  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.
