Shorted-Ended Stepped-Impedance Dual-Resonance Resonator and Its Application to Bandpass Filters

In this paper, a short-ended stepped-impedance dual-resonance resonator is presented. Analysis of its resonance characteristics is carried out, from which the design graph is given. By controlling the impedance and length ratios of the resonator, for the first time, the first three pairs of resonant modes corresponding to three passbands within a single resonator can be controlled freely in a wide reasonable range. First, a bandpass filter (BPF) with the first spurious occurring at 4.2 f₀ is designed. Second, in the dual-/tri-band BPFs design, according to the voltage distributions along the resonator, the feedlines with two arms supply the needed external coupling for the two/three passbands simultaneously, and good impedance matching can be achieved in each passband. Lastly, three filters with one, two, and three passbands are fabricated and measured, which show good agreement with the simulated results.