TM 11-5985-379-14&P
3
section
theory of operation
3.1 GENERAL
f o r communications is not the value of signal
level, but the signal-to-noise ratio,
In the past, most short-range hf communication
circuits used vertical whip antennas. With these
A s an example of typical noise encountered,
antennas, c o m m u n i c a t i o n s are achieved on very
noise as a function of time of day at the frequency
short ranges by ground-wave (surface-wave pro-
of 2-MHz and 1-kHz bandwidth for Vietnam in
pagation), and longer paths are achieved by sky-
July of 1967 (reference CCIR Report 322). Also
wave propagation. S e e figure 3-1. An inherent
c h a r a c t e r i s t i c of radio-wave propagation, using
16-foot whip) signal levels of 10, 25, and 40 dB
whip antennas, is the zone of silence (skip zone)
above 1 microvolt per meter with corresponding
between the point where the ground-wave signal
distances taken from figure 3-2 for various
becomes unusable and the sky-wave signal starts
types of soil conditions. Good ground-wave com-
t o become usable., ( F o r example, see Radio
munications are expected at 25 miles at any time
A m a t e u r ' s Handbook, Ionospheric Propagation,
of the day for good ground conditions, and the range
most editions). Depending upon terrain, ground
m a y be as much as 100 miles for a couple of
conductivity, o p e r a t i n g frequency, noise levels,
hours at midday. However, if the environment is
etc., g r o u n d - w a v e signals (1-kW transmitters)
dense jungle instead of good ground, the maxi-
a r e usable up to about 70 miles over average
mum ground-wave communication range is 1 mile
soil. Also, minimum distances for sky-wave paths,
or less,
using whips, a r e generally 200 miles (E-layer)
during the day and 400 miles (F-layer) at night.
F r o m the above discussion, it is clear that a
skip zone is present when vertical whip antennas
While the skip zone, d e s c r i b e d above, severely
are used. The extent of the skip zone is dependent
limits the usefulness of whip antennas for short-
upon soil conditions. For average environments,
r a n g e communications, conditions become even
the skip zone lies between 70 and 200/300 miles;
worse in an adverse environment, such as a hilly
however, in extreme environments, it may include
or jungle-type terrain. This occurs because of the
the range from 1 to 200/300 miles.
restricted range of ground-wave signals in these
environments.
T h e skip zone is of a very critical range for
m o s t tactical communication systems including
manpack, vehicular, a n d shelter equipments,
ground-wave propagation, as a function of distance,
Most tactical requirements necessitate good com-
for several types of soil conditions and for dense
m u n i c a t i o n s in the 0- to 300-mile range, If hf
jungle environments. These signals assume l-kW
transmitters, operating frequency of 2 MHz (atten-
communications are to be effective in this range,
d i f f e r e n t antennas and propagation modes are
uation is lowest for lower hf frequencies), and a
necessary.
16-foot whip with a minimum-type ground screen,
such as a shelter-mounted whip. Signal levels are
T h e solution to the short-range communication
p l o t t e d in dB above 1 microvolt per meter.
problem is the use of sky-wave instead of
ground-wave propagation on the short paths.
The inverse distance field is the field that would
T h i s r e q u i r e s r a d i a t i o n from the antenna at
b e present if there were no attenuation due to
very high elevation angles (near vertical incid-
the surface over which the signal is propagated.
T h e strongest practical signals occur over sea-
ence) as shown in figure 3-4 arid 3-5, Radiation
water. As the soil conductivity decreases or as the
characteristics of this type are achieved through
t h e use of horizontal antennas mounted above
foliage increases, the signal strength at a distance
d e c r e a s e s rapidly. The important consideration
ground up to a height of about one-quarter
3-1
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