Contribution of the Ψ = constant mode to the modal expansion in a waveguide

The function Ψ 0 = constant belongs to the set of Neumann eigenfunctions of a waveguide cross-section. The contribution of Ψ0 must not be omitted in the expansion of H_z axial component of H. This contribution is shown to be produced by the presence of an aperture, and more precisely by the (transverse) E_c component in the aperture. The presence of Ψ0 in the expansion is necessary to arrive at the correct from of the magnetic Green´s dyadic of the waveguide.