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FM 24-18: Tactical Single-Channel Radio Communications Techniques


Section I. Siting Considerations

4-1. Site Selection

The reliability of radio communications depends largely on the selection of a good radio site. Since it is very difficult to select a site for a radio that satisfies all the technical, tactical, and security requirements, we compromise and select the best site of all those available. It is also good planning to select both a primary site and an alternate site. If, for some reason, radio communications cannot be established and maintained at the primary location, the radio equipment can be moved a short distance to the alternate site.


A radio station must be located in a position that will assure communications with all other stations with which it is to operate and yet maintain a degree of physical and communications security. To obtain efficiency of transmission and reception, the following factors should be considered.

Hills and mountains between stations normally limit the range of radio sets. In mountainous or hilly terrain, select positions relatively high on the slopes (fig 4-1). Avoid a location at the base of a cliff or in a deep ravine or valley (fig 4-2). For operation at frequencies above 30 MHz, and whenever possible, select a location that will allow line-of-sight communications. Try to avoid locations which provide the enemy with a jamming capability, visual sighting, or easy interception.

Figure 4-1. Good sites for radio communications.

Dry ground has high resistance and limits the range of the radio set. If possible, locate the station near moist ground, which has much less resistance. Water, and in particular salt water, greatly increases the distances that can be covered.

Trees with heavy foliage absorb radio waves, and leafy trees have more of an adverse effect than evergreens. Keep the antenna clear of all foliage and dense brush; but try to use available trees and shrubs for cover and concealment and for screening from enemy jamming.

Man-made Obstructions.

Do not select an antenna position in a tunnel or beneath an underpass or steel bridge (fig 4-2). Transmission and reception under these conditions are almost impossible because of high absorption of RF energy.

Figure 4-2. Poor sites for radio communications.

Buildings located between radio stations, particularly steel and reinforced concrete structures, hinder transmission and reception. You should, however, try to use buildings to camouflage your antenna from the enemy.

Avoid all types of pole wire lines, such as telephone, telegraph, and high-tension powerlines, when selecting a site for a radio station. Wire lines absorb power from radiating antennas located in their vicinity. They also introduce hum and noise interference in receiving antennas.

Avoid positions adjacent to heavily traveled roads and highways. In addition to the noise and confusion caused by tanks and trucks, ignition systems in these vehicles may cause electrical interference.

Do not locate battery-charging units and generators close to the radio station.

Do not locate radio stations close to each other.

Locate radio stations in relatively quiet areas. The copying of weak signals requires great concentration by the operator, and his or her attention should not be diverted by outside noises.

4-2. Tactical Requirements

Local Command Requirements.

Radio stations should be located some distance from the unit headquarters or command post that they serve. Thus, long-range enemy artillery fire, missiles, or aerial bombardment directed at the stations as a result of enemy direction finding will not strike the command post area.

Cover and Concealment.

The locations selected should provide the best cover and concealment possible, consistent with good transmission and reception. Perfect cover and concealment may impair communications. The permissible amount of impairment depends upon the range required, the power of the transmitter, the sensitivity of the receiver, the efficiency of the antenna system, and the nature of the terrain. When a radio is being used to communicate over a distance that is well under the maximum range, some sacrifice of communications efficiency can be made to permit better concealment of the radio from enemy observation.

Practical Considerations.

Manpack sets have sufficiently long cordage to permit operation from a concealed position (set and operator) while the antenna is mounted in the best position for communications.

Some sets can be controlled remotely from distances of 30 meters (100 feet) or more. The remotely controlled set can be set up in a relatively exposed position, if necessary, while the operator remains concealed.

Antennas of all radio sets must be mounted higher than ground level to permit normal communications.

Small tactical sets usually have whip antennas. These antennas are difficult to see from a distance, especially if they are not silhouetted against the sky. However, they have a 360° radiation pattern and are extremely vulnerable to enemy listening.

Avoid open crests of hills and mountains. A position protected from enemy fire just behind the crest gives better concealment and sometimes provides better communications.

All permanent and semipermanent positions should be properly camouflaged for protection from both aerial and ground observation. However, the antenna should not touch trees, brush, or the camouflage material.

Use one well-sited, broadband antenna to serve several radios. Appendix D describes a transceiver multiplexer (TD-1288-( )/GRC and TD-1289( )/GRC) that will allow several radio sets to be connected to a single antenna with no degradation of performance. This will allow quicker set-up and tear-down times as well as reducing camouflaging time and materials.

Local Communications.

At brigade level and higher, contact must be maintained between the radio station and the message center at all times, either by local messenger or field telephone. The station should also be readily accessible to the unit commander and to staff members.

Section II.
Transmitter Characteristics and
Operator's Skills

4-3. Importance to Reliable Communications

In addition to proper siting, the reliability of radio communications also depends upon the characteristics of the transmitted signal. The transmitter and its associated antenna form the initial step in the transfer of energy to a distant receiver.

4-4. Operating Frequency

Ground-wave transmission is used for most field radio communications. The range of the ground-wave becomes correspondingly shorter as the operating frequency of the transmitter is increased through the applicable portions of the medium-frequency band (300-3000 kHz) to the high-frequency band (3-30 MHz). When the transmitter is operating at frequencies above 30 MHz, its range is limited generally to slightly more than line of sight. For circuits using sky-wave propagation, the frequency selected depends on the geographic area, season, and time of day.

4-5. The Transmitting Antenna and Power Output

For maximum transfer of energy, the radiating antenna must be the proper length for the operating frequency. The local terrain determines, in part, the radiation pattern and, therefore, affects the directivity of the antenna and the possible range of the set in the desired direction. If possible, several variations in the physical position of the antenna should be tried to determine the best operating position to radiate the greatest amount of energy in the desired direction.

The range of a transmitter is proportional to the power radiated by its antenna. An increase in power output of the transmitter results in some increase in range, and a power decrease reduces the range. Under normal operating conditions, the transmitter should feed only enough power into the radiating antenna to establish reliable communications with the receiving station. Transmission of a signal more powerful than required is a breach of signal security, because the location of the transmitter may be more easily fixed by enemy direction-finding stations. Also, the signal may interfere with friendly stations operating on the same frequency.

    CAUTION: Do not detune a transmitter to reduce power output. Operation with a detuned power output stage can cause damage to the transmitter.

4-6. Transmitting Operator's Skills

The skill and technical abilities of the operators at the transmitter and receiver play important parts in obtaining the maximum range possible. The transmitter, output coupling, and antenna feeder circuits must be tuned correctly to obtain maximum power output. In addition, both the radiating antenna and the receiving antenna have to be constructed properly with regard to both electrical characteristics and conditions of the local terrain. The operator is the main defense against enemy interference. The skill of the operator can be the final determining factor in maintaining command and control communications in the face of enemy efforts to disrupt it.

Section III. Transmission Paths

4-7. Characteristics of the Transmission Path

After the radio signal leaves the transmitting antenna, you must be concerned with the amount of radiated energy that is lost along the transmission path. Selecting the transmission path with the least radiation loss ensures that more energy will be transferred to the receiving antenna.

4-8. Conductivity of the Terrain

The type of terrain between two field radio sets determines ground conductivity and affects the ground-wave. Flat prairie country has high conductivity and there is little absorption of the ground-wave by the earth.

Large bodies of water also have high conductivity. Mountainous, rugged, and broken country usually has low conductivity. In areas where there are large mineral deposits, and in deep ravines and valleys, the ground-wave may be absorbed completely by the soil.

4-9. Location of the Antenna

Large terrain obstructions between the transmitting and receiving stations reduce the reliability of radio transmissions. When you are selecting a site location, select high ground on which to erect the antenna.

4-10. Distance Between Stations

Low-power radio transmitters of limited range must work with receivers located within their range. Higher power transmitters with correspondingly stronger ground and sky waves may reach receiving stations with either or both of these waves, depending upon the distances between the transmitter and the receivers. When sky-wave propagation is used for communications, the skip distance must be considered. At times during the day or night on certain frequencies, the receiving station might lie within the skip zone and will not receive a signal from the transmitter.

Section IV. Receiver Characteristics
and Operator's Skills

4-11. Sensitivity and Selectivity of the Receiver

When the transmitted signal reaches the receiver location, it arrives at a much lower power level than when it left the transmitter. The receiver must efficiently process this relatively weak signal to provide maximum reliability of communications.

Sensitivity is a term used to describe how well a receiver responds to a weak signal at a given frequency. A receiver with high sensitivity is able to accept a very weak signal and amplify and process it to provide a usable output (an output that can be fully understood or that can be used to operate a teletypewriter or other devices). The principal factor that limits or lowers the sensitivity of a receiver is the noise generated by its own internal circuits (for example, tube and resistor noise).

Selectivity is a term used to describe how well a receiver is able to differentiate between a desired frequency and nearby frequencies.

4-12. The Receiving Antenna

In field radio communications, the type, location, and electrical characteristics of the receiving antenna are not as important as they are for the transmitting antenna. The receiving antenna must be of sufficient length; be properly coupled to the input of the receiver circuit; and, except in some cases for HF sky-wave propagation, it must have the same polarization as the transmitting antenna.

4-13. Interference

Interference from Natural Sources.

There are four kinds of radio interference you can expect from natural sources.

  • Atmospheric interference from electrical storms.

  • Solar and cosmic interference from eruptions on the Sun and other stars.

  • Precipitation static from charged particles (rain, sleet, snow, sand, smoke, or dust) in the atmosphere. Dry particles produce greater charges and more static than wet ones.

  • Fading from disturbances in the medium through which radio waves are propagated.

Interferences listed above, except the last, appear in electronic equipment as disturbing noise. This noise shows up as sound in headphones and loudspeakers and as errors in the output of other terminal equipment. There is interference from natural sources at most frequencies, but it diminishes considerably as the frequency is increased. At very high frequencies these disturbances have very little effect on reception.

Man-made Interference.

Man-made interference is generated by electrical devices (such as arc welders), leakage on high-tension lines, television sets, vehicle ignition systems, and sparking brushes on motors and generators and other rotating machines. This interference may be intentional or unintentional. If the interference is intense enough, it will drown out or obscure communications.

Although man-made interference is best eliminated or minimized at its source, some improvements can be made at the receiver. The operator can often make tuning adjustments which will enable the signal to be read through the interference. The use of a directional receiving antenna will eliminate some of the interference if the source is not in the same direction as the transmitting station. In addition, specially designed antenna lead-in wire may eliminate or minimize man-made interference that would normally be picked up on the lead-in wire.

Radio noise waves coming from a man-made source tend to be vertically polarized. Therefore, a horizontally polarized receiving antenna will generally receive less noise than a vertically polarized antenna.

Mutual Interference.

When one communications system interferes with another, or when one particular unit within a given system interferes with other units in the same system, there is a condition of mutual interference.

Mutual interference may appear in several forms: noise, cross talk, and/or harmonic interactions. Some of the common conditions that cause mutual interference are--

  • Spurious, undesired signals.

  • Spurious receiver responses.

  • Rf arcing in transmitters.

  • Impedance mismatch in the antenna system.

  • High-voltage pulse interference.

  • Improper frequency assignments.

Interference originates from many local and distant sources. Frequency relationships, geographical location, faulty adjustment of equipment, and improper operating techniques are important factors contributing to mutual interference. Equipment and systems that are potential generators of mutual interference are radar, radio, radio aids to navigation, and telephones.

4-14. Receiving operator's Skills

Most communications receivers have adjustable controls that, when properly used, are designed to minimize the adverse effects of fading, noise, and interference. The proficient use of these controls, such as the noise limiter and one of the various types of filters, often will permit satisfactory reception of many messages that would otherwise be lost when noise and interference become excessive. On the other hand, maladjustment of these controls, through either ignorance or carelessness, can cause unsatisfactory operation. Therefore, the skill and technical proficiency of the receiver operator play a most important part in radio communications.

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