Novel compact dual-band bandpass filter with improved stopband characteristics

A novel compact microstrip three- pole dual-band bandpass filter is developed with remarkably improved stopband characteristics. Two square open-loop resonators are embedded in a U-shaped resonator to achieve a significant size-reduction of the filter. Meanwhile, microstip open stubs as well as a cross-coupling between nonadjacent resonators are exploited to produce multiple transmission zeros in the stopbands of the filter. As a consequence, wide stopbands with sharp and large attenuations are obtained. Theoretical and measured frequency responses of the designed filter show a good agreement.