Cross-Coupled Wide Stopband SIW Bandpass FilterLoading Snake-shaped Slot

In this paper, a novel fourth-order bandpass filter is proposed and fulfilled, which is based on a cross-coupling structure achieved by etching snake-shaped slots between vias of adjacent SIW cavities. The gap of the vias’ wall between adjacent SIW cavities is used to achieve coupling between the two cavities. The length of the coupling gap can be adjusted to change the resonant frequency of TE101 mode to TE201 mode. The offset distance between the ports and the centerline is disclosed as a key parameter to suppress undesired modes, achieving wide stopband characteristics. The snake-shaped slot is loaded on the vias’ wall between SIW cavities for the enhancement of the electrical coupling, which produces a pair of transmission zeros (TZs) on either side of the passband, enhancing frequency selectivity. The filter design is validated through simulation, fabrication, and measurement of a fourth-order SIW filter with TZs. It is verified that the designed filter shows promising characteristics, with a center frequency of 24.1 GHz, an insertion loss of 0.52 dB, and a return loss exceeding 11.8 dB. It is observed that two TZs appear at 22.82 GHz and 25.15 GHz, respectively, each exhibiting a suppression level better than 20 dB. The designed filter shows potential applications in microwave and radio frequency circuits, as well as in wireless communication systems.