FREQUENCY SELECTION.

TM 11-5820-919-12

Quickly assemble and erected

2-30

SITE SELECTION.

requires two vertical supports

(trees, etc.)

2-31. Site selection is particularly important for short-

range, ground wave communications. The existing

Good gain extended range (0 to

tactical situation will determine how much of a selection

300 miles and beyond)

is available. Generally, the best communications will be

obtained if the transmitting site is clear of all

QUARTER WAVE

Basically bi-directional

obstructions, located in an area with good ground

SLANT WIRE

conductivity (wet or plowed ground) and as high above

Good portability

ground as possible. Several locations in the same

general vicinity should be checked as possible sites. A

Quickly assembled and erected

slight change in antenna location can make a substantial

requires only one vertical support

difference in effectiveness.

Fair gain ; moderately extended

2-32. Avoid areas near sources of electrical

range (to 1000 miles)

interference such as power lines, radar sets, field

hospitals with X-ray machines, etc. Also avoid large

2-26.

FREQUENCY SELECTION.

structures having metallic content bridges, tanks, steel

reinforced buildings, etc.

If enemy jamming is a

2-27. The primary considerations when selecting an

possibility, the site should be selected so that a large

operating frequency are the distance b)between the

object or terrain feature is situated between the radio and

sending and receiving stations, and the time of day'.

the jamming source. This minimizes the effects of the

Refer to Appendix II, figures A24 and A25 which relate

jamming station and makes it more difficult to locate your

the relative effective less of communications for four sub-

transmitter using direction finding equipment.

band segments to the tile of day. These figures are

based on average propagation characteristics for the

2-33.

MODES OF OPERATION.

sub-ands and are intended for rise only as a general

guide. For detailed frequency planning over a specific

2-34. The radio set mode of operation is dependent

time period, propagation prediction charts are able from

upon tactical requirements. The following conditions are

several government agencies. ("Intermediate and Short

examples of the types of conditions which may be

Distance

Sky-Wave

e Propagation

Charts",

encountered.

Headquarters U.S. Army Communications Electronics

Engineering Installation Agency, Ft. Huachuca, Arizona

2-35. NORMAL COMMUNICATIONS. For two-way.

8.5613)

communications under normal circumstances, the radio

set should i be operated on _R. This selection permits

2-28. Another consideration selecting an operating

the radio to transmit and receive voice (or cw

frequency is the atmospheric condition. If rain or other

communications on any of the 280,000 frequency

precipitation is present, signals at the lower end of the hf

settings.

band may be degraded by atmospheric noise while the

higher frequencies will be affected to a lesser degree.

2-36. RADIO SILENCE. When field conditions

Therefore, higher frequencies will provide more

require that no radio transmissions be made, the node

effective communications in inclement weather.

selected should be V-RCV or DRC,V. These modes

render the radio set passive and inhibit the modulator in

2-29. The most useful phenomenon in radio wave

order to prevent transmission. When V-RCV is selected,

propagation is the "skip" effect. This effect occurs

the radio set receives voice (or cw) but cannot transmit.

l)because the layers of the ionosphere become ionized

When DRC: is selected, the radio set receives data

l)y the ultraviolet radiation of the sun. This level of

information but does not transmit data.

ionization determines the highest frequency for skip over

a given distance. Generally, the higher the ionization

2-37. Data Communication. When field conditions

level, the higher the fre(quency that can be used in "skip"

require transmitting and receiving data, the radio set

mode and therefore the broader the range of frequencies

mode selectetd is D-TR. This Mode permits two-way

available

for

rise

2-7