09 Sep Ground Wave
Ground wave propagated signals are signals that generically speaking, travel along or close to the Earth’s surface on their path between the transmitting and receiving antennas. Ground wave signals are the “local” signals we receive — the signals that are always present at your location, day and night, regardless of any particular atmospheric or ionospheric conditions.
Ground wave propagation is a form of signal propagation where the signal travels over the surface of the ground, and as a result, it is used to provide regional coverage on the long and medium wavebands.
The ground wave can propagate a considerable distance over the earth’s surface particularly in the low frequency and medium frequency portion of the radio spectrum.
Ground wave radio signal propagation is ideal for relatively short distance propagation on these frequencies during the daytime. Sky-wave ionospheric propagation is not possible during the day because of the attenuation of the signals on these frequencies caused by the D region in the ionosphere. In view of this, lower-frequency radio communications stations need to rely on the ground-wave propagation to achieve their coverage.
Typically, what is referred to as a ground wave radio signal is made up of a number of constituent waves. If the antennas are in the line of sight then there will be a direct wave as well as a reflected signal. As the names suggest the direct signal is one that travels directly between the two antennas and is not affected by the locality. There will also be a reflected signal as the transmission will be reflected by a number of objects including the earth’s surface and any hills or large buildings that may be present. In addition to this, there is a surface wave. This tends to follow the curvature of the Earth and enables coverage beyond the horizon. It is the sum of all these components that is known as the ground wave. Beyond the horizon, the direct and reflected waves are blocked by the curvature of the Earth, and the signal is purely made up of the diffracted surface wave. It is for this reason that surface wave is commonly called ground wave propagation.
Effect of the Ground
The surface wave is also very dependent upon the nature of the ground over which the signal travels. Ground conductivity, terrain roughness and the dielectric constant all affect the signal attenuation. In addition to this, the ground penetration varies, becoming greater at lower frequencies, and this means that it is not just the surface conductivity that is of interest. At the higher frequencies this is not of great importance, but at lower frequencies, penetration means that ground strata down to 100 meters may have an effect.
Despite all these variables, it is found that terrain with good conductivity gives the best result. Thus soil type and the moisture content are of importance. Salty seawater is the best, and rich agricultural, or marshy land is also good. Dry sandy terrain and city centres are by far the worst. This means sea paths are optimum, although even these are subject to variations due to the roughness of the sea, resulting in path losses being slightly dependent upon the weather. It should also be noted that in view of the fact that signal penetration has an effect, the water table may have an effect dependent upon the frequency in use.
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