Figure 3-1. Ground-Wave and Sky-Wave Propagation From Vertical Antenna.

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,

figure 3-7, the lowest MUF is 3.5 MHz, which

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 (µv/m); lower decile, -10 dB>l µv/m;

ducted, using 0500 hours local time because of the

and upper decile +12 dB >1 µv/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