Editing Line-of-sight propagation (section)

== Impairments to line-of-sight propagation ==
[[File:Fresnel zone disrupted.png|thumb|right| Objects within the [[Fresnel zone]] can disturb line of sight propagation even if they do not block the geometric line between antennas.]]
Low-powered [[microwave]] transmitters can be foiled by tree branches, or even heavy rain or snow. The presence of objects not in the direct line-of-sight can cause diffraction effects that disrupt radio transmissions. For the best propagation, a volume known as the first [[Fresnel zone]] should be free of obstructions.

Reflected radiation from the [[ground plane|surface of the surrounding ground or salt water]] can also either cancel out or enhance the direct signal. This effect can be reduced by raising either or both antennas further from the ground: The reduction in loss achieved is known as ''height gain''.

See also [[Non-line-of-sight propagation]] for more on impairments in propagation.

It is important to take into account the curvature of the Earth for calculation of line-of-sight paths from maps, when a direct visual fix cannot be made. Designs for microwave formerly used {{frac|4|3}}&nbsp;Earth radius to compute clearances along the path.