Independently Controllable External Coupling for Resonant Junctions in Diplexers

It has been a challenge to independently control the external couplings from the common port to the channel filters of a diplexer when a dual-mode resonator is used at the junction. This paper proposes a coupling structure to tackle this challenge and demonstrates the implementation in a microstrip diplexer with a stub-loaded resonator (SLR) junction. A fork-like structure has been used to couple to the two modes of the SLR and control their external coupling independently. This enables the flexible realization of the channel bandwidth in resonator-based diplexers. Two diplexers with the same topology but different channel bandwidths have been designed, fabricated and measured to demonstrate this capability.