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The vane-loaded helical slow-wave structure is widely used in high power, broadband travelling wave tubes (TWTs). Taking the radial thickness of the helix into account, the finite size vane model is suggested for vane-loaded helical slow-wave structures (SWS). The expressions of the dispersion equations, interaction impedance and attenuation constant of this model are derived. Two vane-loaded helical slow-wave structures of the travelling wave tube (TWT) are calculated by the derived equations. The calculated results are compared with the results of measurement and the calculated results of the infinite number of vanes model (INV) that also considered the radial thickness of the helix. The influence on dispersion characteristics is analysed for the thickness of the helix and different centre angles of the vanes. It is interesting to find that the centre angle (2θ) of the vanes has a fixed value when the phase velocity is a minimum.