The vane-loaded helical slow-wave structures is widely used in high power
broadband traveling-wave tubes (TWTs).Considering azimuthal space harmonics brought by the vanes
the finite size vane model is suggested for vane-loaded helical slow-wave structures in this paper.Two vane-loaded helical slow-wave structures of TWT are calculated by the derived dispersion equations and the expressions of interaction impedance.The calculated results are compared with the results of measurement and the calculated results of simple vane model.We find the most suitable center angle(θ) of the vanes is in existence for the first time.