The atmospheric effects of interest for RF propagation are refraction/reflection, scattering, and absorption/attenuation.With the exception of refraction, these effects are all minimal below 30MHz. Between 30MHz and 1GHz, refraction/reflection is the primary concern. Above 1GHz or so, attenuation starts to be a significant factor and refraction/reflection becomes less of an issue except for nearly horizontal paths. Atmospheric multipath also starts to be observed above 1GHz and can cause extreme fading on terrestrial microwave links. The effects of interest for propagation analysis are in the troposphere and, to a limited extent, the tropopause.The stratosphere is well approximated as free-space.
Refractive and scattering effects of the atmosphere include:
• Refraction on horizontal paths resulting in alteration of the radio horizon due to ray curvature.
• Troposcatter, from localized fluctuations in the atmospheric refractive index, which can scatter electromagnetic waves.
• Temperature inversion, abrupt changes in the refractive index with height causing reflection.
• Ducting, where the refractive index is such that electromagnetic waves tend to follow the curvature of the earth.
These effects vary widely with altitude, geographic location, and weather conditions. The effects can permit beyond-the-horizon communication (or interference), or produce blockage and diffraction from terrain that appears to be below the line of sight and multipath fading.
Reference:
An excerpt from John S. Seybold, "Introduction to RF Propagation", John Wiley and Sons Inc., 2005.
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