Compact wide-stopband microstrip bandpass filterbased on stub-loaded stepped-impedance resonators

This study presents compact wide-stopband microstrip bandpass filters using stub-loaded stepped-impedance resonators. The resonators consist of three stepped-impedance sections. By adjusting the parameters of the resonators, one can control the frequencies of the resonant modes as well as the frequency of the transmission zero. When the transmission zero frequency is close to the spurious resonant mode frequencies, one can have a wide-stopband bandpass filter without external filtering circuits or increasing the resonator number for suppressing the spurious passbands. To validate the design concept, three dual-band bandpass filters and one wideband bandpass filter are designed, fabricated and tested.