Design of Microstrip Triplexer with High Isolation Based on Parallel Coupled-Line Filters Using T-Shaped Short-Circuited Resonators

Design approach for simultaneously achieving impedance matching and enhancing the isolation of multiplexer has been proposed. The channel filters constituting the multiplexer utilize the T-shaped short-circuited resonators as resonant components and short-circuited parallel-coupled lines as inter-stage couplings. The embedded open-ended stubs of resonators intentionally placed along the main transmission path of each channel contribute the virtual-shorts by creating transmission zeros thereby making the matching and isolation improvement possible. A triplexer with satisfactorily high output isolation is designed and implemented to validate the concept.