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LESSON 3
ELECTROMAGNETIC INTERFERENCE
| CRITICAL TASKS: | 01-5701.07-0002 01-5701.07-0003 |
OVERVIEW
LESSON DESCRIPTION:
In this lesson, you will learn about the sources, types, and effects of electromagnetic interference (EMI). You will also learn about electromagnetic compatibility (EMC) and electromagnetic pulse (EMP).
TERMINAL LEARNING OBJECTIVE:
| ACTION: | Define and describe the sources, types, and effects of EMI. |
| CONDITION: | Given the material in this lesson. |
| STANDARD: | To demonstrate competence, you must achieve a minimum of 70 percent on the subcourse examination. |
| REFERENCES: | The material in this lesson was derived from the following publications: FM 24-2, FM 24-18, and AR 105-3. |
INTRODUCTION
Maintaining command, control, and communications (C3) in an electromagnetic environment is a major challenge. Commanders who try to control and manipulate the environment gain an advantage for their side to the detriment of an enemy. The frequency spectrum is very crowded because it has finite limits. Thus, C3 tasks are harder due to the number and capabilities of enemy emitters, jammers, and various non-communication emitters. They are also harder due to unintentional interference by friendly forces. You must be aware of the effects of EMI and EMP.
1. Electromagnetic Compatibility. EMC is the process of predicting and controlling potential interference in new equipment through design and frequency sharing methods. It is the ability of C3 systems to operate in their intended working environment without suffering or causing unacceptable degradation due to electromagnetic radiation or response. The signal officer has primary responsibility for spectrum management in his unit. EMC is just one consideration under spectrum management. The signal officer, in coordination with the electronic warfare (EW) officer, establishes and maintains a joint restricted frequency list. That list is based on G3/S3 reports and priorities.
2. Electromagnetic Interference. EMI is any electromagnetic disturbance that interrupts, obstructs, degrades, or limits the performance of electronic or electrical equipment. It can be natural, man-made, or mutual interference.
a. Natural interference has four categories (Figure 3-1). They are atmospheric, solar and cosmic, precipitation static, and fading. Natural interference affects most frequencies with its effects decreasing as the frequency increases.
Figure 3-1. Natural interference.
(1) Electrical storms in the atmosphere cause atmospheric interference.
(2) Eruptions on the sun or other stars cause solar and cosmic interference.
(3) Charged particles in the atmosphere cause precipitation static interference.
(4) The medium through which radio waves are propagated causes fading interference.
b. Man-made interference is generated by electrical devices. High-tension wires, motors, arc welders, and similar items can cause it. EW is another form of man-made interference. An enemy constantly monitors friendly C3 system to gather information on friendly movement, capabilities, identification, and locations. Enemy EW systems intentionally disrupt certain friendly force C3 systems to confuse and degrade operations. Some enemy EW activities will sound like different types of interference caused by static, electrical storms, sun spots, and electrical equipment. Others activities will imitate different EMI types.
c. Mutual interference occurs when one C3 system interferes with another. This may be from local or distant sources. Figure 3-2 shows some of the associated common conditions.
Figure 3-2. Mutual interference.
3. Recognizing Jamming.
a. Radio operators must be able to recognize jamming. This is not always an easy task. Threat jammers may employ obvious or subtle jamming techniques. Operators must know how to discriminate between unintentional interference and enemy jamming.
(1) The two sources of interference are internal and external. If the interference or suspected jamming can be eliminated or substantially reduced by grounding the radio equipment or disconnecting the receiver antenna, the source of the disturbance is most likely external to the radio. If the interference or suspected jamming remains after grounding or disconnecting the antenna, the disturbance is most likely internal. That is, it is caused by a radio malfunction. External interference must be checked further for enemy jamming or unintentional interference.
(2) Unintentional interference may be caused by other radios, some other type of electronic or electrical equipment, or atmospheric conditions. The battlefield is crowded with radios and other electronic equipment. Unintentional interference is virtually unavoidable. The static electricity produced by atmospheric conditions can adversely affect radio communications. Unintentional interference normally travels only a short distance. A search of the immediate area may reveal the source of this type of interference. Moving the receiving antenna short distances may cause noticeable variations in the strength of the interfering signal. These variations normally indicate unintentional interference. Little or no variation often indicates jamming.
b. Some jamming takes advantage of the design characteristics of certain radio equipment, such as the AN/VRC-12 in the new squelch on mode. To activate these radios, a 150-Hz tone must be transmitted to them along with the transmitter carrier signal. These radios lock onto the strongest signal they receive.
c. When an enemy transmits a strong signal to these equipment items, they will lock onto the strong signal. If a friendly signal, along with the 150-Hz signal, is transmitted to these equipment items, they will not receive it as long as the jamming signal is stronger than the friendly signal. This type of subtle jamming is called squelch capture. The operator cannot detect this method of jamming. It may appear as if the radio is faulty because the operator cannot receive any friendly signal. Report suspected enemy jamming and unidentified interference that disrupts communication. This applies even if the effects of the jamming or interference are overcome.
d. An enemy can use powerful unmodulated or noise-modulated signals. You should note that modulated jamming signals are characterized by obvious interference noises. The following procedures help operators determine whether their radios are being jammed.
(1) AN/PRC-77.
(a) Turn the function control from the SQUELCH ON to the OFF position.
(b) A lack of noise may mean an unmodulated jamming signal is jamming a radio. Temporarily disconnect the antenna. If normal static noise returns when the antenna is disconnected, the radio is likely being jammed by an unmodulated signal.
(c) A greater than normal level of noise or an obviously modulated signal may mean a noise-modulated jamming signal is jamming the radio. The operator should temporarily disconnect the antenna. If normal static noise returns when the antenna is disconnected, the radio most likely is being jammed by a noise-modulated signal.
(d) If the above tests indicate there is a high probability the radio is being jammed, the operator should follow the local SOP to reestablish communications and initiate a report informing higher headquarters the incident.
(2) AN/VRC-12 series radio.
(a) Turn the squelch control from the NEW SQUELCH ON mode to the NEW SQUELCH OFF mode.
(b) A radio's lack of noise and lighted call light may mean an unmodulated jamming signal is jamming it. Temporarily disconnect the antenna. If normal static noise returns, and the call light goes off when the antenna is disconnected, the radio is likely being jammed by an unmodulated signal.
(c) A greater than normal level of noise or an obviously modulated signal may mean the radio is being jammed by a noise-modulated jamming signal. Temporarily disconnect the antenna. If normal static noise returns, and the call light goes off when the antenna is disconnected, the radio is likely being jammed by a noise-modulated signal.
(d) If there is a high chance a radio is being jammed, the operator should follow unit SOP to reestablish communications. He should also submit a report to tell superiors about the incident.
(3) You should coordinate with military intelligence units for aid in developing tests for special capacity radios or radios unique to your unit. Examples include the improved high frequency radio (IHFR) and single-channel ground and airborne radio system (SINCGARS) radios. You must make sure your radio operators are trained to use these radios.
4. Overcoming Jamming. An enemy will seek to use new and more confusing jamming. You must be alert to the possibility of jamming. Training and experience are the most important tools in determining when a signal is a jamming signal. Exposure to the effects of jamming in training or actual situations is invaluable. Once you determine jamming is being used, take the following actions. If any of these actions lessen the problem, you should continue normal operations and send a report informing higher headquarters of the incident.
a. Continue to operate. Stop and consider what an enemy does during a jamming operation. Enemy jamming often involves a jamming period, then a brief listening period. He is attempting to determine how effective his jamming has been. What we do while he is listening tells him how effective his jamming has been. If our operation continues in a normal manner, as before the jamming an enemy may assume his jamming was not very effective. If he finds us excitedly discussing our problem on the air, an enemy may assume his jamming was effective. He may also think so if we shut down our operation. There is a rule that applies when being jammed. Unless otherwise ordered, never shut down operations or in any other way disclose to an enemy that you are being adversely affected. This means normal operations continue, even when degraded by jamming.
b. Improve the signal-to-jamming ratio. This involves the relative strength of the desired signal to the jamming signal at the receiver. Signal refers to the communications or emission received. Jamming refers to the hostile or unidentified interference being received. Try to have a signal-to-jamming ratio where the desired signal is stronger than the jamming signal. Then, the desired signal cannot be significantly degraded by the jamming signal. The following helps to improve the signal-to-jamming ratio:
(1) Adjust the receiver. When jamming is experienced, check to make sure the receiver is tuned as precisely as possible to the desired incoming signal. A slight receiver readjustment may provide an improved signal-to-jamming ratio. Specific methods that apply to a certain radio are in the operator's manual. Some of these methods are:
(a) Adjust the beat frequency oscillator.
(b) Adjust the bandwidth.
(c) Adjust the gain or volume control.
(d) Fine-tune the frequency.
(2) Increase the transmitter power output. The most obvious way to improve the signal-to-jamming ratio is to increase the transmitter's power output. To increase power output during jamming, the transmitter must be on less than full power when jamming begins. Using low power as a preventive technique depends on the enemy not being able to detect our radio transmissions. Once the enemy begins jamming our radios, the threat of being detected is moot. Use the reserve power on terrestrial, line-of-sight (LOS) radios to override an enemy's jamming signal. Tactical satellite communications terminals will not increase their transmit power.
(3) Adjust or change the antenna. Adjustments can improve the signal-to-jamming ratio. When being jammed, make sure the antenna is optimally adjusted to receive the desired incoming signal. Specific anti-jamming methods used for a radio set are found in the operator's manual. Some methods are:
(a) Reorient the antenna.
(b) Change the antenna polarization (must be done by all stations).
(c) Install an antenna with a longer range.
(4) Establish a retransmission station. Doing this can increase the range and power of a signal between two or more radio stations. This is often available method to improve the signal-to-jamming ratio.
(5) Relocate the antenna. The signal-to-jamming ratio can often be improved by relocating the antenna and its affected radio set. This may mean moving a few or several hundred meters. Relocate the antenna and its radio so there is a terrain feature between them and any suspected enemy jammer.
c. Use an alternate route for communications. Enemy jamming may prevent us from communicating with a radio station. If communications are degraded between two radio stations that must communicate, there may be another radio station or route available. That radio station or route should be used as a relay between the two other radio stations.
d. Change frequencies. Enemy jamming may not be overcome using the above measures. If so, a net may be directed to switch to an alternate or spare frequency. Dummy stations may continue to operate on the jammed frequency to mask the change to an alternate frequency. This action must be planned and coordinated. During jamming, coordinating a change of frequency is difficult. Radio operators should know when and under what circumstances they are to switch to alternate or spare frequencies. If not done smoothly, the enemy may discover what is happening and try to jam the new frequency.
5. Electromagnetic Pulse. EMP results from the effects of nuclear detonations or from weapons designed to produce the same effect. It can have a major impact on communications. Such detonations can cause changes in atmospheric conditions (a transient effect). They can also damage electronic signal equipment via intense gamma radiation.
a. Our signal and other electronic equipment are increasingly susceptible to EMP. This is due to the extensive use of microcircuits, semiconductors, and transistors.
b. The ability to repair quickly and reintegrate EMP-damaged portions of a communications system with minimum confusion is very important. This requires that procedures to protect equipment become second nature to all personnel. The following are some protection procedures:
(1) Wires and cables should be shielded and properly grounded.
(2) Excess cabling should be taken from its reel and stretched to its full length. It should never be left wound on a reel.
(3) Whenever possible, cables should be buried a depth of a foot or more to partially shield the cable from EMP.
(4) Commercial power sources are also susceptible to EMP. Military communications equipment should not use commercial power when preparing for or conducting operations in a nuclear environment.
6. Unusual Operating Conditions. Radio operations in unusual climatic and atmospheric conditions pose special and unique situations for operators.
b. The major problem in the tropics' hot and humid atmosphere is vegetation. Particularly when wet, it acts like a vertically polarized signal and impedes good signal propagation. The best solution is to use a horizontally polarized antenna. Since vegetation also absorbs radio signals, transmit from a cleared area or one with a light jungle canopy.
c. Deserts provide poor conductivity to ground. A counter-poise antenna (RC-292, OE-254, etc.) should provide the range needed to ensure quality communications among widely dispersed forces. Wind-blown metallic dust passing over a desert's dry surface can create a static discharge of electricity. This problem can be overcome by operating at the upper end of a radio set's frequency range. The static's impact has less effect on communications in that frequency range.
d. Communications in mountainous areas are affected more by the terrain than by atmospheric conditions. Using LOS radios in mountainous terrain is difficult. Doing so requires the use of relay stations and effective antenna placements.
7. Summary.
a. EMC involves predicting and controlling interference. It results in signal systems operating without unacceptable degradation.
b. EMI is a disturbance that interrupts, obstructs, degrades, or limits the performance of electronic or electrical equipment. It can be natural, man-made, or mutual interference.
(1) Natural interference has four categories. They are atmospheric solar and cosmic, precipitation static, and fading.
(a) Electrical storms in the atmosphere cause atmospheric interference.
(b) Eruptions on the sun or other stars cause solar and cosmic interference.
(c) Charged particles in the atmosphere cause precipitation static interference.
(d) The medium through which radio waves are propagated causes fading interference.
(2) Man-made interference is caused by electrical devices. EW is also a form of man-made interference.
(3) Mutual interference occurs when one C3 system interferes with another.
c. EMP is caused by a nuclear detonation. It can damage signal equipment by intense gamma radiation.
d. Radio operations in unusual climatic and atmospheric conditions pose special and unique situations for operators.
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