Miniaturized Single Band Filter with Wide Outer-band Rejection at Higher Frequencies

A miniaturized triple-mode single passband filter with wide out-of-band rejection at higher frequencies is presented. The structure is based on the recently proposed magnetic side wall cavity (MSWC) and a T-shaped resonator (TSR). The MSWC is a two-layered substrate integrated waveguide cavity structure, free from radiation, with a metallic patch placed between the layers and none of the four patch edges connected to the via arrays surrounding the cavity. Together with a metallic post inserted from the top to the bottom, it generates two modes of lower resonant frequencies with four equivalent magnetic side walls. A rectangular slot on the patch is introduced to adjust of the frequency distance between the first two modes, without significantly affecting the third mode. The TSR on the patch is a symmetrical step impedance resonator producing the third-lowest resonant mode. By selecting its position and dimension, the resonant frequency of that mode can be determined or changed. The coupling of the TSR and MSWC can also provide the lowest outer-band transmission zero to improve the passband selectivity. Field cancellation at certain frequencies between resonance points of higher order modes generates other transmission zeros. Proper selection of the feeding points helps suppress higher order modes.