Circular Polarized 2-D Series-fed Patch Antenna Array with Direct Coupling for X-band Airborne SAR Applications

A dual circular polarised 9-element series-fed planar array for X-band airborne synthetic aperture radar (SAR) applications is presented in this paper. As the structure to realise directional radiation and right hand circular polarisation (RHCP) and left hand circular polarisation (LHCP), the antenna array patches are interconnected with series feeding one to another (LHCP). For good impedance characteristics, the patch elements are excited by 50 microstrip line feeding in series with a quarter wave transformer. To achieve combined gain and circular polarisation, a hybrid coupler is used to connect the antenna via a 3-way power divider. The proposed method is validated by fabricating an antenna prototype. The prototype 9-element array antenna measures $3.256 g \times 3.256 g \times 0.0645 g$ (g is guided wavelength at centre frequency 9.65GHz). At each port, the measured $S_{11}-10$ dB reflection bandwidths are 1.4 percent. Furthermore, an $S_{21}-25$ dB isolation was achieved across the operation band. Peak gains are greater than 19.5dBi with an efficiency of 85 percent for both RHCP and LHCP ports. To meet the various performance requirements of a 9-element series-fed array antenna, factors such as realised gain, radiation efficiency, side lobe level (SLL), half power beam width (HPBW), and front to back ratio (FTBR) are investigated. The concept has been validated by the measured results of an antenna prototype. The proposed antenna has been found to be appropriate for dual polarised airborne SAR applications.