a. Surface wave.
(1) As shown in Figure 2-3, the
surface wave travels along the surface of the
ground. A surface wave flows the curvature of
the Earth due to the process of diffraction. As a
surface wave hits an object in its path, it tends to
curve (bend) around the object.
Figure 2-3. Surface wave propagation.
(2) As a surface wave passes over the ground, it induces voltage into Earth. The induced
voltage takes energy away from the surface wave, thereby weakening (attenuating) the wave as it moves
away from the transmitting antenna. To reduce attenuation, the amount of induced voltage must be
reduced. This is done by using vertically polarized waves, which minimize the extent to which the
electric field of the wave is in contact with the Earth. When the wave is horizontally polarized, the
wave's electric field is parallel with the surface of the Earth and constantly in contact with it. As a
transmission is made, the signal (horizontally polarized wave) is completely attenuated within a short
distance from the transmitting site. Conversely, a vertically polarized surface wave has its electric field
perpendicular to the Earth and merely dips onto and off of the Earth's surface. Because of the lower
signal loss, vertical polarization is vastly superior to horizontal polarization for surface wave
propagation.
(3) The amount of attenuation that a surface wave undergoes due to the induced voltage in
the Earth also depends, to a considerable extent, on the electrical properties of the terrain over which the
wave travels. The best type of surface is one which has good electrical conductivity. The better the
conductivity, the less attenuation and the better the propagation. Table 2-1 shows the relative
conductivity of various surfaces of the Earth.
(4) Each type of terrain shown has a different degree of conductivity-the ease at which radio
waves propagate. Salt water has the best degree of conductivity. Because salt enhances conductivity, it
can be used in the field when grounding a communications assemblage or generator. Moist land
surfaces provide fair conductivity, while dry terrain provides poor conductivity and thus, impedes wave
propagation. Jungle terrain is the worst environment, as the jungle vegetation absorbs the radio waves,
reducing transmission range.
SS0130
2-4
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