The European Commission Horizon 2020 ULTRAWAVE, "Ultra capacity wireless layer beyond 100 GHz based on millimeter wave Traveling Wave Tubes", aims to exploit portions of two frequency bands in the millimetre wave spectrum, the D-band (141-148.5 GHz) and the G-band (275-305 GHz) for creating a very high capacity layer. Due to the high atmosphere and rain attenuation, high transmission power is needed to provide a useful transmission range. Traveling Wave Tubes are the only devices that can provide the multi-Watt transmission power above 100 GHz. In this paper, the design of the Double Corrugated Waveguide (DCW), as slow wave structure, for a novel D-band TWT, for wireless communications, will be described.