Compact size and high isolation microstrip diplexers designed with tri-mode stub-loaded stepped-impedance resonators (SL-SIRs) have been proposed. By adjusting the structural parameters of the proposed tri-mode SL-SIR, its first three resonant frequencies can be con-trolled. Thus, it can be utilized to implement a third-order bandpass filter with a transmis-sion zero at the upper side of the passband edge when these resonant frequencies are suitably assigned. In addition, by utilizing multiple coupling paths with a common input coupled-line section, the two passbands of the diplexer can then be fully controlled and, thus, the design is flexible so as to independently realize different channels. For our dem-onstration, a third-order bandpass diplexer formed by only two tri-mode resonators has been designed and fabricated. It is shown that the diplexer occupies an extremely small area, i.e. 0.14λg by 0.12λg, while still keeping good isolation better than 40 dB for the two channels. Good agreement is also achieved between simulation and measurement.
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