Dual-band bandpass filter using open-loop resonators

A novel dual-band microstrip bandpass filter is presented that is based on the open-loop ring resonator structure. The proposed filter´s upper dual-mode passband can be tuned without perturbing the characteristics and centre frequency of the lower passband. Design methodology and experimental results of the prototype circuit are presented. To further improve the filter´s selectivity and out-of-band response an additional transmission zero is introduced using spur-lines embedded into the input/output feedlines. The prototype circuit was validated by measured results.