High-Selectivity Inverted Microstrip Gap Waveguide Bandpass Filter Using Hybrid Cavity and Stub-Loaded Ring Resonant Modes

A high-selectivity hybrid-technology bandpass filter (BPF) at the K band using an inverted microstrip gap waveguide (MGW) approach is reported. The resonant frequencies of the modes TE102 and TE103 (i.e.,
$f_{102}$
and
$f_{103}$
) of its non-perturbed groove gap waveguide cavity resonator—as example TE resonant modes being contained within the extracted stopband range of the mushroom-type EBG cells—are firstly analyzed. By perturbing the referred cavity with a pin-loaded patch, a second-order BPF block with an adjustable transmission zero (TZ) is implemented. Moreover, by adding a source-load coupling structure among its inverted microstrip feeding sections, one more close-to-passband TZ is created. Next, in order to further increase the filter order and the number of stopband TZs, a stub-loaded microstrip ring resonator is introduced. In this way, by exploiting the perturbed- groove-gap-waveguide-cavity-resonator-based second-order BPF block along with the microstrip ring resonator, a three-pole BPF response is achieved. Furthermore, owing to the multiple signal paths being created in this mixed-technology filter, two additional close-to-passband TZs are produced. Consequently, an overall sharp-rejection third-order BPF with four TZs is realized. The RF operational foundations of the devised mixed-technology BPF are detailed. Besides, for practical-validation purposes, a third-order BPF prototype is developed and tested. The measured BPF circuit exhibits a passband centered at 21.21 GHz with 3-dB fractional bandwidth (FBW) of 4.72%, four out-of-band TZs, and stopband power-attenuation levels above 17.68 dB from 18.65 to 25.7 GHz.