A Balanced Bandpass Filter With High-Selectivity and Common-Mode Suppression Based on Inverted Microstrip Gap Waveguide

In this letter, a balanced bandpass filter (BPF) is reported by stacking two cavity resonators based on inverted microstrip gap waveguide (MGW). The proposed balanced feed structure achieves deep and wide common-mode (CM) suppression for the first time in a gap waveguide (GW) structure based on a mushroom-type electromagnetic bandgap (EBG) cell package. A highly selective balanced fourth-order bandpass response with three out-of-band transmission zeros (TZs) and CM suppression is achieved by employing a pin-loaded patch perturbation structure at the center of the upper and lower cavities and a pair of slots on the common ground. A prototype of the stacked GW balanced BPF exhibiting a high selectivity was fabricated and measured. The measured results are in good agreement with the simulated ones.