Miniaturised dual-mode microstrip bandpass filters with wide upper stopband

Dual-mode bandpass filters with miniaturised size and wide upper stopband are presented here. The miniaturisation is realised by resorting to the proposed unilateral and bilateral stepped-impedance open stubs. With adequate design, the loaded capacitance values of a stepped-impedance open stub can achieve 3.2 times the counterpart of a uniform impedance resonator. In addition, the proposed resonator is featured of an additional transmission zero in the upper stopband. To establish the necessary capacitive input/output couplings, filters with only one coupling arm is demonstrated and an enhanced rejection level of the upper stopband is obtained. Results show that the proposed filters cannot only reduce the circuit size up to 68%, but also achieve wide upper stopband as well.