摘要: Radio wave propagation in the VHF to EHF bands at low elevation angles and nearthe earth's surface is almost always affected by refraction. This paper details these effects and the various methods used to model them, from simple e... 展开 Radio wave propagation in the VHF to EHF bands at low elevation angles and nearthe earth's surface is almost always affected by refraction. This paper details these effects and the various methods used to model them, from simple effective-earth-radius factors for standard refraction to parabolic-equation methods for range-dependent ducting environments. Part A introduces and discusses refraction-related propagation mechanisms and their effects. and Part B describes propagation models and uses examples from propagation assessment systems and other propagation software to illustrate many of the effects. Frequencies from about 30MHz to 100GHz are considered. Refraction and Snell's law are discussed and standard and nonstandard propagation mechanisms are defined. Standard propagation mechanisms are free-space spreading, reflection. forward scatter from rough surfaces, spherical earth divergence, optical interference, diffraction, tropospheric scatter, and absorption. Nonstandard mechanisms discussed are evaporation ducts, surface-based ducts from elevated trapping layers. elevated ducts, lateral inhomogeneity, and terrain effects. To establish the significance of nonstandard propagation effects, some statistics on the occurrence of ducting around the world are presented. Specifically. statistics on the strength of evaporation ducts for selected areas and on the frequency of occurrence of surface-based ducts are presented. (MM). 收起