Line-of-sight propagation

The rays or waves may be diffracted, refracted, reflected, or absorbed by the atmosphere and obstructions with material and generally cannot travel over the horizon or behind obstacles.

Additionally, frequencies in the shortwave bands between approximately 1 and 30 MHz, can be refracted back to Earth by the ionosphere, called skywave or "skip" propagation, thus giving radio transmissions in this range a potentially global reach.

Broadcast FM radio, at comparatively low frequencies of around 100 MHz, are less affected by the presence of buildings and forests.

Reflected radiation from the surface of the surrounding ground or salt water can also either cancel out or enhance the direct signal.

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.

For mobile phone services, these problems are tackled using: A Faraday cage is composed of a conductor that completely surrounds an area on all sides, top, and bottom.

For example, mobile telephone signals are blocked in windowless metal enclosures that approximate a Faraday cage, such as elevator cabins, and parts of trains, cars, and ships.

The radio horizon is the locus of points at which direct rays from an antenna are tangential to the surface of the Earth.

Radio wave propagation is affected by atmospheric conditions, ionospheric absorption, and the presence of obstructions, for example mountains or trees.

[citation needed] Assuming a perfect sphere with no terrain irregularity, the distance to the horizon from a high altitude transmitter (i.e., line of sight) can readily be calculated.

Usually, a factor k is used in the equation above, modified to be k > 1 means geometrically reduced bulge and a longer service range.

[6] For example, in normal weather conditions, the service range of a station at an altitude of 1500 m with respect to receivers at sea level can be found as,