Properties of NRD-guide and H-guide higher-order modes: physical and nonphysical ranges

New, interesting modal properties are presented for the waveguiding structure consisting of a rectangular-section dielectric rod sandwiched between parallel conducting plates, which can represent either a nonradiative dielectric (NRD) guide or an H guide. A rigorous quantitative analysis of the dispersion properties, making use of both wavenumber and steepest-descent plots, indicates that higher-order modes can show different anomalous behaviors, as geometrical and electromagnetic parameters vary. A discussion of the physical nature of the waves related to complex wavenumbers allows us to illuminate the performance differences between NRD and H-guides, as regards certain previously unexplored leakage effects