In this work it is reported the results of a slow-wave structure (SWS) loss analysis using the tape-helix model instead of the simpler sheath-helix model in order to consider the space-harmonic effects. The analysis is made rigorous by removing the usual approximation of considering the radial propagation constant to be the same in the different structure regions. This paper presents not only the effect on loss of the dielectric support rods for the helix but also the functional variations of structure loss with frequency, helix radius, helix pitch, helix width, circular guide-to-helix diameter ratio, and helix and guide resistivities. All results shown in this paper are in complete agreement with those reported in the literature.