Novel compact dual-band bandpass filter using square ring resonators

A novel compact microstrip dual-band bandpass filter (BPF) using square ring resonators and good selectivity is proposed in this letter. The characteristics of the dual-mode resonator are investigated by using even-odd mode analysis. It is found that, the even- and odd-mode of resonators can be regulated by adjusting the lengths of open folder and square ring. Meanwhile, four transmission zeros are properly tuned to improve the selectivity of the two passbands and the rejections lower and upper stopband. A prototype of dual-band bandpass filter for WLAN (2.4 GHz) and WiMAX (3.5 GHz) applications is designed and fabricated for demonstration. The good agreement between simulation and measurement is obtained.