A Novel Dual-Mode Dual-Band Bandpass Filter Based on a Single Ring Resonator

A dual-mode dual-band bandpass filter with two transmission poles in both passbands using a single ring resonator is proposed. Two excited ports are placed at the 135
^deg
-separated positions along the ring resonator and coupled with the ring via parallel-coupled lines, leading to synchronous excitation of two transmission poles in dual passbands. After the principle of this initial filter is described, an improved ring resonator with periodic loading of open-circuited stubs is constructed and studied to achieve compact size and adjustable spacing between the two passbands. Finally, a dual-band ring resonator filter with center frequencies at 2.4 and 5.8 GHz is designed and fabricated. Measured results verify the design principle.