D-band (141 - 175.8 GHz) is very attractive for high capacity wireless links. However, the low power available from solid state amplifiers is not sufficient to ensure long links with 99.99% availability in the most common rain zones. Traveling wave tubes have been extensively demonstrated as the solution for satisfying the link budget. At sub-THz frequencies their fabrication is difficult and expensive. Typically, the 40 dB gain needed can be provided by two sections of slow wave structure separated by a sever. Due to the intrinsic low interaction impedance at sub-Thz frequencies, slow wave structures with high number of periods are needed. In case of metal SWS this determines long fabrication time and difficult assembly. This paper presents a short double corrugated waveguides for wide band (141 - 148.5 GHz) TWTs to provide about 26 dB gain and reduced fabrication difficulties.