Design of high-selectivity microstrip bandpass filter using triple-mode stub loaded resonator

In this letter, design of a highly selective six-pole microstrip bandpass filter with low insertion loss is reported. For this purpose, a new type of triple-mode open loop resonator having patch loaded open-circuited stubs is designed and analyzed through even-odd mode resonance conditions. Designed resonator is coupled to input and output ports to obtain a bandpass filter response with three modes and two transmission zeros. Coupling scheme and coupling matrix of the proposed filter is also introduced. In order to increase the selectivity of the filter, six reflection zeros as well as three transmission zeros can be obtained in passband by using two identical triple-mode resonators which are coupled to each other. Both of triple-mode and six-pole bandpass filters are adjusted to the passband covering 1.8-1.9 GHz frequency interval for GSM applications with a total circuit size of 19.9×19.9 mm² and 38.6×15.6 mm², respectively. Proposed triple-mode and six-pole bandpass filters are also fabricated and measured with a good agreement with the predicted S-Parameters.