Radio propagation

Radio propagation is a term used to explain how radio waves behave when they are transmitted, or are propagated.

In free space, all electromagnetic waves (thus including radio) obey the inverse-square law, that is an electromagnetic wave's strength is proportional to 1/(x²), where x is the distance from the source. So doubling the distance from a transmitter means the strength is reduced to a quarter, and so on.

Radio propagation on Earth only rarely fits the simple inverse-square model. A variety of phenomena make radio propagation more complex.

Radio waves at different frequencies propagate differently.

Low frequency waves (around 300kHz) propagate near the ground, and are sometimes referred to as ground waves, as opposed to sky waves as higher frequency waves do. This effect is caused by diffraction by the shape of the Earth.
High frequency waves (around 1000kHz) do not usually tend to hug the ground like lower frequency waves do, and this behaviour lessens the higher the frequency is. Shortwave frequencies (in the 3MHz-30MHz range) tend to bounce off the ionosphere and reflect back to earth and back again, and this enables shortwave frequencies to travel long distances.
Very high frequency waves (around 100MHz) do not generally hug the ground at all, and do not bounce off the ionosphere, and these transmissions continue out into space.
Extremely high frequency waves, higher than VHF (around 1GHz) tend to move at line of sight, and are useful for communications

On earth the last two reach only until the line-of-sight horizon, which depends on the height of the transmitter antenna.

Table of contents

1 Ground plane reflection
2 Diffraction
3 Absorption

Ground plane reflection

Ground plane reflection effects are an important factor in radio propagation. Although free-space radio waves follow the inverse-square law, the interference between the direct beam and the reflected beam often leads to an effective inverse-fourth-power law for ground-plane limited radiation.

A number of formulae have been developed to take into account the interaction of horizon, diffraction and ground-plane effects. In general, transmitters which are higher tend to have higher effective range, which explains the use of tall antenna masts.

Diffraction

Diffraction phenomena by small obstacles are also important at high frequencies. Signals for urban cellular telephony tend to be dominated by ground-plane effects as they travel over the rooftops of the urban environment. They then diffract over roof edges into the street, where multipath propagation, absorption and diffraction phenomena dominate.

Absorption

Low frequency radio waves travel easily through brick and stone. As the frequency rises, absorption effects become more important.

In addition, at microwave or higher frequencies, absorption by molecular resonance in the atmosphere is a major factor in radio propagation. For example, in the 58 - 60 GHz band, there is a major absorption peak which makes this band useless for long-distance use. Beyond around 400 GHz, the Earth's atmosphere is effectively opaque to radio waves.

Heavy rain and snow also present major challenges to microwave reception.