A dual-mode dual-band bandpass microstrip filter using double square-loop structure is proposed in this paper. Each of the square-loop forms a dual-mode resonator with controllable respective passband. Two tuning patches placed symmetrically at the side of the perturbation patch are used to change the higher passband frequency, while while keeping the lower invariable. Several attenuation poles in the stopband are realized to improve the selectivity of the proposed bandpass filter. The filter is evaluated by experiment and simulation with good agreement.
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