Section 3. Theory of operation

TM 11-5985-379-14&P

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

figure 3-3 shows the upper and lower limits of

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

figure 3-3 depicts (for a l-kW transmitter and a

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

Figure 3-2 shows the expected signal levels for

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