621, ,365 , Ii6b'5te no. 29-30; cop. ,3 G2I. 3£5 no. 3$. Op.? ENGINEERING UBRAIX ANTENNA LABORATORY Technical Report No. 30 C0M1NCE ROOM NON-PLANAR LOGARITHMICALLY PERIODIC ANTENNA STRUCTURES by D. E. Isbell THE! SiTY OF iLlI Contract No. AF33(61 61-32 20 Project No. 6(7-4600) Task 40572 WRIGHT AIR DEVELOPMENT CENTER ELECTRICAL ENGINEERING RESEARCH LABORATORY ENGINEERING EXPERIMENT STATION UNIVERSITY OF ILLINOIS URBANA, ILLINOIS ENGINEERING LIBRARY ANTENNA LABORATORY Technical Report No , 30 NON- PLANAR LOGARITHMICALLY PERIODIC ANTENNA STRUCTURES by D.E Isbell 20 February 1958 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 THE LIBRARY OF MAR A ] 195ft UNIYErtSHYOF lUMUJS Digitized by the Internet Archive in 2013 http://archive.org/details/nonplanarlogarit30isbe ii FOREWORD ^ • 3i The material covered in this report was first presented at the Seventh Annual symposium on the USAF Antenna Research and Development Program, Robert Allerton Park (University of Illinois), Monticello, Illinois, in October 1957. iii CONTENTS Page Foreword ii Abstract v Acknowledgement v 1 . Introduction 1 2. Description of the Antenna 2 3. Experimental Results 5 3ol Radiation Patterns 5 3.2 Input Impedance 16 4. Conclusions 19 Bibliography 20 Distribution List IV ILLUSTRATIONS Figure Number Page I . Planar Antenna 3 2 Experimental Model Configuration 4 3. Radiation Pattern Coordinate System 6 4. Typical = 90 Plane Patterns as a Function of ^ 7 5. E Plane Patterns 8 6. H Plane Patterns 10 7. E Plane Patterns 11 8. E Plane Patterns 12 9. Typical (f) = 90 Plane Patterns as a Function of ^ 13 10. E Plane Patterns 14 II. E Plane Patterns 15 12. Typical Impedance Locus for a Single Period 17 13. Boundary Circles of the Impedance Loci as a Function of 4* 18 ABSTRACT A method of obtaining unidirectional radiation patterns over essentially unlimited bandwidths with a single radiating structure is described. The method employed is a natural extension of a technique which has previously been introduced for providing frequency independent bidirectional radiation patterns. Data showing variation in radiation patterns and impedance with changes in antenna geometry are presented. ACKNOWLEDGEMENT The author gratefully acknowledges the assistance of Mr. J. Diefenthaler in obtaining the experimental data necessary to carry out this investigation. This work was performed under the sponsorship of the United States Air Force, Wright Air Development Center, Wright-Patterson Air Force Base, Ohio. NON- PLANAR LOGARITHMICALLY PERIODIC ANTENNA STRUCTURES by D.E. Isbell 1. INTRODUCTION Considerable success has been obtained in the developing of antennas which are based on the proposition, advanced by Professor V.H. Rumsey, formerly of the University of Illinois, that if the shape of an antenna were such that it could be specified entirely by angles its performance would be independent of frequency. This has become known as the "angle 2 3 4 concept," and led to the* conception of the equiangular spiral, ' ' which has been the most successful of the angular antennas to date. Bandwidths in excess of twenty to one have been measured for thet equiangular spiral , and its characteristics were found to remain essentially constant. At the University of Illinois, during the fall of 1955, Professor R.H. DuHamel began the investigation of a new concept of wideband antenna 5 design. It was his plan to design an antenna structure such that its electrical properties would repeat periodically as the frequency was varied and for which the" variation of performance over a period was small or negligible. The result would be a pseu do -frequency- independent antenna for, although the performance would vary with frequency within each period, the variations would be the same over all periods, irrespective of frequency. By incorporating this concept with the angle concept, a slot antenna was devised which proved to have a bandwidth limited by mechanical considerations only. This antenna was introduced in a paper 6 presented by Dr. DuHamel at the 1957 National I.R.E. Convention and shortly 7 thereafter in a technical report to the Wright Air Development Center. The earlier work was, for the most part, restricted to planar config- urations only, and bi-directional patterns were characteristic of these structures. This paper describes the results of an investigation of non- planar models of the periodic antennas demonstrating that unidirectional radiation patterns can also be obtained over a wide bandwidth with these structures. 2. DESCRIPTION OF THE ANTENNA Complete design principles for the class of logarithmically periodic antenna structures are given in the technical report to W.A.D.C, covering 7 the planar antenna models. It will be sufficient here to describe the geometry of the particular structure which has been investigated. Figure 1 shows the antenna constructed as a slot in a ground plane. The general configuration is that of the complement of a serrated "bow tie" antenna, that is, the configuration which results when the metal and non-metal portions of the plane are interchanged. The serrations are in the form of rectangular teeth connected to triangular fins, and are so designed that all dimensions are proportional to their distance from the origin or feed point. The structure is seen to be defined in terms of angles and radii. These radii are chosen such that the ratio of the radius from any point on one tooth to the radius of the corresponding point on an adjacent tooth is a constant. This constant is denoted in the figure as t and determines the period at which the structure repeats. The ratio O determines the slot width and, when taken equal to the square root of T, provides a ratio of tooth to slot width which is the same for all rows of teeth. It can be seen that infinite structures of this type have the property that, when energized at the vertex, the fields at a frequency f will be repeated at all other frequencies given by t f (apart from a change of scale) ; where n may assume any integral value. When plotted on a logarithmic scale, these frequencies are equally spaced, with separation or period of In t, hence the name logarithmically periodic antenna structures. The angles a and (3 are those subtended by the teeth and solid metal portion of the element, respectively. For the purpose of this investigation the structure complementary to that shown in Figure 1 was used. The properties of the antenna were investigated as a function of the angle between the elements as they are rotated about the vertex out of the plane. Figure 2 shows the manner in which the two elements are inclined, and the angle of inclination 4 1 is defined. FIGURE 1 PLANAR ANTENNA Front Side FIGURE 2 EXPERIMENTAL MODEL CONFIGURATION 3. EXPERIMENTAL RESULTS 3.1 Radiation Patterns In the earlier work, radiation patterns and impedances of the planar antennas were investigated as functions of the tooth angle a and the geometric ratio T, The general pattern shape for all models was found to be bi-directional , having approximately equal principal plane beamwidths . The most significant effect of change in parameters investi- gated was the change in pattern beamwidth with change in T. Half power beamwidths were obtained, ranging from eighty to forty degrees, with values of t between .81 and .25. The general effect of increasing the tooth angle was to reduce the amplitude of the cross polarization. In the present investigation, a few of these same antennas have been tested with the elements inclined and it has been found that excellent unidirec- tional broadband performance can be obtained. The effect of rotating the elements of the planar antenna about the vertex out of the plane was to cause it to radiate more in one direction than in the other. The front to back ratio was found to increase as the angle 4* was reduced. The change in the patterns with ^ was found to be gradual down to approximately 100 , with rapid increase in the front to back ratio as the angle was further reduced. The radiation pattern coordinate system relative to the antenna is shown in Figure 3, and Figure 4 shows typical 0=0 plane patterns for the model having the following values affixed to its geometrical parameters: R l = 9 inches T = .81 a = y^~ a = 45° (3 = 45°. These patterns were measured at a frequency of 1400 mc , and four values of ^ are shown. Cross polarization, which is typical for these parameter values, ranges from 4 1/2 db below signal maximum at ^ - 100 to 10 db at 4 1 - 30 . Figure 5 shows E plane patterns of this antenna over a six to one bandwidth for angles of ^ between 100 and 30 . Note that the band- width is independent of the angle 4 J . H plane patterns over this same *=0° x L * / 6 Polar Angle - no E*,* =0 E a ,f*90« y *=90° FIGURE 3. RADIATION PATTERN COORDINATE SYSTEM e = o c e = o< •^=100' ^ = 60° e~o< e = o c ^(bO* f = 30* FIGURE 4 TYPICAL 0= 90° PLANE PATTERNS AS A FUNCTION OF ^ FREQ. 1400 Mc T = 0.81 a = p = 45 V IOO IOOO I200 GO ( 5Cf 4.Q 30* c 3 it 1400 1700 2000 3000 6000 FIGURE 5 E PLANE PATTERNS =0.81 , = (3 = 45 C = 90" E Q Pol . band of frequencies are shown in Figure 6 and are typical of the H plane patterns of all models tested during this investigation. Another of the antennas was constructed, holding a and p fixed and using a geometric ratio t of .7. E plane patterns of this antenna are shown in Figure 7 and, as can be seen, little change has resulted in this variation of T. Similarly, a model with t equal to .5, also with a and 6 unchanged, was constructed and tested. In this case, for the smaller values of 4 1 , the beamwidth and pattern shape variation over a period were found to be considerable. These variations are apparent in the patterns of Figure 8. Although the patterns remain essentially intact over the band shown, they are seen to be inferior to those obtained for the higher values of t from the point of view of frequency independence. The best over-all results were obtained when a substantial decrease in the cross polarization resulted from an increase in the tooth angle. A model was constructed having the following parameter values: R l = 9 inc hes T = .7 a = J~T~ a = 60° P : 30°. Typical pattern shape and cross polarization as a function of 4 1 for this antenna are seen in Figure 9. Cross polarization ranges from 11 db below signal maximum at ^ = 100 to 18 db below at 4* = 30 , and these values are typical of all frequencies measured. Complete E plane patterns for normal polarization are seen in Figure 10. The primary radiation pattern investigation was carried out above 1000 mc in order that the results might be typical of the infinite structure, that is, without contributions due to end effect. At a frequency of 1000 mc , with R equal to nine inches, the antennas are approximately one and one half wavelengths in diameter for ^ = 180 . This does not, however, represent the minimum useful size of these antennas. Actually, satisfactory performance is available down to approximately one half wavelength in diameter. This point is demonstrated in the patterns of Figure 11, which show the performance over a 20:1 band for ^ = 40 and a minimum antenna f IOO' IOOO GO' 5>0< 40 c 10 50° I20O \AOO o c Q) I MOO 2OO0 5000 GOOO T = FIGURE 6 H PLANE PATTERNS 0.81 0=0° a = |3 = 45 v E^Pol I00< 80* 60* 50* 40* 1000 1200 1400 o o c CX 1700 2000 3000 6000 n U Figure 7 E Plane Potterns r = 07 +=90° a = £=45° E 9 Pol. 100* 1000 40* 1200 1400 o if) o c 3 1700 2000 3000 6000 Figure 8 E Plane Patterns t=0.5 * = 90° a=£=45° Eg Pol. 13 e=o e e = o° r- 100 e=o c e-o e \jf~Q£f ^=30 e FIGURE 9 TYPICAL u c Q) cr L i4-00 700 2000 5000 (2»OCO T a FIGURE 10 E PLANE PATTERNS 0,7

= 90° a 60° * = 30° E $ Pol. 16 diameter of less than one-half wavelength. It should also be pointed out that 6000 mc is by no means the upper frequency limit. The high frequency performance can be extended upward, limited only by the precision with which the geometry of the structure can be carried out. The models constructed for this investigation were cut out of l/32 inch copper sheet on a jigsaw, and it was found almost impossible to construct the small teeth near the center accurately by this technique. Since the geometry of the structure requires an infinite number of teeth of zero width in the limit as the center is approached, a small area near the center was left without teeth, thus fixing the upper frequency limit of periodic operation. 3.2 Input Impedance All of the antennas measured in this investigation were self -complemen- tary in the plane, that is, for the infinite planar structure, the antenna is identical to its complement. As has been pointed out in the earlier work, this condition ensures a nearly constant input impedance, for the planar antennas, of approximately 180 ohms but gives no assurance that the impedance will remain constant as 4 1 is made less than 180 . The impedance of all models constructed for this investigation was measured as a function of 4 1 . The impedance characteristics were found to be essentially the same for all models, showing little effect resulting from changes in T and the tooth angle a. The impedance of the antennas is periodic, just as are the radiation patterns. A typical impedance locus for a single period of one of the structures at an inclination of 4 1 = 40 is shown in Smith Chart form in Figure 12. The locus is seen to be approximately circular, with the impedance making two revolutions during the period about a mean resistance l^vel of approximately 80A The mean resistance level of the impedance locus i i ° was found to decrease with decreasing y, ranging from 165n. at f = 180 to approximately 70/1 at 4 1 = 30 . Figure 13 shows the general variation of the impedance with ^. The circles shown are the approximate boundaries within which the impedance loci lie for the values of 4 1 indicated. It is apparent that the variation in impedance with frequency increases as 4* is decreased. 17 FIGURE 12 TYPICAL IMPEDANCE LOCUS FOR A SINGLE PERIOD <\> = 40° 18 FIGURE 13 BOUNDARY CIRCLES OF THE IMPEDANCE LOCI AS A FUNCTION OF *\> 19 4. CONCLUSIONS An experimental investigation of non-planar logarithmically periodic antenna structures has shown that unidirectional radiation patterns which are essentially frequency independent are obtainable over wide bandwidths. These antennas are of practical form, being constructed from a plane metal sheet and, although the method has not, as yet, been employed, the structure suggests easy adaptability to printed circuit techniques. 20 BIBLIOGRAPHY 1. V.H. Rumsey, "Frequency Independent Antennas," 1957 IRE National Convention Record, Part 1, page 114, March 1957. 2. J.D. Dyson, "The Equiangular Sprial Antenna," USAF Antenna Symposium, University of Illinois, October 1955. 3. J.D. Dyson and R.L. Carrel, "An Experimental Investigation of the Equiangular Spiral Antenna," USAF Symposium, University of Illinois, October 1956. 4. J.D. Dyson, "The Equiangular Sp al Antenna," University of Illinois, Antenna Laboratory, Technical Report No. 21, Contract 33 (616)-3220, Wright Air Development Center, September 1957. 5. R.H. DuHamel and W.D. James, "Structures Periodic with Frequency," University of Illinois Antenna Laboratory Quarterly Engineering Report, "Research Studies on Problems Related to ECM Antennas," Contract AF33(616)-3220, Wright Air Development Center, October 1955. 6,7. R.H. DuHamel and D.E. Isbell, "Broadband Logarithmically Periodic Antenna Structures," 1957 IRE National Convention Record, Part 1, page 119, March 1957. Also, University of Illinois Antenna Laboratory Technical Report Number 19. Contract AF 33 (616)-3220, Wright Air Development Center, May 1957. DISTRIBUTION LIST FOR REPORTS ISSUED UNDER CONTRACT AF33(616)-3220 One copy each unless otherwise indicated Armed Services Technical Arlington Hall Station Arlington 12, Virginia Information Agency 4 and 1 repro Commander Wright Air Development Center Wright-Patterson Air Force Base, Ohio ATTN: WCLRS-6, Mr. W.J. Portune 3copies Commander Wright Air Development Center Wright-Patterson Air Force Base, Ohio ATTN: WCLNQ-4, Mr, N, Draganjac Commander Wright Air Development Center Wright-Patterson Air Force Base, Ohio ATTN: WCOSI, Library Director Evans Signal Laboratory Belmar , New Jersey ATTN: Technical Document Center Commander U,S Naval Air Test Center ATTN: ET-315 Antenna Section Patuxent River, Maryland Chief Bureau of Aeronautics Department of the Navy ATTN: Aer-EL-931 Chief Bureau of Ordnance Department of the Navy ATTN: Mr, CH. Jackson, Code Re 9a Washington 25, D„C„ Commander Hq. A.Fo Cambridge Research Center Air Research and Development Command Laurence G„ Hanscom Field Bedford, Massachusetts ATTN: CRRD, R.E. Hiatt Commander TVir Force Missile Test Center Patrick Air Force Base, Florida ATTN: Technical Library Director Ballistics Research Lab. Aberdeen Proving Ground, Maryland ATTN: Ballistics Measurement Lab, Office of the Chief Signal Officer ATTN: SIGNET- 5 Eng. & Technical Division Washington 25, D.C. Commander Rome Air Development Center ATTN: RCERA-1 D. Mather Griffiss Air Force Base Rome, N.Y. Airborne Instruments Lab,, Inc. ATTN: Dr. E.G. Fubini Antenna Section 160 Old Country Road Mineola, New York M/F Contract AF33(616)-2143 Andrew Alford Consulting Engrs . ATTN: Dr, A. Alford 299 Atlantic Ave, Boston 10, Massachusetts M/F Contract AF33(038)-23700 Bell Aircraft Corporation ATTN: Mr. J.D. Shantz Buffalo 5, New York M/F Contract W-33(038)-14169 Chief Bureau of Ships Department of the Navy ATTN: Code 838D, L.E. Shoemaker Washington 25, D.C Director Naval Research Laboratory ATTN: Dr. J.I. Bohnert Anacostia Washington 25, D.C. page 2 DISTRIBUTION LIST (CONT, ) AF33(616)-3220 One copy each unless otherwise indicated National Bureau of Standards Department of Commerce ATTN: Dr. A.G. McNish Washington 25, D.C. 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