Analysis of the spectra, real or complex, of the waves or oscillations in open guiding structures is much less developed as compared with the theory of shielded waveguides which enters many textbooks and monographs in electromagnetics, e.g., [1–4]. In fact, the former is possible only within the frames of the spectral theory of open structures [5, 6] involving mathematically correct statements of non-self-adjoint boundary eigenvalue problems with generalized conditions at infinity that enable one to consider complex modes of all types.