Broadband linearly/circularly polarized microstrip filtering antennas

Two linearly and circularly polarized filtering antennas with low profile, broad bandwidth, and improved gain are proposed. Without increasing the antenna profile, a vertically coupled resonant structure, which can broaden the bandwidth and provide the filtering response, is employed. To prove the concept, a broadband linearly polarized (LP) filtering antenna is designed by integrating the hairpin resonator on the feed, U-slot resonator in the ground and the patch resonator. Then, we design a broadband circularly polarized (CP) filtering antenna, which utilizes a single-feed dual-coupled structure for two coupling routes with 90 degrees phase difference to excite two orthogonal resonant modes without power divider and phase delay line. Both antennas exhibit third-order filtering characteristics with improved bandwidths and flat in-band gain. The equivalent circuits of both antennas are discussed. The LP and CP antennas achieve an impedance band of 28% and 29.6%, respectively. The CP antenna shows a 3-dB axial ratio bandwidth of 17.6%. Both antennas also exhibit the high gain of 9.5 dBic with less than 0.6 dB variation within the band.