A 3-D Millimeter-Wave Filtering Antenna With High Selectivity and Low Cross-Polarization

A three-dimensional (3-D) filtering antenna with features of high selectivity and low cross-polarization is proposed and experimentally verified in this communication. Thanks to the vertically 3-D integration, the high selectivity at either frequency sides of the filtering antenna is achieved by a novel cross-coupling scheme between in-band and out-of-band modes in a single cavity located in the lower substrate, while the low cross-polarization in the far-held is realized due to the symmetric feed and radiators on a cavity-backed dual-slot antenna at the upper substrate. In our design, the cavity-backed dual-slot antenna performs not only a radiator but also the last resonator of the bandpass filter (BPF). A prototype is demonstrated at Ka-band with a center frequency of 31.495 GHz and fractional bandwidth of 1.56%. Two radiation nulls [transmission zeros (TZs)] at either frequency band edges can be observed and a cross-polarization level lower than -30 dB is obtained.