| Summary: | A six-port 3×3 series-fed planar antenna array design for dual polarized X-band airborne synthetic aperture radar (SAR) applications is presented in this paper. The antenna array patches interconnected with series feeding one to another as the structure to realize directional radiation and vertical/horizontal polarization. The patch elements are excited through 50 Ω microstrip line feeding in series with quarter wave transformer for good impedance characteristics. A six-port feeding allows selecting the direction of the traveling waves, and consequently the sense of linear (vertical and horizontal) polarization. A prototype of the antenna is fabricated and validated the proposed method. The dimensions of the fabricated prototype 3×3 array antenna are 3.256λg×3.256λg×0.0645λg (λg is guided wavelength at center frequency 9.65 GHz). The measured S<sub>11</sub> <; -10 dB reflection bandwidths are 1.4% at each port. Furthermore, an interport and intraport isolation S21<;-25 dB was also achieved across the operation band. The measured peak gains are higher than 11.5 dBi with 90% efficiency for all individual ports/polarizations V1, V2, V3 H1, H2, and H3). In order to satisfy the different performance requirement of series-fed array antenna, such as realized gain, radiation efficiency, side lobe level, half power beam width and front to back ratio are also examined. Measured results of an antenna prototype successfully validate the concept. The proposed antenna is found suitable for dual polarized airborne SAR applications.
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