Stub-loaded step-impedance resonator (SLSIR) is a multi-mode resonator and can be applied to implement multi-band or wideband filters. In this paper, odd- and even mode impedance analysis is used to resonant properties of the SLSIR. Only two SLSIRs are applied to design a quint-band bandpass filter (BPF). To find the required five resonant modes, the frequency ratios of the high order modes to the fundamental mode of the SLSIR are calculated depending on the impedance ratio and the length ratio of the SLSIR. Several coupling types of the SLSIRs are considered first to have enough energy for all the five passbands. When forming the quint-band, a pair of the SLSIR are coupled electrically and connected with 0o feeding input/output structure. The center frequencies are designed at 1.38 GHz, 2.58 GHz, 3.69 GHz, 5.36 GHz, and 5.8 GHz, corresponding to the different communication applications. The filter is designed, fabricated, and measured. Simulated and experimental results are in agreement, verifying the design concept.
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