Based on a staggered double-grating slow wave structure with step-shaped grooves,the expressions of the electromagnetic feld in it have been obtained and the formulae of the dispersion equation and coupling impedance have been solved by means of feld matching method.As an example,a staggered double-grating slow wave structure for W band TWT application was used to calculate the characteristics of dispersion and coupling impedance using the formulae and CST-MWS code,and analyze the efect of the step dimensio...