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 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> ≤ −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 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub>, confirming its superiority in suppressing clutter signals. These results make the designed antenna a promising candidate for 5G application.