Modal characteristics in NRD and H-guides with lossy dielectric strips

The modal characteristics of non-radiative dielectric (NRD) and H-guides with lossy dielectric strips are analyzed using complex propagation constants that are rigorously obtained using Davidenko's complex root-finding algorithm. For the dominant mode of the lossy NRD guide, there is no cutoff frequency, and this mode is no longer a trapped surface wave (TSW) due to dielectric loss. The TSW mode is changed to surface-wave-like (SWL) mode, and its effective cutoff frequency is defined. For the first higher-order mode of the lossy NRD guide, the cutoff frequency in dispersion curve is not the actual cutoff frequency because it resides in the reactive region. Thus, an effective cutoff frequency can also be defined for this mode. This mode is also a SWL mode. For the general higher-order modes of a lossy NRD guide, as dielectric loss increases, the spectral gap, which is known to be a nonphysical region in the lossless case, becomes narrower and changes to physical regions, such as forward and backward leaky wave regions. If the dielectric strip is sufficiently lossy, eventually, the spectral gap disappears. Even the TSW region also changes to a backward leaky wave region in a specific frequency range.