Conversion of radiation modes to trapped modes due to lateral inhomogeneity of a simple elevated layer

Abstract

It often happens that tropospheric ducts produced by elevated layers have a modal system in the microwave range consisting of many virtually lossless modes. These weakly attenuated modes have their energy localized about the layer and are insignificantly excited by sources far removed from the layer where the modal height gains are characteristically evanescent. Despite such an unfavorable geometry for direct excitation of these essentially loss‐free modes, they may be indirectly excited within the line of sight region by mode conversion processes associated with lateral inhomogeneity of the layer. In this paper an approximate theory is developed for the mode conversion produced within the line of sight region by large‐scale lateral inhomogeneity of an idealized, vertically homogeneous layer. The theory is applied to a 3‐GHz case study with the following results: (1) lateral inhomogeneity associated with layers of low elevation is more effective for converting to lossless modes than is lateral inhomogeneity associated with layers of high elevation; and (2) lateral inhomogeneity near the horizon is more effective than lateral inhomogeneity near the transmitter for converting to lossless modes.

Applying the Parabolic Equation to Tropospheric Groundwave Propagation: A review of recent achievements and significant milestones

“…In (1), n is the refractive index of the tropospheric homogeneous medium, while k is the wavenumber in a vacuum. The Helmholtz equation can be converted to (2) through paraxial approximation: …”

“…Mode theory requires a great deal of calculation, and RT generally fails to estimate the distribution of field strength precisely. Moreover, mode theory and RT have difficulty in modeling range-dependent environments, such that modeling radio-wave propagation IEEE AntEnnAs & ProPAgAtIon MAgAzInE j u n e 2 0 1 6 in the troposphere is also difficult [2]. Meanwhile, PE methods can efficiently solve these problems.…”