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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.
- Low frequency waves (below 300 kHz) propagate near the ground, and are sometimes referred to as ground waves. This effect is caused by diffraction by the shape of the Earth and guiding by the D layer in the ionosphere.
- High frequency waves (300 - 1000 kHz) are not guided by the ground like lower frequency waves are. The higher the frquency of the wave, the less likely it is to propagate near the ground.
- Shortwave frequencies (in the 3 MHz -30 MHz range) tend to reflect off the ionosphere (from the E, or F layers), back to earth, and sometimes back to ionophere and to earth again. This enables shortwave frequencies to travel long distances. This type of propagation is known as skywave.
- Very high frequency (VHF) waves (30 Mhz - 100 MHz) are not guided by the ground at all, and generally do not reflect off the ionosphere, except is times of extreme solar activity. These transmissions continue out into space.
- Extremely high frequency (EHF) waves, higher than VHF (100 Mhz - 1 GHz) tend to move at line of sight, and are useful for point to point and satellite communications.
- Above 1 GHZ are the microwave and optical waves. These are also useful for point to point and satellite communications.
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.
See also
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