refraction mode is
transmissions can be scattered from the ionosphere back to earth at
distances up to 1,250 miles and at frequencies of 60 megahertz or
This scattering process occurs in the lower part of the
electromagnetic wave with the inhomogeneities of the layer.
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.
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
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
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
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
42. Surface Wave Propagation.
been discussed as they relate to ground-based, line-of-sight
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