IEEE Access (Jan 2023)
A Shared Aperture X/Ku-Band 5-Group- 5-Element Series-Fed Center-Fed Antenna Array With Isolation Improvement for Airborne Synthetic Aperture Radar Applications
Abstract
This research paper introduces a novel dual-band single-polarized (DBSP) high gain shared aperture antenna (SAA) with better isolation is proposed for the use in Airborne Synthetic Aperture Radars (AIR-SARs). The SAA operating in both X-band and Ku-band with center frequencies of 9.3GHz (ideal for soil moisture estimation in agricultural areas) and 13.265GHz (suitable for applications in snow-covered regions, cold areas, and disaster monitoring) with a frequency ratio of 1:1.426. The SAA consists of four groups of 5-element series-fed center-fed planar array square microstrip patches for the X-band and one group of 5-element series-fed center-fed planar array square microstrip patches for the Ku-band. The inter-element spacing between patches is set at $0.7\lambda $ to meet the $\pm 25^{0}$ scan range requirements. To validate the antenna design, a prototype is fabricated and tested for S-parameters, radiation characteristics, and gain measurements. The antenna with return loss of $\text{S}11 < -10$ dB has an impedance bandwidth (BW) (9.1919-9.4674 GHz) (276 MHz with 2.96% BW) in X-band and (13.054-13.513 GHz) (459 MHz with 3.46% BW) in Ku-band. More than 25 dB of isolation has been measured between $\text{S}_{21}=\text{S}_{12}$ for case 3 and are >35dB isolation between in-band ports (P12/P23/P13) for case 4. Additionally, it achieves high gain values of 12.8 dBi for the X-band and 12 dBi for the Ku-band for case 3. The $2\times 2$ planar array (5-elements in each group excited with coaxial probe feed) is connected with 8-way power divider (4-groups are feed with 4 power divider ports and other 4-ports are matched with $50\Omega $ terminations) and achieved a gain of 19 dBi for the X-band and 12 dBi for the Ku-band for case 4. The combined gain is varied between 19dBi to 20dBi with 00 to 250 scan angle. The combined $2\times 2$ planar x-band array Half-Power Beam width (HPBW) values are 16° in the E-plane and 12.6° in the H-plane for the X-band, while for the Ku-band, they are 22.7° in the E-plane and 78.4° in the H-plane. The antenna design shows a side-lobe level (SLL) of −16.2 dB at E-plane/ $\varphi = 0^{0}$ for the X-band and −14.1 dB at E-plane/ $\varphi = 0^{0}$ for the Ku-band. The size of the shared-aperture antenna is 160 mm $\times160$ mm $\times1.6$ mm. This paper presents the first reported SAA X/Ku-DBSP, which holds significant value for AIR-SAR applications. All the measured results were in line with simulated results and matched reasonably well.
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