ACCURATE ANALYSIS OF HELIX SLOW-WAVE STRUCTURES

In this paper, the helix slow wave structure (SWS) of traveling wave t ubes (TWT's) has been analyzed, Dielectric supporting rods of arbitrar y cross section have been considered in this analysis. The inhomogeneo us dielectric loading factor has been accounted for by modeling the di screte support with a number of continuous dielectric tubes of appropr iate effective dielectric permittivity, The helix tape model has been used for the field analysis. Furthermore the thickness of the helix ta pe has been considered. A rigorous solution of the field equations, in cluding the contribution of the space harmonics, was performed to eval uate the phase velocity and the interaction impedance up to millimeter -wave frequencies. The nonuniformity of radial propagation constant ov er the structure cross section has been also included. With respect to other approaches, a closed-form expression of the field constants has been obtained. A study to choose the optimum number of space harmonic s and dielectric tubes to be used in the analysis, has shown how the r esults are more sensitive to the number of space harmonics than to the number of dielectric tubes, beyond a certain number of the latter. Th e validity of this theory has been proved by comparison between measur ements and simulations for helix SWS with different dimensions, rod sh apes and operating frequency band.