A novel substrate integrated waveguide (SIW) planar diplexer with very extremely high isolation is presented. It is formed by two quarter mode substrate integrated waveguide (QMSIW) cavity resonators which are designed individually and combined through a T-juction. The diplexer channels are 13% and 15% relative bandwidths at 2.35 GHz and 3.5 GHz, respectively. Inband return losses are better than 22 dB and 25 dB, and the insertion losses are 0.8 dB and 0.5 dB, in the lower and higher channels. The isolation between the two channels is lower than -42 dB, indicating enough isolation between the two channels. The measured results of the fabricated diplexer agree well with the simulated ones.
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