A compact microstrip dual-mode dual-band filter with source-load coupling

A novel microstrip dual-band filter is proposed in this paper. The filter consists of a microstrip stub-loaded dual-mode resonator and a half-wavelength resonator that are folded into a very compact U-shape configuration. By choosing the step-impedances of the stub-loaded dual-mode resonator, we can change the center frequencies of the dual-bands and the bandwidth of the first passband separately. By adding the half-wavelength resonator, we can control the bandwidth of the second passband freely. The half-wavelength resonator also introduces cross-coupling between the input and output feed lines. As a consequence, two additional transmission zeros are produced, which improve significantly the isolation between the dual passbands. The designed filter operating at 2.45GHz and 5.2 GHz for WLAN applications shows favorable performance.