A Triple-Mode Bandpass Filter With Controllable Bandwidth Using QMSIW Cavity

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 viaholes 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-hand rejection.