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An ultra-wideband (UWB) bandpass filter (BPF) with an electronically tunable notch band based on a folded stepped impedance resonator (SIR) is investigated. The proposed UWB BPF comprises four folded shunt quarter-wavelength short-circuited stubs, one SIR with a metallised via hole to ground and a varactor diode with DC bias. The SIR is studied and employed to generate one desired notched band. The predicted results are compared with measured data and good agreement is reported.