Novel compact quasi-elliptic SIW filter based on quarter-mode cavities

This paper presents a novel substrate integrated waveguide (SIW) filter, based on quarter-mode cavities, which permits to obtain a transmission zero close to the passband. The use of quarter-mode cavities provides a significant size reduction compared to regular SIW cavities, while preserving most of the advantages. The proposed filter comprises four side-coupled cavities, and the use of cross-coupling permits to introduce the transmission zero. The cross-coupling is due to thin metal strip outside the cavities, and it can be easily controlled to locate the transmission zero at the desired frequency. A four-pole filter operating at frequency of 4 GHz, with a transmission zero very close to the passband, was fabricated and measured. The proposed filter has a footprint of 41×28.5 mm2, corresponding to 0.55 λ0×0.38 λ0, where λ0 is the wavelength in vacuum at 4 GHz.