A modal approach is used to analyse the radiation from a dielectric loaded infinite sheath helix, that is excited by a circular sheet of electric and magnetic currents equivalent to the TE11 fields over the cross-section of a circular metallic waveguide of the same diameter. To estimate the far-field radiation pattern of a finite length sheath helix fed by the aperture of a circular waveguide, the near-field solution of the infinite problem, expressed as a sum of discrete modes and a continuous spectrum wave, is integrated over a finite length of the infinite structure using the Kirchhoff-Huygens formulation. The approximations involved are discussed. Measured radiation patterns of multifilar helices fed by the circular waveguide TE11 mode, are compared to computed radiation patterns.