A Circularly Polarized, Highly Selective, Harmonic-Suppressed Filtering Antenna for 5G Application

This research presents a novel circularly polarized filtering antenna with high selectivity and harmonic suppression. The feeding network integrates out-of-phase power dividers and fourth-order filters, which are codesigned with the radiation patch to introduce multiple resonant poles, thus effectively broadening the bandwidth. Circular polarization is attained through equal-amplitude unequal-phase feeding of the power divider. Additionally, cross-coupling introduces radiation zeros on either side of the passband, thereby enhancing passband selectivity. The selectivity at the lower and upper frequencies is quantified as 287 dB/GHz and 269 dB/GHz, respectively. To enhance performance, a periodic square slot structure is incorporated into the coupling section of the filter to achieve a transmission zero in the stopband, which effectively suppresses high-order harmonics. The fabricated filtering antenna demonstrates an S-11 <= -18 dB, axial ratio below 3 dB, and peak gain of 6.45 dBi over the passband from 3.4 GHz to 3.65 GHz (approximately 7.1%). In the extended stopband, it exhibits an attenuation greater than 16.04 dBi at 2.37 f(0), confirming its superiority in suppressing clutter signals. These results make the designed antenna a promising candidate for 5G application.