A new compact microstrip ultra-wideband (UWB) bandpass filter (BPF) with improved out-of-band rejection and good selectivity is proposed using a modified ring multiple-mode resonator (MMR). The initial UWB bandpass filter comprises interdigital coupled lines and a conventional ring MMR. Then, four high-low impedance resonant cells are periodically placed in the inner area of the conventional ring MMR, which have the properties of achieving harmonic suppression and size reduction. To validate

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... design theory, a new compact microstrip UWB BPF with improved out-of-band rejection is designed and fabricated. Both simulated and experimental results are provided with good agreement.