Mode composite waveguide directional coupler

In this work, a mode composite waveguide (MCW) directional coupler is proposed and demonstrated. Due to the dual mode nature of the MCW, the mode conversion between the inner and outer waveguide is involved during power coupling. The coupler operation principle is mathematically formulated in achieving the required power summation and cancellation, and a detailed design methodology is also presented. An MCW backward directional coupler at 10 GHz is fabricated to validate the proposed concept. The desired coupling level of -20 dB and the maximum isolation of -50 dB are obtained in the coupler measurement.