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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.