Ground Wave Propagation

By Jairam Sankar

31810
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Ground wave propagation is particularly important on the LF and MF portion of the radio spectrum. Ground wave radio propagation is used to provide relatively local radio communications coverage, especially by radio broadcast stations that require to cover a particular locality.

Ground wave propagation of radio signal 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, radio communications stations need to rely on the ground-wave propagation to achieve their coverage.

Concept of ground wave propagation

Propagation of EM wave near earth surface (including troposphere).When the Transmit and Receive antenna are on earth there can be multiple paths for communication. If the Transmit and Receive antenna are in line of sight (LOS) then direct path exist. The propagating wave is called direct wave. When EM wave encounters an interface between two dissimilar media, a part of energy will flow along the interface known as Surface Wave. At LF and MF this is predominant mode of energy transfer for vertically polarized radiation. Interaction with the objects on ground will manifest as, Reflection, Refraction, Diffraction, Scattering. Waves are collectively called as Space Wave.

Ground Reflection

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In LOS model, the assumption is that there is only one path for propagation of EM Wave from transmit antenna to receive antenna. The two antennas are kept in free space with no other objects intersecting radiation from transmitter antenna. If two antennas are situated close the ground due to discontinuity in the electrical properties at the air ground interface any wave that falls on the ground is reflected. The amount of reflection depending on factors like angle of incidence, Polarization of wave, Electrical Properties of the Ground i.e conductivity and dielectric constant, the frequency of the propagating wave. Thus, the field at any point above the ground is a vector sum of the fields due to the direct and the reflected waves.

Surface Wave

It is a wave (whether mechanical or electromagnetic) that travels along a surface or an interface between two mediums that have different densities (for mechanical wave) or refraction indexes (for electromagnetic wave) and keeps being trapped between the two mediums. In other words, keeps propagating along the surface unless a discontinuity appears.

Examples:

1) In the case of mechanical waves, they are they waves you see generating on the surface of the water when some energy source (animal) swims above the water surface.

2) In the case of electromagnetic waves, they travel along a real (materials with relative permittivities > 1) or artificial substrates (using a series or metal disks or rods as in Yagi-Uda antennas) without getting radiated, unless a discontinuity appears where reflection and diffraction happens at the edges, giving rise to radiation. Sometimes, the discontinuity is engineered such that the patterns are shaped accordingly.

Effects of 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 sea water is the best, and rich agricultural, or marshy land is also good. Dry sandy terrain and city centers 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 on 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.


Feel interested in ground wave propagation? Check out other basic electronics concepts in the learning corner.

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