A novel compact microstrip bandpass filter based on open stub-loaded triple-mode resonator

A novel compact microstrip bandpass filter with two transmission zeros based on open stub-loaded triple-mode resonator is presented in this paper. By analyzing the characteristics of open stub-loaded triple-mode resonator and its inherent transmission zeros which can be easily moved by changing the length of stub-loaded lines, a bandpass filter is designed, fabricated and tested. In order to achieve compact size the triple-Mode resonator has been folded, with the size reduction of about 36% compared with the conventional dual-mode microstrip open loop filter. There are two transmission zeros at upper (2.57 GHz) and lower side (2.27 GHz) of the passband edge, with the attenuation better than 41 dB and 15dB respectively, and the insertion loss is less than 1.5 dB in passband. Good agreements are obtained between its measured and simulated S-parameters.