Attenuation in an elliptic waveguide

The formula for the anisotropic axial surface impedance, specified by Falciasecca et al. as necessary for agreement with the classical attenuation computation of Chu, gives an erroneous result for a much elongated ellipse. This error is attributed to the form of the asymptotic expansion of the radial Mathieu functions of the fourth kind that is used in the waveguide wall. Corrected expressions for the axial and transverse surface impedances and for the TE and TM mode attenuation are elaborated.