TM 11-5985-379-14&P
Figure 3-1.
Ground-Wave and Sky-Wave Propagation From Vertical Antenna.
tion Agency Technical Report No. 5, Revised June,
w a v e l e n g t h . Such radiation characteristics are
1961, ASTIA AD 262 209. )
omnidirectional in azimuth and provide a l-hop
range of about 300 miles. The antenna gain varies
T h e first step of the analysis determined the
mainly with the height of the antenna above ground.
operating frequency for the conditions cited. Fig-
Because it is highly desirable to have minimum
ure 3-7 shows the maximum usable frequencies
h e i g h t and weight for tactical antennas, the
(MU F). This data is taken from Central Radio
immediate problem becomes one of determining
Propagation Laboratory, Ionospheric Predictions.
t h e minimum effective antenna height required.
National Bureau of Standards (now Environmental
S c i e n c e Service Administration). As shown in
In order to determine the required antenna height,
a minimum acceptable level of performance is
occurred at 0500 local time.
e s t a b l i s h e d as necessary to permit communi-
c a t i o n s in South Vietnam. Figure 3-6 specifies
The next step determines the noise levels. These
the conditions for which communications are
w e r e computed, using CCIR Report 322. Noise
required. June 1965 was picked because of the low
levels at 3.5 MHz for the above location and time
sunspot number and consequently, the requirement
were found to be: median, 1 dB above 1 microvolt
for low-operating frequency. An analysis was con-
per meter (v/m); lower decile, -10 dB>l v/m;
ducted, using 0500 hours local time because of the
and upper decile +12 dB >1 v/m. The required
difficulty in communicating in the hours just before
dawn. The AN/PRC-47 radio was selected because
s i g n a l level (field strength) is then the median
of its extensive use. A service factor of 28-dB
n o i s e level plus the service factor, or 29 dB
s i g n a l - t o - m e d i a n noise was selected to provide
above 1 microvolt/meter. An additional 4 dB is
a d d e d to this figure to ensure field strength
90 percent intelligibility, 50 percent of the time.
down to 45 degrees above the horizon. Figure 3-8
(For example, see Median Signal Power Required
shows the required field strength to be 33 dB above
f o r Reception o f Radio Transmission in the
1 microvolt/meter.
Presence of Noise, US Army Signal Radio Propaga-
3-2
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