Planar diplexer for microwave integrated circuits

A planar microwave diplexer based on the substrate integrated waveguide (SIW) technique is presented in the paper. The SIW-microstrip transition and SIW filter are experimentally investigated, and then a C-band SIW diplexer is designed and fabricated using a standard PCB process. The relative bandwidths of up and down channels are 3% and 4.8% at 5.96 GHz and 5.42 GHz, respectively. The measured results show good channel isolation, moderate insert losses and small return losses in pass-bands. The diplexer takes a planar form and can be easily integrated in microwave integrated circuits.