Radio propagation

From Academic Kids

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.

Since radio propagation is somewhat unpredictable, such services as emergency locator transmitters, in-flight communication with ocean-crossing aircraft, and some telelvision broadcasting have been moved to satellite transmitters. A satellite link, though expensive, can offer highly predictable and stable coverage of a given area.


Contents

Propagation modes

Radio waves at different frequencies propagate in different ways.

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Frequencies_Propagation.gif
Radio Frequencies and their Primary mode of Propagation

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 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.

Early commercial and professional radio services relied exclusively on long wave, low frequencies and ground-wave propagation. To prevent interefence with these services, amateur and experimental transmitters were restricted to the higher frequencies, felt to be useless since their ground-wave range was limited. On discovery of the other propagation modes possible at medium wave and short wave frequencies, these became much more useful for commercial and military purposes, and amateur experimentation was then confined only to authorized frequencies in the range.

Line-of-Sight Propagation

Ground plane reflection effects are an important factor in VHF line of sight 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

Sky-wave propagation is any of the modes that rely on reflection of radio waves from the ionosphere, which is made up of one or more ionized layers in the upper atmosphere. These layers are directly affected by the sun, and its varying activity (sunspot cycle) determines the utility of these modes. Forecasting of sky-wave modes is of considerable interest to amateur radio operators and commerical marine and aircraft communications, and also to shott-wave broadcasters.

Tropospheric Scattering

At VHF and higher frequencies, the atmosphere at a height of around 6 miles (10 kilometres) can scatter some of the normally line-of sight beam of radio frequency energy back toward the ground, allowing over-the-horizon communication between stations as far as 500 miles (800 km) apart.

A special form of tropo scattering relies on reflecting radio waves off the intensely ionized regions generated by meteors. While this mode is very short-duration, often only a couple of seconds per event, it allows remote stations to communicate to a base that may be hundreds of miles (km) away, without the expense and power input required for a satellite link.

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

References

Larry D. Wolfgang et. al, (ed), The ARRL Handbook for Radio Amateurs, Sixty-Eighth Edition , (1991), ARRL, Newington CT USA ISBN 0872591689da:Radioudbredelse

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