Anomalous propagation

Anomalous propagation (sometimes shortened to Anoprop) includes different forms of electromagnetic (EM) wave propagation that are not encountered in a standard atmosphere due to a non standard distribution of temperature and humidity with height in the atmosphere. While technically the term includes propagation with larger losses than in standard atmosphere, in practical applications it is most often meant to refer to cases when signal propagates beyond normal radio horizon.

The first assumption is that an electromagnetic wave is moving through air that cools down at a standard rate with height in the troposphere. Any variation to this stratification of temperatures will modify the path followed by the wave. This can be separated into super and under refraction:

It is very common to have temperature inversions forming near the ground, for instance air cooling at night while remaining warm aloft. This happens equally aloft when a warm and dry airmass overrides a cooler one, like in the subsidence aloft cause by a high pressure intensifying. The index of refraction of air increases in both cases and the EM wave bends toward the ground instead of continuing upward.

On surface-base inversion, the beam will eventually hit the ground and a part of it can be reflected back toward the emitter. In upper air inversion, the bending will be limited to the layer involved but the bending will extend the path of the beam, possibly beyond the usual transmission horizon.

The extreme of this problem is when the inversion is very strong and shallow, the EM wave is trapped within the inversion layer. The beam will bounce many times inside the layer as within a waveguide. In surface-based ducting, the beam will hit the ground many times, causing return echoes at regular distances toward the emitter. In elevated ducts, the transmission can be extended to very large distances.

On the other hand, if the air is unstable and cools faster than the standard atmosphere with height, the wave is higher than expected as can miss the intended receiver.

Other ways anomalous propagation is recorded is by troposcatters causing irregularities in the troposphere, scattering due to meteors, refraction in the ionized regions and layers of the ionosphere, and reflection from the ionosphere.

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Wikipedia