__________________________________________________________ Radio Wave Propagation
Figure 2-12. Surface Wave Propagation
2-52. As a surface wave passes over the ground, the wave induces a voltage
in the earth. The induced voltage takes energy away from the surface wave,
thereby weakening, or attenuating, the wave as it moves away from the
transmitting antenna. To reduce the attenuation, the amount of induced
voltage must be reduced. This is done by using vertically polarized waves
that minimize the extent to which the electric field of the wave is in contact
with the earth. When a surface wave is horizontally polarized, the electric
field of the wave is parallel with the surface of the earth and, therefore, is
constantly in contact with it. The wave is then completely attenuated within
a short distance from the transmitting site. On the other hand, when the
surface wave is vertically polarized, the electric field is perpendicular to the
earth and merely dips into and out of the earth's surface. For this reason,
vertical polarization is vastly superior to horizontal polarization for surface
wave propagation.
2-53. The attenuation that a surface wave undergoes because of induced
voltage also depends on the electrical properties of the terrain over which the
wave travels. The best type of surface is one that has good electrical
conductivity. The better the conductivity, the less the attenuation. Table 2-2
gives the relative conductivity of various Earth surfaces.
Table 2-2. Surface Conductivity
SURFACE
RELATIVE CONDUCTIVITY
Sea water
Good
Flat, loamy soil
Fair
Large bodies of fresh water
Fair
Rocky terrain
Poor
Desert
Poor
Jungle
Unusable
2-54. Another major factor in the attenuation of surface waves is frequency.
Recall from earlier discussions on wavelength that the higher the frequency
of a radio wave, the shorter its wavelength will be. These high frequencies,
with their shorter wavelengths, are not normally diffracted but are absorbed
by the earth at points relatively close to the transmitting site. You can
assume, therefore, that as the frequency of a surface wave is increased, the
more rapidly the surface wave will be absorbed, or attenuated, by the earth.
2-17
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