Radio propagation is a term used to explain how radio waves behave when they are transmitted, or are propagated from one point on the Earth to another.

In free space, all electromagnetic waves (radio, X-rays, visual, etc) obey the inverse-square law which states that an electromagnetic wave's strength is proportional to 1/(x²), where x is the distance from the source. Doubling the distance from a transmitter means the strength is reduced to a quarter, and so on.

Radio propagation on Earth is not only affected by the inverse-square model, but by a number of other factors determined by its path from point to point. This path can be a direct line of sight path or an over-the-horizon (see also radio horizon) path aided by reflection from the ionosphere. A variety of phenomena make radio propagation more complex.

Radio waves at different frequencies propagate in different ways.

The Antenna

The beginning and end of a communication circuit is the antenna. The antenna will provide gain and directivity on both transmit and receive. The take off angle of the antenna is based on the type of antenna, the height of the antenna above ground, and the type of ground below and in front of the antenna. The take off angle will determine the angle of attack on the ionosphere, which will effect if and where the signal will be reflected by the ionosphere.

Ground wave Propagation

Ground Waves are radio waves that follow the curvature of the earth. Ground waves progress along the surface of the earth and must be vertically polarized to prevent short circuiting the electric electric field through the conductivity of the ground. Since the ground is not a perfect electrical conductor, ground waves are attenuated as they follow the earth’s surface. At low frequencies, ground losses are low and become lower with decreasing the frequency. The VLF LF frequencies are mostly used for military communications, especially with ships and submarines.

Line-of-Sight Propagation

Ground plane reflection effects are an important factor in VHF line of site propagation. The interference between the direct beam line-of-sight and the ground reflected beam often leads to an effective inverse-fourth-power law for ground-plane limited radiation.

Sky-Wave Propagation

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.

See also

• Fresnel zone
• Radio frequency
• Schumann resonance
• Cellular telephony
• Rayleigh fading
• E-skip
• FM DX
• Radio electronics