2-17
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
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. Because of this loss by attenuation, the surface wave is impractical for long-
distance transmissions at frequencies above 2 megahertz. On the other hand, when the frequency of a
surface wave 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 very low frequency (vlf) range and using very high-powered transmitters, the surface
wave can be propagated great distances. The Navy's extremely high-powered vlf transmitters are actually
capable of transmitting surface wave signals around the Earth and can provide coverage to naval units
operating anywhere at sea.
SPACE WAVE.—The space wave follows two distinct paths from the transmitting antenna to the
receiving antenna—one through the air directly to the receiving antenna, the other reflected from the
ground to the receiving antenna. This is illustrated in figure 2-13. The primary path of the space wave is
directly from the transmitting antenna to the receiving antenna. So, the receiving antenna must be located
within the radio horizon of the transmitting antenna. Because space waves are refracted slightly, even
when propagated through the troposphere, the radio horizon is actually about one-third farther than the
line-of-sight or natural horizon.
