Since antennas are usually sensitive to the orientation of the
electric field, fading is common to this mode of propagation. Fading
also
results
from
multipath
of
direct
and
ionospherically reflected waves and from variation at the effective
point of reflection, which in turn varies in the region of ground
return.
d. In as much as the characteristics of the ionospheric
propagation
path
depend
upon
solar
ionospheric
propagation is affected by diurnal (day-to-night) and seasonal
changes. Thus, fixed point-to-point communications circuits must be
adaptable (i.e., change operating frequency) if uninterrupted service
is desired. Occasionally sudden ionospheric disturbances, caused by
solar events such as flares, disrupt communications in this mode both
by introducing a prohibitive temporary absorption and by distorting
the transmission through an excessive multipath condition. The rate
of solar events is closely linked to the number of sunspots, and this
appears to show a cyclic regularity of approximately 11 years.
For
example, while 1969 was a year of maximum sunspot activity, 1975, was
the year of the quiet sun, was a year of minimum sunspot activity.
Finally, there are regions near the magnetic poles in which the
ionosphere is highly disturbed as a result of the effects of cosmic
particles and their interaction with the earth's magnetic field.
Communications in these regions, characterized by the phenomenon of
visible aurora, is highly erratic and frequently impossible.
e. Despite the complexity of the ionosphere, its use in
communications has been and still is quite attractive. The advent of
line-of-sight satellite communications, however, is certain to
diminish the once dominant role of the ionosphere for long-range
communications.
Even with the many embellishments recently used to
improve ionospheric communications, this mode lacks the reliability
and emission bandwidth capability necessary to compete successfully
with the newer entries in the field.
f. Transhorizon
propagation
is
also
possible
because
of
refractive bending in the troposphere. The rule that the radio line-
of-sight is equivalent to an optical path between terminals on a
sphere four-thirds the size of the earth, is apparently as good an
approximation as any to the longer line-of-sight distance of radio
transmission paths over optical transmission paths.
On occasion,
reliable transhorizon transmission is found at very high frequency
and ultrahigh frequencies as a result of electromagnetic wave
refraction and trapping atmospheric ducts.
41. Scatter Mode.
a. Propagation in a
scatter mode occurs at frequencies at
which the ionosphere is
essentially transparent insofar as the
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