Novel tunable bandstop filter using end-loaded quarter-wavelength resonators

In this paper, a novel end-loaded quarter-wavelength resonator is investigated for the designs of wideband tunable bandstop filter (BSF). The novelty of the resonator lies in that two tunable capacitors are added to the two ends of the resonator, and then its resonant frequency can be bi-directionally tuned. As a result, the theoretical frequency tuning range can be significantly extended to approximately twice that of the conventional tunable quarter-wavelength resonator. For demonstration, the proposed resonator is employed to design a wideband tunable BSF. As expected, the tuning range is 53.5% for the BSF. Both the simulated and measured results are presented, good agreement has been observed.