Table 2-1. Surface conductivity.
(5) A surface wave component, generally transmitted as a vertically polarized wave, remains
vertically polarized at appreciable distances from the antenna. As mentioned before, vertically polarized
waves do not lose power (attenuate) like horizontally polarized waves. The better the conducting
surface, the less energy lost. Since no surface is a perfect conductor, any loss retards the grounded edge
of a wave front, causing it to bend forward in the direction of travel so that successive wave fronts have
a forward tilt. The Earth's surface guides the wave, and the tilt has the effect of propagating the energy
in the direction of wave travel. Poor conducting surfaces cause a high loss of energy and greater tilt.
The result is total absorption of wave energy. As frequency increases, the angle of tilt increases. A 20
MHz signal propagating over sea water has a very small tilt (one degree). Over dry ground, the same
signal is tilted about 35 degrees.
(6) Frequency is a factor in surface wave attenuation.
(a) The higher a radio wave's frequency, 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. As a surface wave's frequency is increased, the more
rapidly the surface wave will be absorbed, and attenuated, by the Earth. Because of this loss by
absorption, the surface wave is impractical for long-distance transmissions with frequencies above 2
MHz.
(b) When a surface waves frequency is low enough to have a very long wavelength, the
Earth appears to be very small, and diffraction is sufficient for propagation well beyond the horizon. In
fact, by lowering the transmitting frequency into the VLF range and using very high-powered
transmitters, the surface wave can be propagated over great distances.
b. Space wave. Figure 2-4, page 2-6, depicts space wave propagation. The space wave follows
two distinct paths from transmitting antenna to receiving antenna--one through the air directly to the
receiving antenna (direct wave or path), and the other reflected from the ground to the receiving antenna
(ground-reflected wave or path).
2-5
SS0130
Previous Page
