FM 24-18: Tactical Single-Channel Radio Communications Techniques
RADIO OPERATIONS UNDER UNUSUAL CONDITIONS
Section I. Operations in Arcticlike Areas
7-1. Capabilities and Limitations
Single-channel radio equipment has certain capabilities and limitations that must be carefully considered when operating in extremely cold areas. However, in spite of significant limitations, radio is the normal means of communications in such areas.
One of the most important capabilities of radio in arcticlike areas is its versatility. Vehicular-mounted radios can be moved relatively easy to almost any point where it is possible to install a command headquarters. Smaller, manpacked radios can be carried to any point accessible by foot or aircraft.
A limitation on radio communications that radio operators must expect in extremely cold areas is interference by ionospheric disturbances. These disturbances, known as ionospheric storms, have a definite degrading effect on sky wave propagation. Moreover, either the storms or the auroral (for example, Northern Lights) activity can cause complete failure of radio communications. Some frequencies may be blocked out completely by static for extended periods of time during storm activity. Fading, caused by changes in the density and height of the ionosphere, can also occur and may last from minutes to weeks. The occurrence of these disturbances is difficult to predict. When they occur, the use of alternate frequencies and a greater reliance on FM or other means of communications are required.
7-2. Techniques for Better Operations in Arctic-like Areas
Whenever possible, radio sets for tactical operations in arcticlike areas should be installed in vehicles to reduce the problem of transportation and shelter for operators. This will also help solve some of the grounding and antenna installation problems due to the climate.
Because of permafrost and deep snow, it is difficult to establish good electrical grounds in extremely cold areas. The conductivity of frozen ground is often too low to provide good ground wave propagation. To improve ground wave operation, use a counterpoise to offset the degrading effects of poor electrical ground conductivity. When installing a counterpoise, remember to install it high enough above the ground so that it will not be covered by snow.
In general, antenna installation in arcticlike areas presents no serious difficulties. However, installing some antennas may take longer because of adverse working conditions. A few tips for installing antennas in extremely cold areas are listed below.
- The mast sections and antenna cables must be handled carefully since they become brittle in very low temperatures.
- Whenever possible, antenna cables should be constructed overhead to prevent damage from heavy snow and frost. Nylon rope guys, if available, should be used in preference to cotton or hemp because nylon ropes do not readily absorb moisture and are less likely to freeze and break.
- An antenna should have extra guy wires, supports, and anchor stakes to strengthen it to withstand heavy ice and wind loading.
Some radios (generally older generation radios) adjusted to a particular frequency in a relatively warm place may drift off frequency when exposed to extreme cold. Low battery voltage can also cause frequency drift. When possible, allow a radio to warm up several minutes before placing it into operation. Since extreme cold tends to lower output voltage of a dry battery, try warming the battery with body heat before operating the radio set. This minimizes frequency drift.
Flakes or pellets of highly electrically charged snow is sometimes experienced in northern regions. When these particles strike the antenna, the resulting electrical discharge causes a high-pitched static roar that can blanket all frequencies. To overcome this static, antenna elements can be covered with polystyrene tape and shellac.
7-3. Maintenance Improvement in Arcticlike Areas
The maintenance of radio equipment in extreme cold presents many difficulties. Radio sets must be protected from blowing snow, since snow will freeze to dials and knobs, and blow into the wiring to cause shorts and grounds. Cords and cables must be handled carefully since they may lose their flexibility in extreme cold. All radio equipment and power units must be properly winterized. Check the appropriate TM for winterization procedures. A few tips for maintenance in arctic areas are listed below.
As the temperature goes down, it becomes increasingly difficult to operate and maintain generators. They should be protected as much as possible from the weather.
The effect of cold weather conditions on wet and dry cell batteries depends upon the following factors: the type and kind of battery, the load on the battery, the particular use of the battery, and the degree of exposure to cold temperatures. Check SB 11-6 and SB 11-30 for specific information.
Damage may occur to vehicular radio sets by the jolting of the vehicle. Most synthetic rubber shock mounts become stiff and brittle in extreme cold and fail to cushion equipment. Check the shock mounts frequently and change them, as required.
Check the TMs for your radio set and power source to see if there are special precautions for operation in extremely cold climates. For example, normal lubricants may solidify and permit damage or malfunctions. They must be replaced with the recommended arctic lubricants.
Moisture from your breath may freeze on the perforated cover plate of your microphone. Use standard microphone covers to prevent this. If standard covers are not available, improvise a suitable cover from rubber or cellophane membranes or from rayon or nylon cloth.
Breathing and Sweating.
A radio set generates heat when it is operated. When you turn it off, the air inside cools and contracts and draws cold air into the set from the outside. This is called breathing. When a radio breathes and the still-hot parts come in contact with subzero air, the glass, plastic, and ceramic parts of the set may cool too rapidly and break.
When cold equipment is brought suddenly into contact with warm air, moisture will condense on the equipment parts. This is called sweating. Before cold equipment is brought into a heated area, it should be wrapped in a blanket or parka to ensure that it will warm gradually to reduce sweating. Equipment must be thoroughly dry before it is taken back out into the cold air or the moisture will freeze.
These radios present special problems during winter operations because of their continuous exposure to the elements. Proper starting procedures must be observed. The radio's power switch must be off prior to starting the vehicle; a particularly critical requirement when vehicles are slave started. If the radio is cold soaked from prolonged shutdown, frost may have collected inside the radio and could cause circuit arcing. Hence, time should be allowed for the vehicle heater to warm the radio sufficiently 80 that any frost collected within the radio has a chance to thaw. This may take up to an hour. Once the radio has been turned on, it should warm up for approximately 15 minutes before transmitting or changing frequencies. This allows components to stabilize. If a vehicle is operated at a low idle with radios, heater, and lights on, the batteries may run down. Before increasing engine revolutions per minute to charge the batteries, radios should be turned off to avoid an excessive power surge. A light coat of silicon compound on antenna mast connections helps to keep them from freezing together and becoming hard to dismantle.
7-4. Capabilities and Limitations
Radio communications in jungle areas must be carefully planned, because the dense jungle growth significantly reduces the range of radio transmission. However, since single-channel radio can be deployed in many configurations, especially manpacked, it is a valuable communications asset. The capabilities and limitations of single-channel radio must be carefully considered when used by forces in a jungle environment.
The mobility and various configurations in which single-channel radio can be deployed are its primary advantages in jungle areas.
Limitations on radio communications in jungle areas stem from the climate and the density of jungle growth. The hot and humid climate increases the maintenance problems of keeping equipment operable. Thick jungle growth acts as a vertically polarized absorbing screen for RF energy that, in effect, reduces transmission range. Therefore, increased emphasis on maintenance and antenna siting is a must when operating in jungle areas.
7-5. Techniques for Better Operations in the Jungle
The main problem you may have in establishing radio communications in jungle areas is the siting of your antenna. Apply the following techniques to improve your communications in the jungle:
- Antennas should be located in clearings on the edge farthest from the distant station and as high as possible.
- Antenna cables and connectors should be kept off the ground to lessen the effects of moisture, fungus, and insects. This also applies to all power and telephone cables.
- Complete antenna systems, such as ground planes and dipoles, are more effective than fractional wavelength whip antennas.
- Vegetation must be cleared from antenna sites. If an antenna touches any foliage, especially wet foliage, the signal will be grounded.
- Vegetation, particularly when wet, will act like a vertically polarized screen and absorb much of a vertically polarized signal. Use horizontally polarized antennas in preference to vertically polarized antennas.
7-6. Maintenance Improvement in the Jungle
Because of moisture and fungus, the maintenance of radio sets in tropical climates is more difficult than in temperate climates. The high relative humidity causes condensation to form on the equipment and encourages the growth of fungus. Operators and maintenance personnel should check the appropriate TMs for any special maintenance requirements. Some techniques for improving maintenance in jungle areas are listed below:
- Keep the equipment as dry as possible and in lighted areas to retard fungus growth.
- Keep all air vents clear of obstructions so air can circulate to cool and dry the equipment.
- Keep connectors, cables, and bare metal parts as free of fungus growth as possible.
- Use moisture and fungusproofing paint (MFP) to protect equipment after repairs are made or when equipment is damaged or scratched.
Dismounted patrols and units of company size and below can greatly improve their ability to communicate in the jungle by using expedient antennas. While moving, you are generally restricted to using the short and long antennas which come with your radios. However, when you are not moving, these expedient antennas will allow you to broadcast farther and to receive more clearly. Keep this fact in mind; however, an antenna that is not "tuned " or "cut" to the operating frequency is not as effective as the whips that are supplied with your radio. Circuits inside the radio "load" the whips properly so that they are "tuned" to give maximum output. Whips are not as effective as a tuned doublet or tuned ground-plane (namely, RC-292 type), but the doublet or ground-plane must be tuned to the operating frequency. This is especially critical with low-power radios such as the AN/PRC-77.
Expedient 292-Type Antenna.
The expedient 292-type antenna was developed for use in the jungle and, if used properly, will increase your ability to communicate. In its entirety, it is bulky and heavy and is not generally acceptable for dismounted patrols and small unit operations. You can, however, carry only the mast head and antenna sections, mounting these on wood poles or hanging them from trees. Or, you can fabricate a complete expedient 292-type antenna (fig 7-1) using WD-1, wire, and other readily available material. You can use almost any plastic, glass, or rubber objects for insulators. Dry wood is acceptable when nothing else is available. See figure 3-26 for types of insulators which may be used. Below are some points on making this antenna.
Use the quick-reference chart (table 7-1) to determine the length of the elements (1 radiating and 3 ground planes) for the frequency you will be using. Cut these elements (fig 7-1A) from WD-1 field wire (or similar wire). Cut spacing sticks (fig 7-1B) the same length. Place the ends of the sticks together to form a triangle and tie the ends with wire, tape, or rope. Attach an insulator to each corner. Attach a ground-plane wire to each insulator. Bring the other ends of the ground-plane wires together, attach them to an insulator (fig 7-1C), and tie securely. Strip about 3 inches of insulation from each wire and twist them together.
Tie one end of the radiating element wire to the other side of insulator C and the other end to another insulator (fig 7-1D). Strip about 3 inches of insulation from the radiating element at insulator C.
Cut enough WD-1 field wire to reach from the proposed location of the antenna to the radio set. Keep this line as short as possible: excess length reduces efficiency of system. Tie a knot at each end of this cable pair to keep it from unraveling. Identify one wire in the pair and tie a knot at each end to identify it as the "hot" lead. Remove insulation from the "hot" wire and tie it to the radiating element wire at insulator C. Remove insulation from the other wire and attach it to the bare ground-plane element wires at insulator C. Tape all connections and do not allow the radiating element wire to touch the ground-plane wires.
Attach a rope to the insulator on the free end of the radiating element and toss the rope over the branches of a tree. Pull the antenna as high as you can, keeping the lead-in routed down through the triangle. Secure the rope to hold the antenna in place.
At the radio set, remove about 1 inch of insulation from the "hot" lead and about 3 inches of insulation from the other wire. Attach the "hot" line to the antenna terminal (doublet connector, if so labeled). Attach the other wire to the metal case--the handle, for example. Be sure both connections are tight or secure.
Set up correct frequency, turn on set, and proceed with communications.
Table 7-1. Quick Reference Chart
Figure 7-1. Expedient 292-type antenna.
This is another antenna that is easy to carry and quick to set up (fig 7-2). The two radiating wires are cut to the length shown in table 7-1 for the operating frequency. For the best results, the lead-in should extend at least 1.8 meters (6 ft) at right angles (give or take 30ø) to the antenna section before dropping to the radio set.
The easiest way to set up this antenna is to measure the length of the radiating elements from one end of the lead-in (WD-1) and tie a knot at that point. The two wires are separated: one is lifted vertically by a rope and insulator, the other is held down by a rock (or other weight) and a rope and insulator. The antenna should be as high as possible.
Attach the other end of the lead-in to the radio set as described in the paragraph on the expedient 292-type antenna.
Figure 7-2. Expedient patrol antenna.
7-8. Capabilities and Limitations
Radio is usually the primary means of communications in the desert. It can be employed effectively in desert climate and terrain to provide the highly mobile means of communications demanded by widely dispersed forces. However, desert terrain provides poor electrical ground and counterpoises are needed to improve operation. (Refer to chapter 3 for information on counterpoises.)
7-9. Techniques for Better Operations in the Desert
For the best operation in the desert, radio antennas should be located on the highest terrain available. Transmitters using whip antennas in the desert will lose one-fifth to one-third of their normal range due to the poor electrical grounding characteristic of desert terrain. For this reason, it is important to use complete antenna systems such as horizontal dipoles and vertical antennas with adequate counterpoises.
7-10. Equipment Considerations
Some radios automatically switch on their second blower fan if their internal temperature rises too high. Normally, this happens only in temperate climates when the radios are transmitting. This may disturb soldiers unaccustomed to radio operation in the desert environment. Operation of the second fan, however, is quite normal. RF power amplifiers used in AM and SSB sets are liable to overheat severely and burn out. Such equipment should be turned on only when necessary (signal reception is not affected). Since the RF power amplifiers take approximately 90 seconds to reach the operating mode, the SOP of units using the equipment should allow for delays in replying. Dust affects communications equipment such as SSB/AM RF power amplifiers and radio teletypewriter sets. The latter especially are prone to damage due to the vulnerability of the oil lubrication system (which attracts and holds dust particles). Dust covers, therefore, should be used whenever possible. Some receiver-transmitter units have ventilating ports and channels that can get clogged with dust. These must be checked regularly and kept clean to prevent overheating.
Wet cell batteries do not hold their charge efficiently in intense heat. Electrolyte evaporates rapidly and should be checked weekly (more often, if warranted). Add distilled water as needed. Extra containers of distilled water should be carried in the vehicle. Maintenance of vehicle batteries beyond adding water must be done only by authorized motor pool personnel according to applicable regulations (see FM 90-3). Dry battery supplies must be increased, since hot weather causes batteries to fail more rapidly.
7-12. Electrical Insulation
Wind-blown sand and grit will damage electrical wire insulation over a period of time. All cables that are likely to be damaged should be protected with tape before insulation becomes worn. Sand will also find its way into parts of items such as "spaghetti cord" plugs, either preventing electrical contact or making it impossible to join the plugs together. A brush, such as an old toothbrush, should be carried and used to clean such items before they are joined.
In deserts with relatively high dew levels and high humidity, overnight condensation can occur wherever surfaces such as metals exposed to air are cooler than the air temperature. This condensation can affect such items as electrical plugs, jacks, and connectors. All connectors likely to be affected by condensation should be taped to prevent moisture from contaminating the contacts. Plugs should be dried before inserting them into equipment jacks. Excessive moisture or dew should be dried from antenna connectors to prevent arcing.
7-14. Static Electricity
Static electricity is prevalent in the desert. It is caused by many factors, one of which is wind-blown dust particles. Extremely low humidity contributes highly to static discharges between charged particles. Poor grounding conditions aggravate the problem. Be sure to tape all sharp edges (tips) of antennas to cut down on wind-caused static discharges and the accompanying noise. If you are operating from a fixed position, ensure that equipment is properly grounded. Since static-caused noise diminishes with an increase in frequency, use the highest frequencies that are available and authorized.
7-15. Maintenance Improvement in the Desert
In desert areas, the maintenance of radio sets becomes more difficult because of the large amounts of sand, dust, or dirt that enter the equipment. Sets equipped with servomechanisms are particularly affected. To reduce maintenance downtime, keep the sets in dustproof containers as much as possible. It is also important to keep air vent filters clean to allow cool air to circulate to prevent overheating.
Preventive maintenance checks should be made frequently. Also, you should keep a close check on lubricated parts of the equipment. If dust and dirt mix with the lubricants, moving parts may be damaged.
7-16. Capabilities and Limitations
Operation of radios in mountainous areas have many of the same problems as in northern or cold weather areas. Also, the mountainous terrain makes the selection of transmission sites a critical task. In addition, the terrain restrictions encountered frequently make radio relay stations necessary for good communications.
7-17. Maintenance Improvement in Mountainous Areas
Because of terrain obstacles, radio transmissions will frequently have to be by line of sight. Also, the ground in mountainous areas is often a poor electrical conductor. Thus, a complete antenna system, such as a dipole or ground-plane antenna with a counterpoise, should be used.
The maintenance procedures required in mountainous areas are very often the same as for maintenance in northern or cold weather areas. The varied or seasonal temperature and climatic conditions in mountainous areas make flexible maintenance planning a necessity.
7-18. Radio Communications in Urbanized Terrain
Radio communications in urbanized terrain pose special problems. Some problems are similar to those encountered in mountainous areas. There are problems of obstacles blocking transmission paths. There is the problem of poor electrical conductivity due to pavement surfaces. There is also the problem of commercial power lines interference.
VHF radios are not as effective in urbanized terrain as they are in some other areas. The power output and operating frequencies of these sets require a line of sight between antennas. Line of sight at street level is not always possible in built-up areas.
HF radios do not require or rely on line of sight as much as VHF radios. This is true because operating frequencies are lower and power output is greater. The problem now is that HF radio sets are not organic to small units. To overcome this, the VHF signals must be retransmitted.
Retransmission stations in aerial platforms can provide the most effective means if they are available. Organic retransmission is more likely to be used. The antenna should be hidden or blended in with surroundings.
This will help prevent the enemy from using it as a landmark to "home in" his artillery bombardment. Antennas can be concealed by water towers, existing civilian antennas, or steeples.
Here are some other steps that should be taken within urbanized terrain:
- Park radio-equipped vehicles inside buildings for cover and concealment.
- Dismount radio equipment and install it inside buildings (in basement, if available).
- Place generators against buildings or under sheds to decrease noise and provide concealment (adequate ventilation must be provided to prevent heat buildup and subsequent failure of generator).
7-19. Radio Operations in a Nuclear, Biological, and Chemical Environment
One of the realities of fighting on the modern battlefield is the presence of nuclear weapons. Nearly everyone is aware of the effects of nuclear blast, heat, and radiation. The ionization of the atmosphere by a nuclear explosion will have degrading effects on communications because of static and the disruption of the ionosphere.
There is also another effect of a nuclear explosion that is even a greater danger to our radio communications. This effect is called electromagnetic pulse (EMP). EMP is a strong pulse of electromagnetic radiation, many times stronger than the static pulse generated by lightning. This pulse can enter the radio through the antenna system, power connections, and signal input connections. In the equipment, the pulse can break down circuit components such as transistors, diodes, and integrated circuits. It can melt capacitors, inductors, and transformers. EMP can destroy a radio.
Defensive measures against EMP call for proper maintenance, particularly the shielding of equipment. When the equipment is not in use, all antennas and cables should be removed to decrease the effect of EMP on the equipment. Effective grounding is a must to reduce effect of EMP. See Appendix J, Electromagnetic Pulse (EMP).
EMP is a danger to our equipment: contamination is a danger to our personnel. In fact, contamination from any faction of the nuclear, biological, and chemical (NBC) environment has adverse effects on both equipment and personnel. Appendix K deals with this situation in more detail.
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