Compact diplexer with high isolation based on mixed-mode triangular substrate integrated waveguide cavities

A novel compact diplexer based on mixed-mode isosceles right triangular (IRT) substrate integrated waveguide (SIW) cavities is presented. A dual-mode IRT SIW junction cavity is direct-coupled with two single-mode IRT SIW cavities for lower and upper channels, respectively. The diplexer has two third-order filtering channels, centred at 16 and 18.3 GHz with 400 and 422 MHz bandwidths. The in-band measured return and insertion losses are 19 dB/1.3 dB in the lower band and 17 dB/1.36 dB in the upper band. Due to the orthogonality of TE102 and TE201 modes in dual-mode IRT SIW cavity, high isolation (>45 dB) between two channels is obtained. Besides, the proposed diplexer based on the triangular SIW cavity also has advantages of low cost and easy integration with other planar circuits.