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"Educational Note: Understanding VHF Propagation"

Beyond line-of-sight propagation: Learn the basics of the common and uncommon modes of VHF communications and some of the viable applications and use cases of these modes

Very High Frequency (VHF) is widely used in broadcasting, in voice and data communications, in avionics, and even in some types of radar. Although it is commonly believed that VHF propagation is mostly "line of sight," true line of sight propagation is in fact relatively rare in terrestrial applications. Signals at VHF normally experience refraction, reflection, diffraction, and scattering, with these effects often increasing with distance.

In addition to these common propagation modes, there are several uncommon modes at VHF. This educational note introduces the four most important of these modes: tropospheric ducting, sporadic E, meteor burst, and "Earth-Moon-Earth" (EME). Tropospheric ducting and sporadic E are not commonly used for communications due to their unpredictable appearance, but they are both sufficiently common that they can create issues with regard to interference, direction finding, and signals intelligence applications. Other modes, such as meteor burst and EME, can however provide reliable long-distance communication at VHF, and thus represent potential backup communications methods at the regional or global level.

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