Miniaturized tri-band bandpass filter using modified triple-mode resonators with multiple transmission zeros

A compact microstrip tri-band bandpass filter (BPF) using the modified triple-mode resonators to reduce the size is presented. The center frequencies of the first and the third passband can be conveniently controlled by tuning the dimensions of the short stub and square ring, whereas the second passband frequency is fixed. Moreover, the filter has been implemented with multi-transmission zeros to improve the selectivity and upper-stopband performance. To verify the proposed approach, a prototype of tri-band BPF centred at 2.4/3.5/5.2 GHz for WLAN/WiMAX applications is designed and fabricated for demonstration, indicating good agreement with the theoretical expectation.