Study of the double rectangular waveguide grating slow-wave structure

A slow-wave structure (SWS) with two opposite gratings inside a rectangularwaveguide is presented and analysed. As an all-metal slow-wave circuit, thisstructure is especially suited for use in millimetre-wave travelling wave tubes(TWTs) due to its advantages of large size, high manufacturing precision and goodheat dissipation. The first part of this paper concerns the wave properties of thisstructure in vacuum. The influence of the geometrical dimensions on dispersioncharacteristics and coupling impedance is investigated. The theoretical results showthat this structure has a very strong dispersion and the coupling impedance for thefundamental wave is several tens of ohms, but the coupling impedance for --1 spaceharmonic wave is much lower than that for the fundamental wave, so the risk ofbackward wave oscillation is reduced. Besides these, the CST microwave studio isalso used to simulate the dispersion property of the SWS. The simulation resultsfrom CST and the theoretical results agree well with each other, which supports thetheory. In the second part, a small-signal analysis of a double rectangularwaveguide grating TWT is presented. The typical small-signal gain per period isabout 0.45 dB, and the 3-dB small-signal gain bandwidth is only 4%.