Low-profile circularly-polarized filtering antenna with improved bandwidth and gain

A novel circularly-polarized (CP) microstrip antenna with a low profile, broad bandwidth, high gain and filtering characteristics is presented. The antenna has a resonator-based vertically coupled structure, which consists of a patch, an embedded slot resonator and a hairpin resonator. Such a structure can effectively broaden the impedance bandwidth without increasing the profile of the antenna. The CP characteristics are achieved by employing a single-fed dual-coupled feeding technique that an intrinsic and broadband 90^o phase difference can be realized between the two coupling paths. This CP technique effectively excites the TM10 and TM01-mode of the patch without using any power dividers and phase delay lines, resulting in the reduction of circuit complexity and insertion loss, and therefore the improved gain response. In addition, owing to the coupled resonant structures, the antenna exhibits the 3^rd-order filtering features with improved impedance and axial ratio (AR) bandwidths, flat in-band gain and out-of-band rejection. The measured results verify the design, showing an impedance bandwidth of 29.6%, and 3-dB AR bandwidth of 17.6%. The antenna also exhibits a high gain of over 9.5 dBic.