Design of Ka-Band Filter-Antenna Based on Characteristic Mode Theory Using Gap Waveguide Technology

In this letter, a Ka-band filter-antenna based on gap waveguide (GWG) is proposed. First, a patch of the filter-antenna with a flat in-band gain and good out-of-band rejection, is analyzed using characteristic mode theory (CMT). Second, a dispersion diagram of a novel mushroom unit cell with four metalized vias is presented. Third, a resonator based on a substrate-integrated waveguide with high quality, along with a GWG feeding line, is added to excite the desired modes. Finally, for integration and measurement considerations, transition structures are designed from the GWG to the microstrip line (MSL) and from the MSL to the coupling slot, and the proposed filter-antenna worked at Ka band is obtained. The measurement results show that the impedance bandwidth of the proposed filter-antenna ranges from 34.3 GHz to 36.9 GHz and the in-band gain is about 8.5 dBi, while the out-of-band rejection levels at the low and high-frequency nulls are about 45 dB and 32 dB, respectively.