A Technique for an Array to Generate Circular Polarization with Linearly Polarized Elements

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1 EEE TRANSACTONS ON ANTENNAS AND PROPAGATON, VOL. AP-34, NO. 9, SEPTEMBER A Technique for an Array to Generate Circular Polarization with Linearly Polarized Elements Abstrut-A method is presented for generating circularly polarized radiation from an array composed of linearly polarized elements having unique angular and phase arrangements. With this technique the complexity, weight, and RF loss of the array feed can be significantly reduced, especially in a large array. This uniquely formed array has the capability of generating excellent circular polarization (CP) over a relatively wide frequency bandwidth. n addition, the array is capable of scanning its main beam in the principal planes to relatively wide angles from its broadside direction without serious degradation to its CP quality. Another feature of this uniquely arranged array is a reduction in the occurrence of mutual coupling. The discussion in this paper places special emphasis on the microstrip type of radiator, although the technique presented can be adapted to most types of linearly polarized antenna elements. A. NTRODUCTON N ARRAY THAT GENERATES circularly polarized radiation is conventionally constructed by using circularly polarized elements. Each element generally requires two feed ports with a hybrid to obtain the required two orthogonal polarizations and a 9" phase differential. For wider bandwidth applications, often four feed ports with ", go", 18" and 27" phase differentials are necessary. For example, in the case of a microstrip radiator, relatively wide impedance and axial ratio bandwidths (21 percent) can be achieved by using a thicker substrate with four-probe feeds [l]. These four feeds are needed to suppress the undesired modes formed in the thick substrate. For a large array, such a multiple-probe element feed system would require many cables, hybrids, power dividers and, consequently, would be more expensive, heavier, and more prone to RF loss. n the case of the microstrip radiator, although a printed transmission line feed system (instead of cables) can be used, a significant amount of radiation leakage is anticipated from the multiplefeed printed lines. A circularly polarized element can also be obtained by using a single feed with some perturbation introduced into the radiator. One example is a single-probe feed conical horn with passive perturbation (tuning) probes orthogonally located in the circular feed waveguide. Another example is a single feed square microstrip patch antenna with a tilted slot [2] introduced at the center of the patch as a perturbation. These Manuscript received October 7, 1985; revised February 12, This work was carried out by the Jet Propulsion Laboratory, California nstitute of Technology, under a contract with the National Aeronautics and Space Administration. The author is with the Jet Propulsion Laboratory, California nstitute of Technology, Pasadena, CA EEE Log Number perturbation techniques for generating circular polarization (CP), however, have very limited axial ratio bandwidth, generally in the order of one-half percent. This paper presents the theoretical and experimental results to demonstrate that an array, which generates circular polarization with wide axial ratio bandwidth (> 1 percent), can be constructed by single feed linearly polarized elements [3]. The circular.polarization is achieved by having a basic 2 x 2 subarray with unique element angular and phase arrangements. As shown in Fig. 1, both the element angular orientation and feed phase are arranged in the ", 9", O", 9" or OD, 9", 18", 27" fashion. With such a system, not only is the feed complexity reduced, but also the bandwidth performance is improved. The reason for reduced feed complexity is because this technique only requires a single feed for each element while four feeds might be needed for each element in a conventional array withwideaxial ratio bandwidth requirement. Due to the orthogonal orientation of neighboring elements, the mutual coupling effect is found to be significantly reduced from that of a conventional array. Finally, this uniquely arranged array can scan its main beam in the principle planes from its broadside direction to relatively wide angles without serious degradation of its CP quality. The concept presented here is good for many different types of antenna elements, such as microstrip patches, dipoles, openended waveguides, horns, etc. The discussion in this paper, however, places special emphasis on the microstrip radiator as a result of a specific project requirement. A brief discussion on the dipole radiator is also presented. n. TECHNCAL BACKGROUND The reasonthat a circularly polarized array canbe constructed by linearly polarized elements is primarily attributed to a four-element subarray with unique angular and phase arrangements. This basic subarray has its elements arranged in a 2 X 2 square or rectangular grid configuration with element angular orientation and feed phase arranged in either a ", 9", O", 9" or a ", 9", 18", 27" fashion. An example that uses microstrip patches is illustrated in Fig. 1. The purpose of different angular orientations of the patches is to generate two orthogonally polarized fields, while different feed phases are used to provide the required phase delays for CP generation. t is well known that circular polarization can be achieved in the broadside direction of an array composed of two linearly polarized elements with angle and phase arranged in a O", 9" fashion as shown in Fig. 2(a). The CP in this arrangement, however, becomes very poor at angles greater X/86/9-1113$ EEE

2 .... '1 114 JEEE TRANSACTONS ON ANTENNAS AND PROPAGATON, VOL. AP-34, NO. 9, SEPTEMBER 1986 U *= 18 lq/= 1 1 *= 9 9 q/= \c= Fig x 2 microstrip arrays that generate CP with LP elements. (a) O", go", On, 9" arrangement for narrow-band application, (b) O", go", 18", 27" arrangement for wide-band application. (b) Y 4 L Ax Z (a) (b) Fig. 2. (a) Two-element microstrip array with On, 9" arrangement. (b) ts spatial phase delay A$' = k&x sin. than 5 ' off broadside direction in the x - z plane as illustrated in Fig. 3. This is caused by the spatial phase delay (A$' = kdx sin, see Fig. 2(b)) formed betweenthetwo orthogonally polarized elements. This spatial phase delay, which disturbs the required 9" phase differential, contributes to the poor CP quality at angles off broadside. With the 2 x 2 subarray, shown in Fig. 1, this spatial phase delay no longer exists. This is because, within the two principal planes, the spatial phase delay in one row or column is opposite to that of the other row or column and, consequently, they cancel each other. A calculated principal plane pattern of the 2 x 2 subarray is illustrated infig. 4, where drastic polarization improvement over that of Fig. 3 can be observed. Due to symmetry, the calculated two principal plane patterns (4 = " and 9") are identical to each other and, therefore, only one is shown here. The phenomenon of the 2 X 2 subarray can also be explained theoretically. From Fig. 5, the far-field patterns in THETA (8) the two principal planes, the x - z plane or the y - z plane, Fig. 3. Calculated CP patterns of the two-element array shown in Fig. 2. can be obtained by a direct summation of the fields from all Element spacing is.87 wavelength. four patches. Let the total far fieldin the x - z plane be denoted by F,, the horizontal field vector from patch 1 by H,

3 HUANG: CRCULAR POLAREATOW L = 9" r! t -X THETA (8) Fig. 4. Calculated principal plane (4 = " or 9") CP patterns of the 2 X 2 array shown in Fig. 1. Element spacing is.87 wavelength. the vertical field vector from patch 2 by V2, etc. The total far field can then be written as follows: ~~z=he-jkodsinoejoa + V4e -jkod sin eej9o -(H ejoo + F/ -jkod sin le + ( ~, ~ j+ " j9)ejkod sin 2e + v jkodsinbej9oo +H jkodsinljejo 2e 3e Since this is a uniformly excited array, H = H3 and V2 = V4, therefore: F =(HeJO"+ l/e;9)(e-jkodsinb+ejk~dsin xz 1 =(Hejkooo+ VejkO9O")2 cos (kodsin e). (1) The two terms in the first bracket together represent a pure circularly polarized wave, and the cosine term is a twoelement array factor. This total field F, is thus equivalent to that generated from two circularly polarized elements. Within the x - z plane of the array shown in Fig. 5, the radiated far field is essentially equivalent to that generated from two circularly polarized elements formed by collapsing the toprow two elements with the bottom-row two elements. For a microstrip array with a relatively thick substrate, there is a distinct advantage if the array has its 2 X 2 subarrays arranged in the ", 9", 18", 27" fashion for both its element orientations and feed phases as shown in Fig. l(b). n this fashion, the axial ratio bandwidth of the array can be increased substantially. This is because most of the radiation impurity (due to higher order modes of the thick substrate) from the " element cancels that from the 18" element, and likewise for the 9" and 27" elements. This phenomenon is illustrated in Fig. 6. The solid arrows are edge field due to fundamental TMlo mode which generates the co-pol radiation. The dashed arrows are edge field due to TMo2 mode which is shown here only as an example and is one of the many modes that contribute to cross-pol. Fig. 6(a) shows that, in the ", go", ", 9" arrangement, both co-pol and cross-pol fields from the two diagonal elements reinforce each other, while (b) Fig. 5. Geometrical layout of the 2 x 2 array for the demonstration of the derivation of (1). Fig. 6(b) demonstrates that, in the ", 9", 18", 27" arrangement, only the co-pols are reinforced but the cross-pols cancel. t is this cancellation of the cross-pols that permits the wide axial ratio frequency band operation. t is found that the above theory for the principal plane fields of the 2 x 2 subarray does not hold true in the diagonal plane (4 = 45" cut). The calculations have revealed a very high cross polarized field, as shown in Fig. 7, at theta (e) angles not very far from the broadside direction. This phenomenon can be best explained by referring to Fig. 8. n this figure, the diagonal plane far field can be thought of as the field generated from a three-element linear array, the center element being composed of two horizontally polarized subelements with 9" phase delay: and each of the two end elements being composed of a single vertically polarized element with " phase delay. This three-element array has a nonuniform amplitude distribution with the center element (horizontal polarized) amplitude equal to two, and each of the end element (vertically polarized) amplitudes equal to one. This causes polarization and phase imbalance at angles off the broadside direction in the diagonal plane. The phase imbalance is contributed by the spatial phase delay, similar to that described in Fig. 2. This high cross-pol level in the diagonal plane, however, can be suppressed in a larger array due to its narrower beam and due to an averaging effect so that the imbalances are averaged out. n other words, in a large array, as shown in Fig. 9 or 1, the diagonal plane's polarization imbalance is much less than that of Fig. 8. The

4 ~ 1116 EEE TRANSACTONS ON ANTENNAS AND PROPAGATON, VOL. AP-34, NO. 9, SEPTEMBER 1986 ;. --* = f=,l: n = += t (a) (b) =9 Fig. 6. (a) O", go", ", 9" arrangement showing cross-pols reinforce each other. (b) ", go", 18", 27" arrangement showing cross-pols cancel each other. Solid arrows represent co-pols, dashed arrows represent cross-pols. '.. v ; - co-pol t- ---CROSS-POL Fig. 7. Calculated CP patterns in the diagonal plane of the 2 X 2 array shown in Fig. 1. Element spacing is.87 wavelength. reader can see this by performing the same exercise as shown in Fig. 8 for Figs. 9 and 1. To demonstrate this, a 4 X 4 array as shown in Fig. 9, and a 2 X 8 array as shown in Fig. 1, have been constructed and tested. Both arrays are composed of the unique 2 X 2 subarray with linearly polarized microstrip elements. The calculated and measured data of both arrays (to be presented in the next section) have shown very good CP quality in the diagonal plane, as well as in all other planes. t is important to realize that any array with N horizontal and N vertical elements can generate CP at a particular angle by proper adjusting each element's phase. However, without proper element location and orientation as presented in this paper, the calculation has shown that the array will have high \ '\ AMP = 1 PHASE Fig " diagonal plane confguration. cross-pol radiation (such as 3 dl3 below co-pol peak) in the direction away from the co-pol peak as shown in Fig. 3. This high cross-pol is only a result of improper arrangement of elements and has nothing to do with each element's phase. One important advantage found to be associated with the type of array discussed here is that the mutual coupling of the array isignificantly less than that of a conventionally arranged array. This is due to the fact that all the adjacent elements of this uniquely arranged array are orthogonally oriented and hence cause very little coupling between immediate neighboring elements. The monolithic array with microstrip elements is playing an important role in the advance of phased array technology. t is more feasible to build a linearly, rather than circularly, polarized monolithic array with quarter-wave long microstrip elements, as illustrated in Fig. ll(a). This is because, by using

5 HUANG: CRCULAR POLARZATON 1117 r-fbr.lu Fig X 4 microstrip array composed of the basic 2 X 2 subarrays with LP elements. Fig X 8 microstrip array composed of the basic 2 x 2 subarrays with LP elements. r L. P. ELEMENTS 7 Fig. 11. Monolithic array configurations. (a) LP CP application. the quarter-wave elements, more area can then be realized [4] between elements for the allocation of a phase shifter, a feed line circuit, etc. For an array to radiate circularly polarized radiation, this quarter-wave element, with a conventional array arrangement, can no longer be used since this element can only generate linear polarization. One can certainly resort to a very small circularly polarized element loaded with very high dielectric constant material. However, such an element generally has a greater amount of electrical loss and can only be used for narrow-band operation. With the uniquely arranged 2 X 2 subarray discussed in this paper, it is possible, by still employing the quarter-wave elements as shown in Fig. 1 le), to construct a circularly polarized monolithic array with wider bandwidth capability.

6 1118 TRAh'SACTONS EEE ANTENNAS ON AND PROPAGATON, VOL. AP-34, NO. 9, SEF'"BER THEORETCAL AND EXPERMENTAL RE~ULTS To verify the theoretical prediction, a 2 X 2 microstrip array with honeycomb substrate has been constructed for the operating frequencies from 8 to 9 MHz. The array, as pictured in Fig. 12, has an element spacing of.87 wavelength with a substrate thickness of one inch and is fed with the O", 9, 18", 27" arrangement. The relatively wide element spacing isaimed for a particular program requirement and bears no special significance here. Both calculated and measured patterns of this array are illustrated in Figs. 13 and 14 with the calculation based on the multimode cavity theory [5]. t can be seen in Fig. 13 that the array is indeed circularly polarized in thetwoprincipalplanes (Q = " or 9"). and the agreement between calculation and measurement is rather good. Fig. 14 shows that the diagonal cut (4 = 45") pattern has a very high cross-pol level at angles immediately off the broadside direction as discussed in the previous section. The agreement between calculation and measurement for this figure is good within the angular region of - 6" -t- 6". The poor agreement outside of that region is attributed to the fact that the calculation is performed on an infinite ground plane while the measurement is carried out on a 3 in x 3 in finite ground plane. Fig. 15 shows two measured principal plane patterns for the same array by using a spinning dipole at frequencies of 821 MHz and 876 MHz. t demonstrates that the main beam peak axial ratio of less than.2 db is essentially unchanged within a 6.5 percent bandwidth. Within the 12 percent bandwidth from 8 to 9 MHz, the worst axial ratio was measured to be.6 dl3 at the main beam peak. One interesting observation of the measured result is that the array patterns are very symmetrical while the patterns of a single linearly polarized element, measured in the presence of other elements, are rather asymmetrical, as shown in Fig. 16. The asymmetry of the single element is caused by the higher order modes present in the relatively thick substrate with off-center feed and also caused by mutual coupling effect. These higher order modes, as discussed in the previous section, are mostly canceled in the array environment with the ", 9", 18", 27" arrangement. The same 2 x 2 array has also been tested in the ", go", ", 9" arrangement where it shows relatively poor axial ratio bandwidth of about 1 percent. The author would like to point out that the ", 9", ", 9" arrangement should be used only with relatively wide-band elements such as dipoles. t can be used with thin microstrip elements only when a narrow bandwidth is required. To demonstrate that the CP performance in the diagonal plane of a larger array can be substantially improved over that of the 2 x 2 subarray, a 4 x 4 microstrip array composed of four of the 2 X 2 subarrays has been fabricated and tested. The array, as illustrated in Fig. 9 and pictured in Fig. 17, has all linearly polarized elements with element spacing of.55 wavelength. Each of its 2 x 2 subarray has the O", 9, 18", 27" element angular and phase arrangements. This microstrip array, designed for operation at 2.27 GHz, has a substrate thickness of.125 inch and a relative dielectric constant of 2.17 for the substrate material. Both calculated and measured patterns of the array are shown in Figs. 18 and 19. Fig. 18 Fig. 12. UHF 2 X 2 microstrip arraywith -inthickhoneycombsubstrate and.87 wavelength element spacing. Each element is linearly polarized with single probe feed. 1u MEASURED - co-pol " a co-pol --- x-pol xxxx x-pol -9-6" -3" " 3" 6" 9" THETA ( 8 ) Fig. 13. CP patterns in the principal plane of the 2 x 2 array shown in Fig. 1L. shows a principal plane cut, while Fig. 19 shows the diagonal plane cut. Both figures indicate very good agreement between calculated and measured results. The diagonal plane pattern does show substantial improvement of the cross-pol level over that of the 2 x 2 subarray illustrated in Fig. 14. n all the other planes (between the diagonal and the principal planes), both measured and calculated data have shown that the CP quality falls between that of the diagonal and the principal planes. t is important to point out that the comparison between Fig. 14 and Fig. 19 for the diagonal planes may be somewhat misleading because the 2 X 2 array of Fig. 14 has element spacing of.87

7 HUANG: CRCULAR POLARZATON MEASURED - co-pol QQQO CO-POL --- x-pol xxxx x-pot THETA e Fig. 14. CP patterns in the diagonal plane of the 2 X 2 array shown in Fig db ' -72O -36' 72O O wavelength while the 4 x 4 array of Fig. 19 has element spacing of.55 wavelength. However, additional calculation for a 2 X 2 array with.55 wavelength element spacing has also shown a high cross-pol radiation with a similar peak level. The only difference in this case is that the cross-pol peak is moved further away from the broadside direction than the case of.87 wavelength spacing. Therefore, one can conclude that the larger array willindeedimprove cross-pol levelinthe diagonal plane regardless of its element spacing. To demonstrate that the concept presented in this paper will work for a variety of array configurations, in addition to the above 4 X 4 array, a 2 X 8 microstrip array has also been fabricated and tested. This array also allows an easy demonstration of its main beam scan capability. The array, as sketched in Fig. 1 and pictured in Fig. 2, is composed of the exact same element and element spacing as that of the 4 x 4 array. Both calculated and measured patterns of the none-scan case are illustrated in Figs. 21, 22, and 23 for pattern cut in the Q = O", 45' and 9" plane, respectively. All three figures not only have demonstrated good agreement between calculation and measurement, but also show excellent CP quality within the main beam region especially in the broadside direction. Since the 4 x 4 and the 2 x 8 arrays are built on relatively thin substrates, both arrays show relatively narrow impedance bandwidths of approximately 3 percent. However, due to the cancellation of higher order modes, as discussed previously, much wider axial ratio bandwidths have been obtained. The measured 1 db axial ratio bandwidth for either array is

8 112 EEE TRANSACTONS ON ANTENNAS AND PROPAGATON, VOL. AP-M, NO. 9, SEPTEMBER db -2 db THETA (8) THETA ( 8 1 Fig. 16. Measured LP patterns of a single element in the 2 x 2 array shown in Fig MEASURED - co-pol co-pol THETA [ 8 ) Fig x 4 microstrip array with uniformly excited LP elements. Frequency = 2.27 GHz, substrate thickness =.125 in, relative dielectric Fig. 18. Principal Plane cp Patterns of the 4 x 4 array shown in Fig. 17 constant = 2.17, and element spacing =.55 wavelen,&.

9 HUANG: CRCULAR POLARZATON MEAS URED - CO-POL CO-POL - MEAS URED co-pol --- x -POL co-pol xxxx x-pol -1 db db -2-3 THETA e Fig. 19. Diagonal plane CP patterns of the 4 x 4 array shown in Fig $,- 8 L 3 6 ' THETA ( e ) Fig. 21. Principal plane (6 = O", see Fig. 1) CP patterns of the 2 X 8 array shown in'fig MEASURED CO-POL --- x -POL co-pol xxxx x-pol -1 db -2-3 Fig X 8 microstrip array with uniformly excited LP elements. Frequency = 2.27 GHz, substrate thickness =.125 in, relative dielectric constant = 2.17, and element spacing =.55 wavelength. -4c )O THETA ( e ) Fig. 22. Diagonal plane (4 = 45', see Fig. 1) CP patterns of the 2 x 8 array shown in Fig. 2. J 9

10 EEE TRANSACTONS ON ANTENNAS AND PROPAGATON, VOL. AP-34, NO. 9: SEPTEMBER MEASURED - co-pol co-pol --- x -POL xxxx x-pol MEASURED - co-pol co-pol --- x -POL xxxx x-pol -1 db -2-3 t -4 THETA ( e ) Fig. 23. Principal plane (4 = 9", see Fig. 1) CP patterns of the 2 X 8 array shown in Fig. 2. approximately 7 percent. This further substantiates the theory that an array built on the basis of the technique presented here can have a relatively very wide axial ratio bandwidth. The 2 x 8 array also has adjustable phase shifters so that its main beam can be scanned off the broadside direction in the x - z plane (4 = " plane). To avoid the formation of grating lobes, the phase shifting for beam scanning is performed at each element level and not at the subarray level. The scanned patterns are shown in Figs with scan angles 19 equal to 25', 4", and SO", respectively. The reader can refer to Fig. 21 for the " scan case. As can be seen, the CP quality does degrade as scan angle increases. However, the degradation is not serious and is very similar to that of a conventional array with circularly polarized elements. The agreements between calculation and measurement are generally quite good in Figs However, there is certain amount of disagreement in the main beam positions at large scan angles, for example, in Fig. 26. This disagreement is primarily attributed to the fact that the calculation is performed on an infinite ground plane while the measurement is carried out on a finite ground plane [6]. The size of the finite ground plane is 28-in by 22-in with the 28-in length oriented in the scan plane. The measured results have also shown, as expected, that the array main beam peak gain reduces as it scans off the broadside direction. For the 25 ', 4', and 5" beam scans, the peak gain reductions are 1 db, 2.2 db, and 4.4 db, respectively. Since it has been pointed out in the previous sections that the concept presented in this paper should work for different types of antenna radiators in addition to the microstrip patches, a calculation is performed on a 2 X 2 array of half-wave dipoles. This dipole array, as depicted in Fig. 27, is arranged in the ", 9", ", 9" fashion with element spacing of.5 wavelength. The reason that O", 9", ", 9" arrangement is used rather than O", 9, 18", 27" is simply because the dipole THETA e ) Fig. 24. Principal plane (4 = O", see Fig. 1) CP patterns of the 2 x 8 array shown in Fig. 2 with main beam scanned to 25" off broadside direction. 1 m V (L z W a W P 3-2c w (L -3( - MEASURED co-pol --- x -POL co-pol xxxx x-pol -4( THETA ( e Fig. 25. Principal plane (6 = ", see Fig. 1) CP patterns of the 2 x 8 array shown in Fig. 2 with main beam scanned to 4" off broadside direction. is a relatively wider band element and no cross-pol needs to be cancelled by the ", 9", 18", 27" arrangement as required for the thick microstrip element. The patterns of the array are calculated by using the moment method technique [7] with the principal plane pattern shown in Fig. 28(a) and the diagonal plane pattern shown in Fig. 28(b). Both patterns demonstrate similar CP characteristics as that shown in Figs. 13 and 14 for

11 HUANG: CRCULAR POLARZATON 1123 MEA5 URED co-pol --- x -POL co-pol xxxx x-pol THETA 1.81 Fig. 26. Principal plane (4 = ", see Fig. 1) CP patterns of the 2 X 8 array shown ioot in Fig. 2 with main beam scanned to SO" off broadside direction. - O db -1 O 9oo Fig X 2 array of linearly polarized dipole elements with ", 9", O", 9" arrangement. Element spacing =.5 wavelength. the 2 X 2 microstrip array. The sharp difference in the pattern shapes is primarily a result of different element spacings. The similarity in CP characteristics between the microstrip array and the dipole array indicates that linearly polarized dipoles can also be used in a larger array to generate circularly polarized radiation. By utilizing the moment method [8], the mutual coupling between the immediate neighboring elements has been calculated and found to be significantly less than that of a conventional array composed of circularly polarized crosseddipole elements. V. CONCLUSON Both theoretical and experimental results have indicated that an array consisting of linearly polarized elements can indeed -3 THETA 1. e (b) Fig. 28. Calculated CP patterns of the 2 X 2 dipole array. (a) Principal plane. (b) Diagonal plane. generate circularly polarized radiation. The requirement is that the array be composed of the basic 2 X 2 subarray with unique element angular and phase arrangement. Based on this concept, a 2 X 2 UHF array, a 4 X 4 S-band array, and a 2 x 8 S-band array have been constructed with linearly polarized microstrip elements. Their measured results have shown- not only that the array can generate circular polarization with wider axial ratio bandwidth and very symmetrical beam pattern but also that the mutual coupling effect can be significantly reduced. n addition, the 2 X 8 array has demonstrated that its main beam can be scanned in the principal plane to relatively wide angles without serious degradation to its CP quality. Certainly, since linearly polarized elements are being used, the feed complexity, weight, cost, and RF feed circuit loss can all be reduced, especially in a very large array. Although the discussion

12 1124 EEE TRANSACTONS ON ANTENNAS AND PROPAGATON, VOL. AP-34, NO. 9, SEPTEMBER 1986 presented in this paper has concentrated on the microstrip type of radiator, with some discussion of the dipole radiator, it is believed that the concept can be applied to essentially any type of antenna element. ACKNOWLEDGMENT The author wishes to express appreciation to the Jet Propulsion Laboratory Director s Discretionary Fund which made this research effort possible. The author also would like to thank Dr. Ken Woo for his support and to Mr. William Folwell and Mr. Alex Miyadi for their assistance in performing the experiment. microstrip antennas and applications, EEE Tram. Antennas P~OPQgat., vol. AP-29, pp , Jan [6] S. Rengarajan, Finite ground plane effects on planar arrays of microstrip patch elements, in EEE Antennas Propagar. SOC. Symp. Dig., June 1984, pp [7] J. Richmond, Computer analysis of threedimensional wire antennas, ElectroSci. Lab., Ohio State Univ., Rep. 2784, Dec [S --, Mutual impedance between coplanar-skew dipoles, EEE Trans. Antennas Propagat., pp , May 197. REFERENCES [l] T. Chiba, Y. Suzuki, and N. Miyano, Suppression of higher modes and cross polarized component for microstrip antennas, in EEE Antennas Propagat. SOC. nt. Symp. Dig,, May 1982, pp [2] J. Kerr, Microstrip polarization techniques, in Proc. Antenna Appl. Symp., Urbana, L, [3] J. Huang, C.P. microstrip array with wide axial ratio bandwidth and single feed L.P. elements, in EEE Antennas Propagat. Soc. nt. Symp. Dig., June 1985, pp [4] R. Stockton and M. Balint, 15 GHz microstrip array development. Ball Aerospace Syst. Div. Final Rep. RADC-TR-843. Feb [S W. Richards, Y. Lo, and D. Harrison, An improved theory for

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