Comments, on "Finite-element analysis of waveguide modes: a novel approach that eliminates spurious modes" [with reply]

For the original article see ibid., MTT-35, no.2, p.117-23 (1987). The commenter explains that the formulation proposed in the above-named work by T. Angkaew et al. seems to enable one to identify guided modes with real propagation constants, but care has to be taken when attributing the eigenvalues having a nonzero imaginary part to spurious modes. In replying, the authors explain that equation (15) was derived from the assumptions that the permittivity and permeability tensors are Hermite tensors and that the propagation constant beta is real.<>