Abstract-Two compact dual-band bandpass filters (BPFs) with closely spaced passbands are presented in this paper. Each of the filters consists of a stub loaded resonator, to which shorted lines are coupled. The ratio of the center frequencies of two passbands can be easily adjusted from 1.2 to 1.1 by changing the gap of the coupled line. In addition, seven transmission zeros (TZs) can be yielded to obtain high passband selectivity and enhance the out of band performances. As an example, two filters are designed, fabricated and measured. Both filters exhibit the merits of high passband selectivity, very low center frequency ratio, and wide stopband suppression.

In this paper, a stub-loaded penta-mode resonator (PMR) with its analysis, characterization and applications for bandpass filter (BPF) is presented. Even-/odd-mode analysis method is employed to analyze the PMR which exhibits five transmission poles (TPs) and three inherent transmission zeros (TZs). By changing the dimension parameters of the resonator, TPs and TZs can be flexibly controlled, and wideband/dual-wideband/tri-band BPF has been designed successfully utilizing the same configuration. For validation, all these three filters are fabricated and measured, and the measured results are in good agreement with the simulated ones.

In this article, a new class of dual-/tri-band bandpass filters (BPFs) using a quintuplemode resonator (QMR) is proposed. The classic odd-/even-mode analysis method is used to analyze the two filters due to their symmetrical structure. Owing to characteristics of the QMR, the dual-/tri-band BPFs based on the same topology are designed. The bandwidths (BWs) of the passbands are controllable. A dual-band BPF with center frequencies of 2.1 GHz/3.43 GHz and a tri-band BPF with center frequencies of 2.35 GHz/3.44 GHz/5.2 GHz are designed, fabricated, and measured. The fabricated filters are compact in size, and measured results are in good agreement with the simulated ones.

A compact wideband bandpass filter (BPF) with stopband suppression by utilizing transversal signal interaction concepts is proposed in this article. Two transmission paths from Port I to Port II are separately constructed by multi-mode step impedance resonator (SIR) and shorted coupled lines. The proposed configuration generates two controllable transmission poles, and wideband characteristic can be realized. Moreover, multiple transmission zeros are implemented by signals superposition of two transmission paths and stub loaded fans resulting in steepness sideband and broad upper stopband suppression up to 100 GHz. For clarification, the designed wideband centered at 4.5 GHz with fractional bandwidth of 14.2% is designed, assembled and measured. The circuit size of prototype BPF only occupies 0.94 cm 2 , and the presented BPF is evaluated by test results and simulated predictions with good agreement.

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