Dual Polarized Microstrip Antenna Array for X-Band Radar Applications1

Víctor J. Marrero-Fontánez, Rafael A. Rodríguez-Solís* Department of Electrical and Computer Engineering

University of Puerto Rico, Mayagüez, PR, 00681-9042 victor.marrero@ieee.org, rarsolis@ieee.org

I. Introduction In present-day radar systems, the need for lightweight antennas of high efficiency has generated much attention the study of compact microstrip technology. These antennas should exhibit low profile as well as low cross polarization (XP) radiation and low sidelobe levels (SLL) in some designs. Very little is documented for series-fed arrays and aperture-coupling arrays for frequencies above C Band, especially arrays with high polarization purity. There is even less published work for the combination of both. Achieving cross-polarization suppression on the order of 30 dB or more makes the use of microstrip-fed and proximity coupled patches difficult. This suggests resorting to aper-ture-coupled patch technology, which provides better control of the cross-polarization level [1]. The design and development of an aperture-coupled dual-polarized antenna array for the CASA Student Test-Bed [2], operating at a frequency of 9.5 GHz and based on the design approach from [3], is presented in this paper. II. Linear Array A series-fed aperture-coupled antenna array operating at of 9.5 GHz was designed using a substrate with a relative permittivity, εr, of 1.2 and a thickness, h, of 1.75mm for the patch antenna, and a substrate with εr=6.15 and h=0.625mm was used for the feed lines. The array is designed to achieve a sidelobe level lower than 20 dB and a cross-polarization lower than -30 dB. Due to the similarity of the results for both polarizations (port H and port V), the results presented are from port H active only. The linear array consists in connecting in series the 4–port radiating element in series. Two linear arrays of 12 elements composed of two sub-arrays of six elements each were simulated. The first array consists of six 4-port elements, terminated with 50 Ω ports to simulate load resistors. The second one consists of five 4-port elements, loaded with the patch load element, as seen in Figure 1.

1 This work was sponsored by the NSF Engineering Research Center EEC-0313747

21441-4244-0878-4/07/$20.00 ©2007 IEEE

Figure 1. Layout for a sub-array of six elements using the PL.

The phase offset at the end of each sub array ranges from 2 to 4 degrees, depending how far the element is from the feed point. As we move farther away from the input ports, less power is radiated by the elements and a larger phase offset is observed. For an array of 12 elements, 6 elements per sub-array, terminated with 50 Ω ports, the Front-to-Back Ratio (FBR) is -16.76 dB. The SLL and XP are 21.7 dB and -29.6 dB respectively, just .4 dB shy of our 30 dB specification limits for cross-polarization. The input impedance for the ports H, H-180º, V and V-180º is 45.01 + j7 Ω, giving a return loss around -20.9 dB. Figure 2 shows the radiation pattern for this array.

Figure 2. Radiation pattern for linear array of 12 elements loaded with resistive loads of 50Ω.

For the array using the PL for its last radiating element, the input impedance of the array at 9.5 GHz is about 45.96 + j5.29 Ω, giving a return loss of approximately -23.16 dB. Figure 3 shows the radiation pattern for the array. It can be seen that the gain of the array drops to 16.24 dB, and the FBR is about -16.24 dB, as expected. Significant changes are seen in the cross-polarization, SLL and HPBW. XP=-37 dB, which is around 7.5 dB bet-ter than the array with out the PL. The SLL and HPBW change significantly due to the changes in the array current distribution caused by the patch load. The SLL is 5.55 dB higher for the array with the PL, 16.17 dB for the array with PL and 21.72 dB for the ar-ray without it. The HPBW is incremented using the PL from 6º to 9º. A second design used curved feed lines under the patches. Figure 4 presents the differ-ence between the feed lines of such array. The input impedance for the array at 9.5 GHz is 45.28 + j0.56 Ω giving a return loss of -26 dB. Figure 5 shows the radiation pattern the array with its feed lines modified and using loading resistors. The FBR is -16.78 dB, al-most the same as the original array with loading resistors. The XP is -31.56 dB, about 1.5

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dB above specification limits for cross-polarization. On the other hand, SLL is lowered in these arrays significantly with values of 28.3 dB.

Figure 3. Radiation pattern for linear array of 12 elements with PL

Figure 4. Original structure for the radiating elements of the array (left), and radiating element with curved feed lines (right).

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Figure 5. Radiation pattern for linear array of 12 elements with curved feed lines ended with load-ing resistors.

The input impedance of the array at 9.5 GHz is about 45.28 + j0.56 Ω, giving a return loss of approximately -26 dB. Figure 6 shows the radiation pattern for the array. Note that the gain of the array drops to 15.98 dB, and the FBR is about 15.93 dB. Just as with the original array configuration, the addition of the PL makes significant changes in the XP, SLL and HPBW. The XP achieved is -37.63 dB, which is around 6 dB better than the array without the PL. The HPBW is incremented using the PL from 7º to 10º, and the SLL exhibits a radical difference of almost 12 dB lower for the array with the radiating element, 16.53 dB for the array with PL and 28.3 dB for the array without it.

IV. Conclusions The data presented indicates that for better cross-polarization a radiating element must be used, thus sacrificing the sidelobe level of the array. If good SLL is desired, resistive loads instead of the PL must be used. The fact that the addition of the PL changes the current distribution of the array, a compromise must be found in order to achieve desir-able SLL and XP with degrading the input impedance of the array. Changing the last element with the PL not only raise the SLL but also broaden the main beam of the array causing an increment in the HPBW of 3º.

Figure 6. Radiation pattern for linear array of 12 elements with PL and feed lines modified.

References [1] D. M. Pozar, “Microstrip Antennas”, Proceedings of the IEEE, Vol. 80, No. 1, Jan 1992 [2] J. M. Trabal, et. al., “Puerto Rico Student Test Bed Applications and Systems re-quirements Document development”, 9th International Conf. on Engineering Education, July 2006 [3] A.Vallecchi and B. Gentili , “Design of a Dual-Polarized Series-fed Microstrip Arrays with Low Losses and High Polarization Purity”, IEEE Trans. Antennas Propg., Vol. 53, No. 5, May 2005

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