In this letter, a novel compact dual-mode tri-band microstrip bandpass filter (BPF) is proposed using three sets of dual-mode resonators, i.e., the quarter-wavelength resonator (QWR) which is designed for the first passband, and two sets of stub loaded resonators (SLR) for the second and third passband, respectively. The center frequencies and bandwidths of three passbands can be flexibly tuned and designed. Multiple transmission zeros can be generated to improve the selectivity. Meanwhile, the BPF can obtain more compact size by adopting the folded resonators and extended feed lines. Finally, a tri-band microstrip BPF prototype with fractional bandwidths 18.3% at 1.8 GHz, 5.5% at 3.6 GHz and 12.2% at 5.25 GHz is designed. The measured results are in good agreement with the full-wave simulation results.
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