A dual-band filter using dual-capacitively loaded substrate integrated waveguide (SIW) cavities is proposed. Two conductive posts are embedded in the SIW cavity and the capacitive effect generated by the loaded posts lowers the first two resonant frequencies. Compared with the unloaded SIW cavity of the same size, the first resonant frequency is lowered by 39% (from 4.12 to 2.51 GHz), and the second resonant frequency is lowered by 20% (from 6.63 to 5.30 GHz). The two loaded posts also have the capability to tune the two resonant frequencies independently, thus two passbands of the filter using the loaded SIW cavity can be flexibly controlled. The proposed filter is fabricated through a multilayer printed circuit board (PCB) process to experimentally validate the predicted behavior.

A compact triple-mode bandpass filter using a modified quarter-mode substrate integrated waveguide (QMSIW) cavity is presented. It is realized by exploiting the first three resonant modes of the QMSIW cavity. Specifically, the dominant mode (TE₁₀₁) is shifted toward the second resonant mode (TE₂₀₂) through the perturbation induced by the metallized via-holes array. The resonant frequency of the third resonant mode (TE₃₀₃) is lowered toward the TE₂₀₂ mode owing to the capacitive loading effect of the loaded circular conductive posts array. In this way, by adjusting the via-holes array and the conductive posts array, the resonant frequencies of the first and third resonant modes can be independently tuned while the second resonant mode remains unchanged. As a result, the bandwidth of the resulting filter can be flexibly controlled at a preassigned center frequency. Moreover, to suppress the spurious frequencies generated by the higher order modes, three cascaded defected ground structures were introduced below the feedline to distinctly widen the stopband and increase the out-of-band rejection.

Triple-mode and quadruple-mode bandpass filters using triangular half-mode substrate integrated waveguide (HMSIW) are presented. To realize the two multi-mode filters, the triangular HMSIW cavity is perturbed by introducing via-hole arrays at different positions of the structure. The perturbation causes mode shifting of certain resonant modes near to the other resonant modes which are not affected in order to form the passband. The first three resonant modes and four high order resonant modes (from the third resonant mode to the sixth resonant mode) in the perturbed triangular HMSIW are respectively utilized to design the triple-mode and quadruple-mode filters. Both filters are fabricated and measured to provide an experimental validation on the predicted performances of the proposed filters.