Quad-mode dual-band bandpass filter based on a stub-loaded circular resonator

In this paper, a quad-mode dual-band bandpass filter (BPF) only using a stub-loaded circular resonator is presented. The odd- and even-mode resonant frequencies can be easily computed and flexibly controlled by the radius of the circle and the two sets of stubs, as all odd- and even-mode equivalent circuits are half-wavelength resonators (double-ended short-circuited uniform transmission line resonator). Introducing the resonator makes the filter be analyzed and designed easily. To further improve the isolation between the two passbands, a dual-band BPF is designed by introducing the source-load coupling technique; the simulation and test S-parameter curves show that the designed BPF has four controllable transmission zeros. The good agreement of measured data with simulation results verifies the proposed design.