Design of a Triple-Mode Bandpass Filter Using a Closed Loop Resonator

In this study, a novel third-order bandpass filter, which is based on a rectangular closed loop resonator, is presented. By adding a series resonator to the conventional loop resonator, the resonator's even resonant mode is split into two modes, while the odd resonant mode is not affected. Therefore, by varying the values of the series resonator elements, the resonant frequencies of two even modes can be determined independent of the odd-mode resonant frequency. In the proposed triple-mode filter design, instead of using a lumped series resonator, a T-shaped transmission line is coupled to the resonator via a small gap. To verify the design method, a filter is designed at 2.4 GHz with a bandwidth of 100 MHz. The improved performances of the proposed triple-mode filter are compared with those of the conventional dual mode filter.