Design and Implementation of Compact Hy- Brid Four-mode Bandpass Filter with Multi- Transmission Zeros

Abstract—This paper presents a novel compact microstrip hybrid four-mode bandpass filter (BPF) with good selectivity and multitransmission zeros. By adding an external resonator to a triplemode stub-loaded resonator, four modes are generated in the desired passband and a hybrid four-mode BPF is implemented. Oddand even-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 3.4GHz for wireless communication was designed and fabricated. Three transmission zeros are obtained at 3.2GHz, 3.55 GHz and 3.73GHz, which improve selectivity and outof-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.