THE UNIVERSITY OF MICHIGAN 7300-3-T RADANT ANALYSIS STUDIES Interim Report No. 3 1 February through 31 May 1966 C.T. Tai June 1966 Contract No. AF 33(615)-2811 Project 4161, Task 416103 Project Engineer, S. Pitts AVWE Prepared for Air Force Avionics Laboratory, AVWE Research and Technology Division, AFSC Wright-Patterson Air Force Base, Ohio

THE UNIVERSITY OF MICHIGAN 7300-3-T FOREWORD This report was prepared by The University of Michigan under Contract No. AF 33(615)2811, Task 416103, Project 4161. The work was administered under the direction of theAir Force Avionics Laboratory, Research and Technology Division, Air Force Systems Command: E. M. Turner, Technical Monitor; S. Pitts, Project Engineer. This report covers the work conducted from February through May 1966. iii

THE UNIVERSITY OF MICHIGAN — 7300-3-T TABLE OF CONTENTS FOREWORD iii ABSTRACT vi I INTRODUCTION 1 II SCATTERING BY TWO PERPENDICULAR DIPOLES LINKED BY A PAIR OF TRANSMISSION LINES 2 m DESIGN OF THE DIPOLE PANEL 6 IV CONICAL MANIPOLE 16 V FUTURE WORK 19 ACKNOWLEDGEMENTS 20 DD 1473 V

THE UNIVERSITY OF MICHIGAN 7300-3-T ABSTRACT In this third interim report, the theory of the scattering property of two crossed dipoles is briefly outlined. It is shown that the two components of the scattered field should be of the same order of magnitude when the transmission line is matched to the dipoles. The impedance of a dipole placed on a dielectric panel has been measured. A significant shift of the resonant frequency has been observed. The result of a preliminary experimental study of twenty pairs of dipoles mounted on a conical surface is also included in this report. vi

THE UNIVERSITY OF MICHIGAN 7300-3-T I INTRODUCTION During this period (1 February - 31 May) our main activity involves a theoretical study of the scattering property of two perpendicular dipoles linked by a pair of transmission lines. This investigation is considered to be a necessary step prior to the study of a panel made of such units. An outline of the treatment for a single unit is included in this report. Experimental work on the dipole panel is on the way and the design is described here. In order to choose the proper frequency band for the experiment, an investigation is made to determine the effect of a dielectric panel on the impedance of a dipole. The data is presented here. Previous work on broadband antennas' indicated that a combination of dipoles with various lengths exhibits certain broadband characteristics. A preliminary investigation was conducted to apply the technique to a conical dielectric surface. Although the problem is not the main topic of this project, it is felt that information concerning this structure would be of general interest to radant study. The result, which is decribed below, indicates that twenty pairs are not sufficient. Like the manipole case, the minimum number of pairs to cover a 6:1 band presumably should be doubled. Broadband Antenna Techniques Study, Technical Report ECOM-01263-3, Radiation Laboratory, University of Michigan, February 1966. I 1

THE UNIVERSITY OF MICHIGAN 7300-3-T II SCATTERING BY TWO PERPENDICULAR DIPOLES LINKED BY A PAIR OF TRANSMISSION LINES The problem under consideration is illustrated in Fig. 1. Following the theory of receiving antennas, it can be shown that the shortcircuit current induced at the terminals of antenna No. 1 is given by (S) I1 Y11 h1 E 1) where Y = self-admittance of antenna No. 1, defined in the presence of antenna No. 2 h = effective height of antenna No. 1 E = incident E-field evaluated at the terminals of antenna No. 1. We assume here that the incident E-field is parallel to the axis of antenna No. 1. Since the mutual impedance for two perpendicular dipoles is zero, hI is the same as the effective height of an isolated dipole. For a half-wave dipole, the value of h1 measured in the broadside direction is given by 1X z (2) 1 r (S) Since the incidence wave is perpendicular to antenna No. 2, I = 0. 2 Denoting the terminal currents and voltages by I, I2 and V, V2, one finds, by the application of the superposition theorem, that I1 Y1 h E1 + Y1 (3) I2 = Y22 V2 (4) L I 2

THE UNIVERSITY OF MICHIGAN 7300-3-T 7 y No. 2 X Ei tv No. 1 FIG. 1: SCATTERING OF A PLANE WAVE BY TWO PERPENDICULAR DIPOLES LINKED BY A PAIR OF TRANSMISSION LINES I, 3

THE UNIVERSITY OF MICHIGAN 7300-3-T Because of the constraint due to the transmission line, the terminal currents and voltages must satisfy the following equations: za zb I VH~~~ ~ Z b~~~~~~~~a)Q2~ ) ~(5) 2 b Za 2 where Z = j cot a ZCOtk Zb -j Z0 csc k1 Z = characteristic impedance of the line = length of the line k = 27r/X = wave number By eliminating V1 and V2 in (3) and (5) we can determine I and I2 The results are I = 11 Y 1 h E1 1- Yla - 1 YZ1 Z' ( = -( Z.) ~I (7) 2 1-Y22Z,/ Once the terminal currents are known the scattered field can be determined. Assuming again a sinnsoidal current distribution for half-wave dipoles, one obtains /7r n \ -jkR (s) j 7r I cos (cos 0 ) n A (s) 0 n 2 n e A R 0 (8) n 2r7 sin 0 R n n n 4 - -- I

THE UNIVERSITY OF MICHIGAN 7300-3-T where n = 1, or 2: and 0 and 0 denote, respectively, the angle between the direc1 2 tion of observation and the axis of the dipole. It is interesting to observe that if one considers the ideal case such that Yl1 Z- Y22 Zo- 1 (9) then 1 I= 2 h E (10) 1 2Z 1 1 -jkI I =-E I (11) 2 1 This is an expected result for a matched system, when there is no exterior coupling between the antennas, as for the case of two perpendicular dipoles. It is seen from the above simple analysis that the scattering field due to the two dipoles would be of comparable strength. The unit, therefore, may be considered as a polarization transformer as far as the scattered field is concerned. One expects that a similar property will prevail when a number of these units is arranged in the form of an array. An experiment involving two dipole arrays, one perpendicular to the other has, therefore, been designed to determine the effectiveness of the polarization transform as a function of the line separation under the actual condition that the line is not matched to the antennas. 5

THE UNIVERSITY OF MICHIGAN 7300-3-T m DESIGN OF THE DIPOLE PANEL The proposed structure will consist of two half-wave dipole arrays. The two arrays will be cross-polarized and separated by a variable electrical length. Initial trial separations will be X/4, and X/2 and 3X/4. A two-wire transmission line will connect each pair of cross-polarized dipoles. Each array will be fabricated from a fiberglass panel copper-clad on one side. The fiberglass panel will be approximately 36" square and 1/16" thick. Excess copper will be chemically removed from the panel leaving approximately 400 small copper strips imprinted on the fiberglass. These strips will then form 200 half wave dipoles. The approximate size and spacing of the dipole elements is shown in Figure 2. A preliminary experimental investigation has indicated that this dipole configuration is resonant at approximately 3 GHz. Two panels will comprise the structure. The fiberglass panels are connected physically and the dipoles electrically by the 200 transmission lines. The orientation of the panels is shown in Figure 3. As noted above, an experimental investigation was conducted to determine the impedance of a single dipole imprinted on the fiberglass board. This study was needed to obtain a better feeling for the characteristic impedance of the transmission line that is to inter-connect the two panels of the radant structure. Initially, a thin cylindrical half-wave dipole was constructed and attached to a balun as shown in Figure 4. Impedance data was collected for this configuration to confirm correct adjustment of the balun for approximately 3 GHz. The final impedance plot for this configuration is shown in Figure 5. The balun was then connected to a single copper-strip dipole imprinted on a 4 inch square fiberglass panel as shown in Figure 6. The length of the dipole was adjusted until the configuration was resonant at approximately 3 GHz. The resulting dipole length I I I, 6

tHE UNIVERSITY OF 7300-3-T MICHIGAN 36"t 36" FIG. 2: ARRANGEMENT OF DIPOLE ELEMENTS OF FIBERGLASS PANEL. 7

THE UNIVERSITY OF MICHIGAN 7300-3-T Dipoles located on outside surfaces of panels FIG. 3: ORIENTATION OF DIPOLE ARRAYS (only 9 dipole pairs shown for clarity) 8

THE UNIVERSITY OF MICHIGAN 7300-3-T ).032" Dia. FIG. 4: CYLINDRICAL X /2 DIPOLE AND BALUN 9

THE UNIVERSITY OF MICHIGAN 7300-3-T NO. GC 19509 NAME TITLE 7300-3-T DWG. NO. 60-N GENERAL RADIO COMPANYATE SMITH CHART FORM 5301-7560-N GENERAL RADIO COMPANY, WEST CONCORD, MASSACHUSETTS IMPEDANCE OR ADMITTANCE COORDINATES /.0 tt.# 0of gw RADIALLY SCAL' gRAMETERS o P < 38 8 t. S ^ d ~- ^?.?.? -. ~...~....8..8...x.....o.. I CEINTER 10 o 0,sv9 1,.o CENTER~~~~~~~~~~~~ O CS~~~~~~~~~~~~~~~~~~~~~ 10 ~~ \P

THE UNIVERSITY OF 7300-3-T J MICHIGAN 1A/6" I ""I Fiberglass Panel with dipole and balun on opposite sides, FIG. 6: COPPER STRIP X/2 DIPOLE AND BALUN 11

THE UNIVERSITY OF MICHIGAN 7300-3-T was approximately 1.3". Impedance measurements of this configuration indicate the impedance to be approximately 50 ohms at resonance as can be seen from Figure 7. To substantiate the above result, a second copper-strip dipole Was constructed on a second piece of 4" square fiberglass. This half-wave dipole had a length of 1. 951 and was connected to a length of coax cable as shown in Figure 8. Data collected for this configuration, Figure 9, indicated the impedance at resonance to be approximately 50 ohms and it shared also a reduction in resonant frequency from that of an equal length free space half-wave dipole. It should be remarked that there is so far no theoretical treatment about the effect of a dielectric panel on the impedance of dipole. The drastic shift of the resonant frequency is, rather unexpected. ~ 12

THE UNIVERSITY OF MICHIGAN 7300-3-T NO. GC 19509 NAME TITLE 7300-3-T DWG. NO.. DATE SMITH CHART FORM 5301-7560-N GENERAL RADIO COMPANY, WEST CONCORD, MASSACHUSETTS IMPEDANCE OR ADMITTANCE COORDINATES

THE UNIVERSITY OF 7300-3-T MICHIGAN 1/16" panel with coax feed on _ 1.95" 4" FIG. 8: COPPER STRIP X/2 DIPOLE AND UNBALANCED FEED 14

THE UNIVERSITY OF MICHIGAN 7300-3-T No. GC 19509 NAME TITLE 73003-TDWG. NO. SMITH CHART FORM 530147560-N GENERAL RADIO COMPANY WEST CONCORD, MASSACHUSETTS DATE IMPEDANCEOR ADMITTANCE COORDINATES 15

THE UNIVERSITY OF MICHIGAN 7300-3-T IV CONICAL MANIPOLE As mentioned in the Introduction, a preliminary study has been conducted to investigate the impedance characteristics of a conical manipole. The manipole previously investigated under another contract was designed to give an omnidirectional azimuth pattern over a very broad frequency range. The manipole antenna may be compared to a monopole over a ground plane where the single element is replaced by many elements. The elements may vary in length by a factor of 10 to 1 with the longest being X/4 for the lowest frequency and others being X/4 long for selected frequencies up to the highest frequency of interest. The present antenna consists of 20 pairs of elements supported by a fiberglass cone. There is, of course no ground plane present and the cone is chosen as representative of the tip of a radome. The elements were trimmed such that they differ in length by 10 percent, i. e., the longest element is resonant at 300 MHz, the second longest at 330 MHz, the third longest at 363 MHz, etc. Each pair of equal length elements was oriented in a dipole configuration as shown in Figure 10. The distribution of pairs is random with an approximate spacing of 9~. VSWR data has been collected for the frequency range of 300 to 2100 MHz. This data is shown in Table I. This is obvious from this data that in order to increase the band width the number of pairs must be increased considerably. Since a 6:1 band antenna has successfully been designed using 43 monopoles' it is very likely that at least 40 pairs are needed for the conical structure too. See reference quoted in Introduction. 16

THE UNIVERSITY OF MICHIGAN 7300-3-T 30~ (Approx.) 20 Pairs of Elements'~~ \^ ~/ (Random Distribution) Balun FIG. 10: CONICAL MANIPOLE CONFIGURATION 17

I THE UNIVERSITY OF MICHIGAN 7300-3-T TABLE I Frequency (MHz) 300 350 400 450 500 550 600 650 700 750 VSWR 3.9 7.8 2.5 2.3 1.6 2.3 3.4 9.3 5. 1 4.8 Frequency (MHz) 800 850 900 950 1000 1050 1100 1150 1200 VSWR 3.8 2.3 1.4 2.7 4.9 2.8 2.8 2.9 2.7 Frequency (MHz) 1250 1300 1350 1400 1450 1500 1550 1600 1650 VSWR 1.9 1.2 1.2 1.5 1.3 2.2 3.9 2.5 3.3 Frequency (MHz) 1700 1750 1800 1850 1900 1950 2000 2050 2100 VSWR 3.0 3.6 5.1 3.7 1.8 2.9 3.4 4.9 3.3 18

THE UNIVERSITY OF MICHIGAN 7300-3-T V FUTURE WORK During the next period, we shall concentrate on the experimental part of the dipole panel. In particular, the polarization transform characteristics of the panel will be investigated. The effect of the length and the characteristic impedance of the transmission line connecting the dipoles will be studied. For the conical manipoles, more pairs will be inserted into the structure. It is hoped that the result, both on impedance and pattern, will be comparable to what we have found for the manipoles. If desirable, the effect of a dielectric coating on the conical antennas will also be investigated experimentally. Finally, the apex angle to yield the best omnidirectional pattern will be determined. I, -,, I 19

THE UNIVERSITY OF MICHIGAN 7300-3-T ACKNOWLEDGEMENTS The author is pleased to acknowledge the assistance of J. E. Ferris, R. L. Wolford, J. P. Bosel and K. M. Jagdmann for their work on the experimental phases of this study. 20

UNCLASSIFIED I Security Classification DOCUMENT CONTROL DATA- R&D (Security cleasification of title, body of abstract and indexing annotation must be entered when the overall report ie claaassified) 1. ORIGINATIN G ACTIVITY (Corporate author) 3a. REPORT SECURITY C LASSIFICATION The University of Michigan Radiation Laboratory UNCLASSIFIED Department of Electrical Engineering 2b. GROUP Ann Arbor, Michigan 3. REPORT TITLE Radant Analysis Studies - Interim Report No. 3 4. DESCRIPTIVE NOTES (Type of report and inclusive date ) Interi m Technical Report 1 February through 31 May 1966 5. AUTHOR(S) (Lset name, first name, inttfl)) Tai, Chen To 6. REPORT DAT.E a"'. TOTAL NO. OF PAGES 7b. NO. OF REFS June 1966 20 -08e. CONTRACT OR GRANT NO. *4. ORIGINATOR'S REPORT NUMBER(S) AF 33(615)-2811 7300-3-T b. PROJECT NO. 4161 C. 9b. OTHER RtPORT NQ(9) (Ay other numbere that may be assigned thie reportj) d. Task 03_ 10. AV IL A BIALITY/I.MITA!ION NOTICES. Agencies of the Dept. of Defense, their contractors, or government agencies may obtain copies from DDC, Cameron Station, Alexandria, Va 22314 11. SUPPL.EMENTARY NOTES Ig. IPONSORINO MILITARY ACTIVITY Air Force Avionics Laboratory, AVWE Research and Technology Div AFSC Wright-Patterson AFB, Ohio 45433 13. ABSTRACT The theory of scattering properties of two crossed dipoles is briefly described in this third interim report. It is shown that the two components of the scattered field should be of the same order of magnitude when the transmission line is matched to the dipoles. The impedance of a dipole placed on a dielectric panel has been measured. A significant shift of the resonant frequency has been observed. The result of a preliminary experimental study of twenty pairs of dipoles mounted on a conical surface is also included in this report. DD I"JAN6 1473 UNCLASSIFIED Security Classification

UNCLASSIFIED I Security Classification 14. LINK A LINK B LINK C KEY WORDS ROLE WT ROLE WT ROLE T WT ANISOTROPIC PANEL RADANT STRUCTURE EXPERIMENTAL STUDY ANTENNAS SCATTERING i i. ii.... i i i~~~~~~~1 INSTRUCTIONS 1. ORIGINATING ACTIVITY: Enter the name and address of the contractor, subcontractor, grantee, Department of Defense activity or other organization (corporate author) issuing the report. 2a. REPORT SECUI;TY CLASSIFICATION: Enter the overall security classification of the report. Indicate whether "Restricted Data" is included. Marking is to be in accordance with appropriate security regulations. 2b. GROUP: Automatic downgrading is specified in DoD Directive 5200.10 and Armed Forces Industrial Manual. Enter the group number. Also, when applicable, show that optional markings have been used for Group 3 and Group 4 as authorized. 3. REPORT TITLE: Enter the complete report title in all capital letters. Titles in all cases should be unclassified. If a meaningful title cannot be selected without classific.,tion, show title classification in all capitals in parenthesis immediately following the title. 4. DESCRIPTIVE NOTES: If appropriate, enter the type of report, e.g., interim, progress, summary, annual, or final. Give the inclusive dates when a specific reporting period is covered. 5. AUTHOR(S): Enter the name(s) of author(s) as shown on or in the report. Enter last name, first name, middle initial. If military, show rank and branch of service. The name of the principal author is an absolute minimum requirement. 6. REPORT DATE: Enter the date of the report as day, month, year; or month, year. If more than one date appears on the report, use date of publication. 7a. TOTAL NUMBER OF PAGES: The total page count should follow normal pagination procedures, ie., enter the number of pages containing information. 7b. NUMBER OF REFERENCES: Enter the total number of references cited in the report. 8a. CONTRACT OR GRANT NUMBER: If appropriate, enter the applicable number of the contract or grant under which the report was written. 8b, 8c, 8& 8d. PROJECT NUMBER: Enter the appropriate military department identification, such as project number, subproject number, system numbers, task number, etc. 9a. ORIGINATOR'S REPORT NUMBER(S): Enter the official report number by which the document will be identified and controlled by the originating activity. This number must be unique to this report. 9b. OTHER REPORT NUMBER(S): If the report has been assigned any other repcrt Cnmbers (either by the originator or by the sponsor), also enter this number(s). 10. AVAILABILITY/LIMITATION NOTICES: Enter any limitations on further dissemination of the report, other than those imposed by security classification, using standard statements such as: (1) "Qualified requesters may obtain copies of this report from DDC." (2) "Foreign announcement and dissemination of this report by DDC is not authorized.' (3) "U. S. Government agencies may obtain copies of this report directly from DDC. Other qualified DDC users shall request through (4) "U. S. military agencies may obtain copies of this report directly from QDC. Other qualified users shall request through t, (5) "All distribution of this report is controlled. Qualified DDC users shall request through it If the report has been furnished to the Office of Technical Services, Department of Commerce, for sale to the public, indicate this fact and enter the price, if known. 1L SUPPLEMENTARY NOTES: Use for additional explanatory notes. 12. SPONSORING MILITARY ACTIVITY: Enter the name of the departmental project office or laboratory sponsoring (paying for) the research and development. Include address. 13. ABSTRACT: Enter an abstract giving a brief and factual summary of the document indicative of the report, even though it may also appear elsewhere in the body of the technical report. If additional space is required, a continuation sheet shall be attached. It is highly desirable that the abstract of classified reports be unclassified. Each paragraph of the abstract shall end with an indication of the military security classification of the information in the paragraph, represented as (TS), (S), (C). or (U). There is no limitation on the length of the abstract. However, the suggested length is from 150 to 225 words. 14. KEY WORDS: Key words are technically meaningful terms or short phrases that characterize a report and may be used as index entries for cataloging the report. Key words must be selected so that no security classification is required. Identifiers, such as equipment model designation, trade name, military project code name, geographic location, may be used as key words but will be followed by an indication of technical context. The assignment of links, rules, and weights is optional. UNCLASSIFIED Security Classification

UNIVERSIIt OF MICHIGA 3 9015 03527 1942