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.