Design, fabrication, and cold test of a high frequency system for an H-band sheet beam travelling wave tube

The design, fabrication and cold test of a high frequency system (HFS) for an H-band (220-325GHz) sheet beam travelling wave tube is presented in this article. The HFS was composed of a 90-period double-staggered grating waveguide and a pair of identical L-shaped couplers with Bragg reflectors and matching steps. The HFS was manufactured by nano-computer numerical control machining and its electromagnetic properties were measured by using a vector network analyzer. The measured S-parameters were in good agreement with the simulated ones, which predicted a 3 dB bandwidth of ∼47.0 GHz. The maximum value of the measured transmission coefficient S 21 was -4.9 dB and the in-band port reflection S 11 was around -15.0 dB. Based on the cold testing-based HFS, simulations of the beam wave interaction predicted a stable output power of over 55.1 W in the frequency range of 230-280 GHz. In addition, the instabilities of the beam wave interaction were investigated in the simulation