phase velocities to be investigated. The attenuation of the radiating guide is very small until the frequency reaches the point where c/v becomes less than unity. The 3 1.05- - > o -4- 1.0 .9 5 Radiating Guide 1 in Region of Coupling (5/16 inch Holes) Radiating Guide End Fed Exciting Guide 1 (Calculated) 8500 9000 9500 Frequency — Mc. 10,000 10,500 Figure 2 c/v vs Frequency for the Radiating Guide with and without Coupling (Experimental) and for the Exciting Waveguide (Calculated) o- ■ 8700 Mc. 03 Q ^J^8300 Mc. — | -J- —-^^8600 Mc. 1 show this apparatus and the other components used in the measurement. The attenuation of the exciting guide is defined as the ratio of its output power to its input power. The slide screw tuner (See Figure 6) is adjusted to match the line where the 20 db coupler is located. The coupler gives a reference signal which is proportional to the input power. ©; cr"5 <0 I h of < II o< H Vw CD en *G * ®K 6 *^. 5 a to ** © X 90 *""vc MlTy 0f lium9 UBRABY 12 13 o 01 14' 10 15 20 25 30 35 Length of Coupling Region — Inches 40 Figure 8 Attenuation in Exciting Guide versus Length of Coupl in; Region Frequency - 10,100 mc 10 15 20 25 Distance A'cng Aperture -Inches 30 35 40 Figure 9 Aperture Amplitude Distribution (Frequency = 10,100 mc. , Length of Coupling - "10 inches L5 5 10 15 20 25 Distance along Aperture — Inches 30 35 40 Figure 10 Aperture Amplitude Distribution Frequency = 10,100 mc. Length of Coupling = 28 inches 10 15 20 25 30 Distance along Aperture — Inches 35 40 Figure II Aperture Amplitude Distribution (Frequency = 10,100 mc ? Length of Coupling - 25 inches) 16. 10 15 20 25 Length of Coupling Region — Inches Figure 12 Attenuation in Exciting Guide versus Length of Coupling Region fFrequency = 9700) 9700 Mc 9700 Mc Length of Coupling Region - 4 inches 10 15 20 25 JO Distance along Aperture — Inches 40 Figure 13 Aperture Amplitude Distributions (Frequency = 9700 mc) 17 u 9100 Mc 5 m Q \ ^ — -. f "X 1 '° O 3 C g ' 5 ■ \/ \ 20 1 1 1 1 1/ 1 1 1 \ / 35 40 Fi 10 15 20 25 30 Length of Coupling Region-Inches gure 14 Attenuation in Exciting Guide versus Length of Coupling Region (Frequency = 9100 mc] Length of Coupling Region * 2 8 in. I I I I L 5 10 >5 20 25 Distance along Aperture- Inches t jO 35 40 Figure 15 Aperture Amplitude Distributions (Frequency = 9100 mc) 18 attenuation curves. Also note that the standing wave present in the radiating guide when the length of coupling corresponds to a minimum of attenuation is larger than when it corresponds to a maximum. 2.5 Radiation Patterns The study of radiation patterns is made with an antenna similar to the one shown in Figure 1, except that the radiating waveguide is 32 5/8 inches long. The coupling mechanism is a series of 5/16 inch diameter holes spaced 23/64' inch (same as in Sections 2.3 and 2.4). Figures 16. to 20 show the radiation patterns corresponding to various lengths of coupling over the frequency range of 9,100 mc to 9,500 mc. An attenuation curve (from Section 2.3) is also shown for each frequency. An examination of the major lobes of these patterns indicates that selective excitation of approximately a single mode is possible by proper choice of the length of the coupling region. From the patterns for 12 inches of coupling it appears that, as frequency increases, the major lobe begins to split, as if the excitation of a second mode increases as frequency increases, in this range. The presence of this strong second mode was pointed out in Section 2.3 from examination of the attenuation curves in this frequency range. Also, as frequency increases, the patterns show a decrease in the front to back lobe ratio. Another point of interest is that Royal theoretically determined that a cosinusoidal distribution of normal electric dipoles along the axis of the aperture would give a good end-fire pattern. The measurement of electric field in the aperture in Section 2.4' shows that a good approximation to this distribution is obtained when the length 19 10 15 20 \ 1 \. 12 Inches T-,., r \ 22 Inches 9100 Mc. Attenuotion in Exciting Guide Versus Length of Coupling — 5/16" Dio. Holes v / \ V \ 10 20 Length of Coupling- Inches 30 40 Figure 16 Patterns for Selected Lengths of Coupling 20 6 Inches 12 Inches 10 i5 20 21 Inches 9200 Mc. 23 Inches \ Attenuation in Exciting Guide Versus Length of Coupling — 5/16" Dia. Holes \ VjX \ [y \, \/ V !0 20 30 Length ci Ccuphng- inches 40 Figure 17 Patterns for Selected Lengths of Coupling 21 6 Inches 12 Inches 21 Inches 9300 Mc. 23 Inches < 15 20 Attenuation in Exciting Guide Versus Length of Coupling — 5/16" Dia. Holes 10 20 Length of Coupling - Inches 30 Figure 18 Patterns for Selected Lengths of Coupling 22 5 Inches 17 Inches 21 Inches 9400 Mc 23 Inches o 10 20 Attenuation in Exciting Guide Versus Length of Coupling — 5/16" Dia. Holes 10 20 Length of Coupling 30 Figure 19 Patterns for Selected Lengths of Coupling 40 23 5 Inches 12 Inches 21 Inches 23 Inches 9500 Mc. en 5 o I c o 20 Attenuation in Exciting Guide Versus Length of Coupling — 5/16" Dio Holes 10 20 30 Length of Coupling - Inches 40 Figure 20 Patterns for Selected Lengths of Coupling 24 of coupling is the same as for the first minimum of attenuation in the exciting guide. For the frequency range of these patterns, this length is approximately 12 inches. 3. CONCLUSIONS It is shown that the mode purity and amplitude distribution of the fields in the radiating waveguide of the axially excited traveling wave slot antenna are dependent upon the guide phase velocities, the size of the coupling holes, and the length of the coupling region. It is also shown that a good end-fire pattern can be obtained by proper choice of the length of the region of coupling. 4'. BIBLIOGRAPHY 1 . Royal, D.E. , "Axially Excited Surface Wave Antennas, "Technical Report No. 7, Antenna Section, Electrical Engineering Research Laboratory, University of Illinois; prepared under Contract AF33(616)-3lO with Air Materiel Command, Wright-Patterson Air Force Base, Ohio; 10 October 1955. 2. Miller, S.E. , "Coupled Wave Theory and Waveguide Applications," Bell System Technical Journal, May 1954'. 25 DISTRIBUTION LIST FOR REPORTS ISSUED UNDER CONTRACT AF33(6 16 ) •• 3220 One copy each unless otherwise indicated Commander Wright Air Development Center Wright Patterson Air Force Base, Ohio Attn: Mr EM Turner, WCLRS-6 4 copies Commander Wright Air Development Center Wright-Patterson Air Force Base, Ohio Attn: Mr. N. Draganjac, WCLNT-4 Armed Services Technical Information Knott Building Agency 4th and Main Streets 5 copies Dayton 2, Ohio 1 repro Director Ballistics Research Lab, Aberdeen Proving Ground, Maryland Attn: Ballistics Measurement Lab. 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