( US Army Strategic Communications Command ... - Survival Books
( US Army Strategic Communications Command ... - Survival Books
( US Army Strategic Communications Command ... - Survival Books
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FM 11-23.<br />
b. For radio frequencies up to about 30 mega- however, the wavelength decreases and the effect<br />
hertz (MHz), the wavelength of the transmitted of these local turbulences, which can be compared<br />
signal is large compared to the size of the turbul- to small whirlpools and bubbles in a fast moving<br />
ences encountered, and there is little effect on the stream, become increasingly important. They are<br />
transmitted signal. As the frequency is increased, the principal factors in tropo transmission.<br />
Section II.<br />
TROPO SCATTER<br />
B-4. Propagation<br />
a. Scatter Volume Turbulences. The magnitude<br />
a. As a radio wave passing through the tronumber<br />
of turbulences<br />
of the received signal depends, in part, on the<br />
poshere encounters a<br />
the transmitted<br />
turbulence,<br />
energy en-<br />
a small amount<br />
of energy is scattered away from the main<br />
counters<br />
beam<br />
in the common scatter volume. Only a<br />
of radiation. The scatter effect is the same as if small<br />
ergy<br />
portion of the resultant<br />
is directed<br />
scatter beam en-<br />
each turbulence received the signal and then rer- ergy is directed<br />
towards<br />
towards the<br />
the receiving<br />
receiving<br />
station.<br />
station.<br />
adiated it. Although the word scatter implies that<br />
aditedit.<br />
Thus, thelarger<br />
Although<br />
the scatter volume,<br />
the number<br />
the greater<br />
the spreading of the reradiated<br />
of turbulences<br />
energy<br />
encountered<br />
is<br />
and the<br />
equally<br />
probable in in all directions,<br />
directions, scattering occurs<br />
occurs<br />
more refraction of useful energy occurs. The size<br />
chiefly<br />
chiefly in<br />
in a forward<br />
forward direction<br />
direction<br />
andtherefore<br />
and, therefore<br />
the<br />
the<br />
gof the scatter volume is determined by the scatter<br />
term forward scatter is sometimes used when re- angle. As the scatter angle is increased<br />
ferring to tropo propagation. A very small portion<br />
rapidly<br />
a<br />
because<br />
moun forward t of<br />
most<br />
scatter<br />
of the<br />
ed energy<br />
of the transmitted radio wave energy reaches<br />
transmitted<br />
the<br />
energy<br />
disof the transmitted radio termincontinues wave energy reaches the<br />
out into space. Also, the amount of<br />
distant radio terminal. scattering required to return the signal to earth<br />
b. The intersection of two radio beams aimed at (receiving antenna) is increased. The distance bea<br />
common point within the troposphere forms a tween terminal stations also affects the height at<br />
common scatter volume around the intersected which the scatter volume occurs. As the circuit<br />
area (fig. B-2). The size of the scatter volume is distance is increased, the height of the scatter<br />
determined mainly by the scatter angle formed volume is also increased. Therefore, the received<br />
by the intersecting beams.<br />
signal level decreases as the circuit distance increases,<br />
if power output and other parameters re-<br />
B-5. Signal Reception<br />
main constant.<br />
The signal level received by the distance station a. Atmospheric Variations. Since tropo propais<br />
determined by several factors. Changes in any gation depends on turbulences in the atmosphere,<br />
of these factors will cause fluctuations in the changes in atmospheric conditions will affect the<br />
strength of received signals.<br />
received signal level. Daily and seasonal variations<br />
of temperature and air pressure cause longterm<br />
fading. In addition to long-term fading, the<br />
tropo signal is also characterized by very rapid<br />
fading. This rapid fading is caused by propagation<br />
over multiple paths from the scatter volume.<br />
Since the turbulent condition is constantly chang-<br />
COMMON<br />
ing, the path lengths and levels of the individual<br />
SCATTER<br />
VOLUMSTE<br />
signals are also changing. However, the composite<br />
signal level at any one time is the sum of all<br />
the signals received from each of the turbulences<br />
SCATTER ANGLE in the volume and although rapid fading may<br />
BEA M-1,, / nu / take place, no complete fadeout occurs.<br />
B-2<br />
FM 11-367-8-2<br />
Figure B-2. Geometry of tropospheric scattering.<br />
cM2 . Propagation Efficiency. A characteristic of<br />
tropo propagation is its relatively low efficiency.<br />
The scatter volume may be pictured as a very<br />
inefficient relay station, located above the horizon,<br />
receiving transmitted energy and reradiating<br />
it. Since most of the transmitted energy is not
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