Reconfigurable Dual-Band Balanced Filter Using Quad-Mode Cavity Resonator

A reconfigurable dual-band balanced filter using quad-mode cavity resonator is proposed in this letter. A rectangular cavity embedded with four pairs of metal posts is adopted as the quad-mode resonator, microstrip feeding lines could generate two pairs of degenerate modes of the cavity under differential-mode (DM) excitation, and then form two passbands. Meanwhile they could be highly rejected under common-mode (CM) excitation. Tuning screws are inserted in the center of cavity to provide frequency and external coupling modulation between the two passbands, and then the frequency of the lower band could be adjusted independently. From the measured results, the center frequency of the lower band varies from 9.41 to 9.65 GHz, while the upper band is fixed at 10.98 GHz. Over the frequency tuning range, the dual-band response shows insertion losses of 0.52 and 41 dB CM suppression. The proposed reconfigurable filter is potential for applications in base stations and satellite communication systems requiring multi-channel, high anti-interference properties.