Coupling Tunable D-band Directional Coupler for Millimeter-wave Applications

The directional coupler is an important device for the millimeter-wave applications such as the electron cyclotron system in the magnetic confinement fusion research. A coupling tunable D-band directional coupler was designed based on a novel coupling grid structure proposed in this paper. The designed directional coupler has excellent performance with ultra-wideband. The coupling can be tuned from -28.2 dB to -33.2 dB at 140 GHz by changing the angle of the coupling grid, and the dynamic range of the coupling is about 5 dB. The return loss is smaller than -15 dB in the whole D-band from 110 GHz to 170 GHz. A 3-dB coupler using the similar coupling structure was also designed. The coupling is 3.3144 dB at the center frequency of 140 GHz.