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Design of Compact Bandpass Filters Using Quarter-Mode and Eighth-Mode SIW Cavities

The contribution of this paper is to propose novel balanced and dual-band bandpass filters (BPFs), using quarter-mode (QM) and eighth-mode (EM) substrate integrated waveguide (SIW) cavities, for the size reduction of the overall circuit. Two balanced BPFs, which separately demonstrate a third-order Chebyshev response and a fourth-order quasi-elliptic response, are realized by properly choosing, feeding, and coupling those QMSIW and EMSIW cavities. Not only desired differential-mode operation within the passband, but also good common-mode suppression in a certain frequency range have been achieved. Furthermore, a dual-band BPF with improved out-of-band rejection is also realized by properly constructing the coupling topology of four EMSIW cavities. An additional source-to-load coupling path is introduced in this dual-band BPF to obtain more transmission zeros (TZs). The sizes of resonant cavities utilized in the above designs are only one-fourth or one-eighth of those rectangular ones in conventional designs, which lead to a significant reduction in the overall circuit size. All proposed designs have been fabricated and measured to verify simulation predictions. Based on the author’s knowledge, it is the first time using QMSIW and EMSIW for balanced filter designs, and dual-band filter designs with TZs.