In this paper, a novel end-loaded quarter-wavelength resonator is investigated for the designs of wideband tunable bandpass filter (BPF) and bandstop filter (BSF). The novelty of the resonator lies in that two varactors 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 double that of the conventional tunable quarter-wavelength resonator. For demonstration, the proposed resonator is applied to design wideband tunable BPF and BSF. As expected, the tuning ranges are 52.4% and 53.5% for the BSF and BPF, respectively. Good agreement can be observed between the simulated and measured results.
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