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The excitation of a semi-infinite sheath helix by the TE11 mode of a circular metallic waveguide with the same diameter is studied using the Wiener-Hopf technique. Expressions are presented for the TE11 reflection coefficient at the waveguide end and for the relative power in the sheath helix discrete modes. Numerical results corresponding to the current density at the sheath helix surface and to the far-field radiation patterns are presented. These are compared to previous results obtained from the modal analysis of an infinite sheath helix that is excited by a current sheet. Experimental patterns corresponding to a finite length multifilar helix show a satisfactory agreement with the theoretical patterns when the scanning mode is dominant.