Compact dual-wideband bandpass filter with good selectivity using side-coupled λ/4 shorted SIR

A compact dual-wideband bandpass filter featuring two controllable center frequencies and passbands using a single side-coupled shorted λ/4 stepped-impedance resonator (SIR) is presented. The resonant frequencies of the two passbands can be adjusted by tuning the impedance ratio and length of the SIR. The bandwidth of the two passbands is determined by the coupling strength between the meandering feedlines and the resonator. The generation mechanism of filter’s transmission zeroes was investigated by equivalent models of the filter and the bending parts of the open-end meandering feedlines. Owing to the cross-coupling between the two open-end meandering feedlines and the phase shifted characteristic of the open-end meandering feedlines, several transmission zeroes can be generated to improve the selectivity. To verify the design concept, one dual-wideband bandpass filter has been designed, fabricated, and measured. The simulated and measured results are presented with good agreement.