Abstract: The vaneloaded helical slowwave structure is widely used in high power, broadband travelingwave tube(TWT). The tape helix model is suggested for vaneloaded helical slowwave structures in this paper. And the dispersion equations, the expressions of interaction impedance and attenuation constant are obtained. Some helical slowwave structures of TWT are calculated by the derived equations and sheath model. The calculated results accord with measured values very well. The influence on dispersion characteristics is analyzed for different radius of metal envelope.