Abstract: The vaneloaded helical slowwave structures is widely used in high power, broadband travelingwave tubes (TWT's). Taking the radial thickness of the helix into account, the tape helix model is suggested for the vaneloaded helical slowwave structures in this paper for the first time. By taking the discrete supports bars into some continuous dielectric tube regions, the dispersion equations and the expressions of interaction impedance are obtained. Besides, the radial propagation constant of every layer in the dielectricloaded region is differentiated. Some helical slowwave structures of TWT are calculated by the derived equations, and the calculated results accord with measured values very well. Compared with measured values, the calculated results of the model considering the r