refraction mode is
concerned.
Yet, with
sufficient power,
transmissions can be scattered from the ionosphere back to earth at
distances up to 1,250 miles and at frequencies of 60 megahertz or
higher.
This scattering process occurs in the lower part of the
(E-layer)
and
involves
the
interaction
of
the
electromagnetic wave with the inhomogeneities of the layer.
The
origin of these inhomogeneities is uncertain, although turbulence and
influence of ionization caused by meteors have been implicated. The
energy scattered is relatively weak and variable, but, with the use
of high power and high-gain antennas, an exceptionally reliable
service can be established in this mode. It is especially noteworthy
that this mode functions in polar regions at the time when the usual
refraction mode used in ionospheric circuits becomes erratic.
The
primary deficiencies are very narrow emission bandwidth (on the order
of 5 kilohertz), high investment cost, and high interference
potential due to both high transmitted power and the use of
frequencies at which refraction mode can frequently be effective.
The ionospheric scatter mode is no longer used for military
communications.
b. An
important
scatter
mode
involves
scattering
from
irregularities of the neutral lower atmosphere. Communications using
so-called tropospheric scatter is possible for distance from 60 to
750 miles in the frequency range from 100 to 9000 megahertz. As was
true for ionospheric scatter, high-gain antennas and high transmitted
powers are required to overcome high transmission losses.
These
transmission losses do not negate this mode since an equivalent line-
of-sight path may not be possible, as for circuits spanning water or
inaccessible land areas. The consideration frequently outweighs the
penalty paid for the increased transmission loss.
c. The tropospheric scatter mode is also distinguished by rapid
fading of the transmitted signal.
To cope with fading, special
reception techniques are used.
With efficient modulation schemes,
the tropospheric scatter mode can be exceptionally reliable and it
has sufficient emission bandwidth to support low-grade television
transmission.
Proper antenna design, and consideration of other
users in terms of the space and the spectrum of interest, can reduce
interference to manageable proportions. This mode has been utilized
for communications with elevated terminals such as aircraft.
42. Surface Wave Propagation.
a. The importance of surface waves at certain frequencies has
been discussed as they relate to ground-based, line-of-sight
propagation.
At frequencies in the broadcast band, e.g., 1000
kilohertz, the total transmitted field that is received by line-of-
sight cannot be accounted for by the direct component alone or
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