Figure 14.
Idealized sketch of the stratified nature of the
atmosphere.
c. Another important feature of the earth's environment is its
surface, 71 percent of which is covered by relatively higher
conductive water. This surface forms a lower boundary to the earth-
ionosphere cavity, the existence and dimensions of which are of
profound significance if the entire subject area of radio frequency
propagation is to be understood.
The earth's magnetic field at
certain frequencies is also a significant factor in propagation. For
example, fading of radio transmissions at 20 megahertz from an
orbiting satellite is due to the combined effect of the ionosphere
and the earth's magnetic field.
Furthermore, the orientation and
strength of the earth's magnetic field make a significant factor in
propagation at least at certain frequencies.
d. Land surface is also a factor in the use of ultrahigh
frequency and other higher spectrum portions.
Widely varying
elevations of the earth may limit the line-of-sight needed for
systems such as microwave or radar.
On the other hand, knife-edge
defraction from rough surface areas may be employed to advantage in
some cases.
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