Abstract: A corrugated elliptical waveguide medium comprises a corrugated hybrid mode excitation member having an elliptical transverse cross-section for propagating electromagnetic energy therethrough. The excitation member is provided with longitudinally spaced apart parallel corrugations with the teeth of the corrugations defining an inner ellipse and the grooves of the corrugations defining an outer ellipse. The depths of the corrugation grooves on the major and minor axes of the ellipsis are dimensioned such that the tangential electric and magnetic field components of the energy in a circumferential direction are zero on the inner ellipse.