Coupling tunable D-band directional coupler for millimeter-wave applications

A coupling tunable D-band directional coupler is designed based on a novel coupling grid structure proposed in this letter. 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 use the similar coupling structure is also designed. The coupling is 3.3144 dB at the center frequency of 140 GHz.