ELECTROMAGNETIC RADIATION

Radio-frequency (rf) transmitting systems with high-power transmitting tubes and high-gain antennas have increased the possibility of injury to personnel working in the vicinity.

An electromagnetic radiation hazard exists when electronic equipment generates a strong enough electromagnetic field to fall in a category listed below:

• Causes harmful or injurious effects to humans and wildlife
• Induces or otherwise couples currents and/or voltages of magnitudes large enough to initiate electroexplosive devices or other sensitive explosive components of weapons systems, ordnance, or other explosive devices
• Creates sparks large enough to ignite flammable mixtures or materials that must be handled in the affected areas

These hazardous situations can be caused by a transmitter or antenna installation. These generate electromagnetic radiation in the vicinity of personnel, ordnance, or fueling operations in excess of established safe levels. Sometimes the existing electromagnetic radiation levels increase to a hazardous level. When personnel, ordnance, or fueling evolutions are located in an area that can be illuminated by electromagnetic radiation, hazardous situations may occur.

Electromagnetic radiation is hazardous to personnel in two ways. It can cause rf burns; and it can cause biological, thermal, and neurological effects to personnel (RADHAZ). Because of the differences in characteristics and safety precautions required for each of the two types, they will be discussed separately.

An rf burn hazard is a hazardous condition caused by the existence of radio frequency (rf) voltages in places where they are not intended to be. Any ship with high-power hf transmitters is susceptible. Potentially hazardous voltages have been found in many areas. Some of these areas are lifelines, vertical ladders, ASROC launchers, gun mounts, rigging for underway replenishment, and boat davits. Another of these areas is on aircraft tied down on carrier and helicopter flight decks.

Whether or not an induced voltage creates an rf burn hazard depends on whether personnel will come into contact with the object being energized. Generally, only the voltage between an object and the deck is important. The rf burn occurs when a person comes into contact with a source of rf voltage in a manner that allows rf current to flow through the area of contact. Resistance of the skin to the current flow at the areas of contact causes heat. The effect of the heat on a person at the point of contact ranges from noticeable warmth to a painful burn.

The most useful and widespread technique in the reduction of rf burn hazards is the proper bonding and grounding of all metallic objects in the rf radiation field.

In some cases, the rf burn hazard can be eliminated only through the use of restrictive operating procedures. These procedures govern the simultaneous use of transmitting and cargo equipment. Techniques such as operation of transmitters at reduced power and the prohibition of simultaneous use of certain combinations of antennas, frequencies, and cargo handling equipment are used.

Figure 3-35 shows typical rf radiation hazard warning signs.

Figure 3-35. - Typical rf radiation hazard warning signs.

Most studies on the subject of radiation hazards (RADHAZ) have emphasized the impact of electromagnetic radiation on man. Man is singled out because of the biological, thermal, and neurological effects that occur in human organs and other biological tissues. Certain organs of the body are considered to be more susceptible than others to the effects of electromagnetic radiation. Presently available information and experience indicate that the eyes and testes are the most vulnerable body organs. The overwhelming danger to date appears to be the hazard from thermal effects, which are a function of intensity of radiation and frequency. This is particularly true in the range of 1 to 3 gigahertz. Thermal effects appear to taper off in severity outside this range.

When the body is irradiated by energy from a point source, the total body surface is usually not exposed. The larger the area exposed and the larger the radiation power density, the higher the body temperature rise and the greater the hazard. Microwave radiation from a radar source will "cook" you internally, just as a microwave oven cooks a chicken.

An injury of great concern is that to the lens of the eye. Exposure of the lens to high-intensity microwaves may cause cataracts. Current medical evidence indicates that a significant temperature elevation of the lens is required for cataract formation. If exposure is limited to 10 milliwatts per centimeter squared, the lens temperature is not elevated to levels at which cataracts occur.

In addition to thermal effects, nonionizing radiation is known to produce nonthermal effects. An association of a biological hazard with the nonthermal effects has not been demonstrated.

A peculiar effect experienced by some personnel is the sensation of sound when they are exposed to pulsed microwave fields. This occurs at levels below stated hazard limits and is not, by itself, considered dangerous.

Q.38 Electromagnetic radiation is hazardous to personnel in what two ways?
Q.39 What is the most useful and widespread technique to reduce rf burn hazards?