A compact quad-band bandpass filter utilising the proposed quad-mode resonator is presented. The proposed resonator, which has four tunable symmetrical resonant poles, can be treated as a two-side open-circuit stepped-impedance resonator with short-circuit stubs in its low-impedance sections. With the multi-paths propagation mode configuration of the two resonators, transmission zeros among each passband are generated to improve passband selectivity and achieve high isolation. As an example, a quad-band bandpass filter centring at 1.9/2.8/4.3/5.2GHz is designed, fabricated and measured. The measurement results agree well with the full-wave EM simulated responses.
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