A novel microstrip dual-mode bandpass filter with harmonic suppression

A novel dual-mode microstrip bandpass filter (BPF) with harmonic suppression is presented in this letter. The filter is composed of a perturbed square-ring resonator and two end-open L-shaped microstrip lines for the input and output of the resonator. The harmonic suppression of the BPF may be realized by choosing a proper length for the L-shaped microstrip line. The BPF has been investigated numerically and experimentally. Measured results show that the stop-band of the filter is increased up to near the third-order harmonic of the square-ring resonator due to harmonic suppression, and the rejection level is kept below -27dB