The electromagnetic field analysis of a helical slow-wave structure (SWS) is carried out based on a tape-helix model incorporating the effects of space-harmonic propagating modes and the surface current on the helix over the actual metallic area of the tape. Using this analysis, closed-form expressions are derived for the shunt capacitance per unit length and the shunt conductance per unit length of the transmission-line equivalent circuit of the structure. The analysis is interpreted for the circuit attenuation constant contributed by the loss of the dielectric helix-support rods. The analysis is accurate, amenable to easy computation, and validated against published results. The analysis is subsequently used for investigating the dielectric loss in an SWS due to the backward-wave (-1) space-harmonic mode of propagation.