A Planar Differentially Fed Circularly Polarized Filtering Antenna With Enhanced Axial-Ratio Bandwidth

A planar differentially fed circularly polarized filtering antenna with enhanced axial-ratio (AR) bandwidth is proposed. The proposed design consists of a square substrate integrated waveguide cavity (SSIWC), a truncated square patch with an etched U-shaped slot, and four parasitic shorted patches. The SSIWC, serving as a feeding cavity, and two metal posts are employed as perturbation elements to excite the degenerate modes in SSIWC. Initially, the energy couples from the SSIWC to the truncated square patch through a small square slot so as to obtain circular polarization with two AR minima. Then, four shorted patches are arranged around the truncated square patch to concurrently generate a third AR minimum and a radiation null, thus improving the AR bandwidth and the upper passband edge's roll-off rate. Finally, a U-shaped slot is utilized, introducing an extra radiation null to further enhance the upper stopband's suppression level and simultaneously optimizing the impedance and AR matching of the whole antenna. A fabricated prototype was measured, demonstrating an overlapped bandwidth (where |${\it {S}}_\text{dd11}$| ≤ −10 dB and AR ≤ 3 dB) of 4.63 GHz to 5.05 GHz (8.6%), and an average realized gain of 6.6 dBi with good filtering response.