This paper presents a novel compact microstrip hybrid four-mode bandpass filter (BPF) with good selectivity and multi-transmission zeros. By adding an external resonator to a triple-mode stub-loaded uniform impedance resonator, four modes are generated in the desired passband and a hybrid four-mode BPF is implemented. Even- and odd-mode theory is introduced to investigate the mode characteristics of triple-mode resonator in detail. Circuit model and coupling matrix are built to further explain the proposed methodology. A microstrip BPF with central frequency of 3.4 GHz for wireless communication was designed and fabricated. Three transmission zeros are obtained at 3.2 GHz, 3.55 GHz and 3.73 GHz, which improve selectivity and out-of-band rejection of the filter. The measured results of the fabricated filter represent good in-band and sharp sideband characteristic, which match well with simulated results.
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