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For specific forms of excitations, the normal modes and mode coefficients of a lossless helically corrugated circular multimode waveguide are determined from the appropriate boundary conditions. Because of the multiplicity of the roots of the characteristic dispersion equation obtained, care must be exercised in the evaluation of the equation and in the interpretation of the ω-β diagram. For the TE11 mode excitation, it is shown that increasing either the depth or the width of the corrugation enhances the conversion into the TM11 mode, whereas increasing the pitch reduces the TM11 mode conversion. Mode conversion always increases with increasing frequency. The theoretical results are in agreement with the results of measurement.