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

APPENDIX J
ELECTROMAGNETIC PULSE

J-1. Detrimental Effects

During nuclear warfare, communications will be more critical to military operations than at any other time. Thus, every commander must understand the effects that nuclear detonations have on electronic equipment. Analysis of communications problems yields two distinct categories of detrimental effects:

  • Degradation of the signal resulting from changes in the medium characteristics (a transient effect).

  • System damage caused by radiation or from intense fields generated by gamma rays.

The first should be approached with an eye toward various or alternative post-effect countermeasures. The second must be minimized through prior planning and precautionary procedures. This appendix discusses the electromagnetic pulse (EMP), its effect on communications and electronics equipment, and protective measures to enhance the survivability of communications systems. Although an explanation of EMP can be quite technical, it is discussed in this appendix in nontechnical terms. The focus is on what commanders and communications personnel need to know about EMP and how they can best use measures to reduce the consequences that may result from it.

J-2. Characteristics of EMP

EMP is produced by the radiation from a nuclear burst. Of the gamma rays, X-rays, and neutrons which comprise nuclear radiations, gamma rays are the dominant cause of EMP. Gamma rays radiate outward from the burst and strip electrons from the atoms in the air. Because of the high energy of the original gamma rays, the electrons are projected away from the burst point at approximately the speed of light. Thus, a virtual wall of fast-moving electrons is generated as the radiation sweeps outward in an expanding spherical wave. This process creates a region of high charge and strong currents (called the source or deposition region) which may extend for several thousand meters from the point of burst for a small-yield, low-altitude detonation, to several thousand kilometers for a high-yield, high-altitude burst. Within this region, the charge separation and currents produce strong electric and magnetic fields; and, if the current density is unproportional, a broad bandwidth, high amplitude radio frequency electromagnetic pulse is radiated. The outer atmospheric burst (height of burst (HOB) more than 50 km) and the near-surface burst (HOB less than 2 km) are the only ones of concern to ground-based systems as far as EMP is concerned.

J-3. Strength of the EMP

The strength of EMP is best appreciated by comparing it to various electromagnetic energy generators. Several electromagnetic energy sources and associated power densities are listed below along with the power density needed to drive a typical radio receiver. Clearly, EMP can deliver to a radio receiver a billion times the power necessary for reception. It is not difficult to understand the detrimental effect this can have. See table J-1 for an energy-comparison chart.

Table J-1. Electromagnetic energy comparison.

J-4. Frequency Analysis

A frequency analysis of EMP reveals frequency radiation from the extremely low frequency (ELF) band to the super high frequency (SHF) band. Most of the energy is concentrated in the high frequency (HF) and very high frequency (VHF) ranges.

J-5. Area Coverage

The relative source strength and area coverage of radiated fields for different heights of burst are summarized in table J-2. These strengths are what a land-based communications system would experience. Where the source region is not touching the ground in some cases, the strength is low or has no effect.

Table J-2. EMP strength and ground coverage.

J-6. Susceptibility

Modern communications and electronics equipment are increasingly susceptible to EMP due to the extensive use and application of microcircuits, semiconductors, and transistor technology. These devices normally cannot handle the voltage and current surges that result from EMP coupling. System, component, or subcomponent susceptibility can also be related to size and level of power handling capability. Normally, the smaller the individual component, the lower its power handling capability and, consequently, the greater its susceptibility.

J-7. EMP Protective Measures

Communications and electronics equipment in the inventory today has generally not been designed to withstand the effects of EMP and remain operational. To enhance its survivability and permit mission accomplishment, measures must be taken to reduce the vulnerability of communications equipment to EMP. While progress is being made in some cases to ensure EMP hardening in the procurement stage of future equipment, most communications equipment presently in the inventory will have to be used unhardened. Thus, measures to reduce vulnerability to EMP must be implemented by the users in the field. This responsibility requires commanders at all levels to incorporate SOPs and directives concerning methods of installation and operation of communications equipment to enhance survivability and mission accomplishment. During field exercises and tests, maximum emphasis should be placed on compliance with EMP related directives. These measures can only be effective if employed as routine actions.

Current demand for reliable communications is extremely high. Degradation of communications systems must be avoided and it is the responsibility of all personnel associated with communications to reduce EMP susceptibility as much as possible. Initially, all planning should provide for alternate means for passing priority traffic in an EMP environment. Since only a portion of a total communications system is likely to be affected by EMP, routing must consider the hardiest systems for the highest priority traffic.

The capability to quickly repair and reintegrate damaged portions of the system with minimum confusion and maximum efficiency is of primary concern. All equipment not absolutely required in primary systems should remain disconnected and stored within a sealed shelter or other shielded enclosure for protection from the EMP. This measure will reduce the likelihood of all equipment being simultaneously damaged by EMP and will provide for backup components to reinstall affected systems. Rigid operator/ user training should be stressed for implementing reroute plans and equipment restoration.

To prevent or reduce "coupling" with the energy from EMP, wires and cables (to include output coaxial types) should be shielded and properly grounded. In addition, the lengths of cables should be kept as short as possible. Excess lengths of cables in use should never be left wound on a cable reel but should be taken off the reel and pulled out in a straight line to avoid having loops in the cables. When cables are not needed, or are installed for possible operation, they should not be connected to the equipment. Burying the cables a foot or more will reduce their exposure to EMP fields and subsequently reduce the collected energy. Power cables should be treated in a similar manner and the same protective measures taken. Shields on all cables should be physically connected to the grounding systems where provided on communications equipment.

Antennas and their connecting cables should be disconnected from radio sets when not in use. Selection of the proper antenna, to include operation required, size, installation, and polarization, should be paramount in planning procedures. Narrow bandwidth antennas requiring high power for operation reduces the EMP coupling effect.

Communications equipment should never use commercial power. All commercial power facilities are extremely susceptible to EMP and their use would provide a direct path to vulnerable communications components. Communications shelter doors, access panels, vents (if possible), and all other apertures should be kept closed or shielded to reduce the direct entry of EMP fields into the shelter. Honeycomb screens are available for vents which allow the vents to open during operation while simultaneously shielding against EMP.

Maintenance of communications equipment should be continuous. Although cables and wires with damaged shielding or connectors may be operational, they should be replaced to ensure that proper shielding is maintained. Power system filters should be checked periodically and repaired/replaced if necessary. Particular attention should be paid to a rapid repair capability, with necessary parts and facilities. Equipment spare parts should be checked periodically to ensure their level of fill conforms to established directives. Alternate or backup equipment should be tested periodically to ensure proper operation. Additional measures should not preclude the use of visual, sound, and extensive messenger services (air, motor, and foot) to replace systems rendered nonoperational by EMP damage.

All users of communications equipment and systems must remember that the EMP threat is real and the use of nuclear weapons on the battlefield of the future will require viable measures to enhance communications survivability and mission accomplishment.

J-8. Guidance

The following reference material provides guidance for EMP protective measures:

  • FC 50-16.

  • FC 50-17.



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