Two novel compact 2.6/5.2 GHz diplexers with high common-mode suppression were designed and fabricated on the FR4 substrate. The diplexers were designed based on two open loop rectangle ring (OLRR) resonators and the two different resonant frequencies could be easily obtained by tuning the lengths of OLRRs. In the past, the traditional balun diplexer needed matching circuit to transfer signal to radio frequency (RF) transceiver with low loss because of the existence of unmatched impendences between the balun diplexer and RF transceiver. In order to improve their efficiency and low down the cost of the fabricated circuits, we would propose a method to tune the output impendence of the balun diplexer and the diplexer could have matching output impendence with the RF transceiver. The proposed balun diplexer had the structure of two stub-loaded microstrip lines, and the needed output impendence could be changed by tuning the widths of microstrip lines and stubs and by changing the length and position of stubs. For that, if the output impendence is well tuned, the designed balun diplexer will have the higher efficiency and low cost because the matching circuit is not necessary.
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