Design of Compact Quadruplexer Based on the Tri-Mode Net-Type Resonators

A compact size and high isolation microstrip quadruplexer based on the tri-mode net-type resonators is proposed in this letter. The quadruplexer is composed of four folded tri-mode net-type resonator filters with one input and four output coupled-line structures. By adjusting the structural parameters of a single tri-mode net-type resonator, its first three resonant frequencies can be controlled and, thus, it can be utilized to implement a third-order bandpass filter when these resonant frequencies are suitably assigned. The proposed third-order bandpass quadruplexer is constructed by only four tri-mode resonators, showing a small circuit size. In addition, no additional five-pole matching junction is needed for the quadruplexer design. As a result, the proposed quadruplexer occupies an extremely small area, i.e., 0.27 λ₀ by 0.09 λ₀ (0.42 λg by 0.13 λg), while still keeping good isolation better than 40 dB for each channel.