Effects of ground planes and reflectors on operator exposures

• Pages: 58-61

Appendix D – Effects of ground planes and reflectors on

operator exposures

Simple models can be used to assess the effects of large nearby metallic objects on the

exposure of operators and other personnel. In particular, the effect of metallic walls and floors

(as in a shielded room) can be analysed to give exposure information. Radiofrequency (RF)

heaters are often placed inside shielded rooms to reduce RF interference with communication

sources for compliance with national FCC regulations, such as the FCC in the United States.

A simple analogy to antenna theory is useful for estimating the increase in absorbed

power due to the energy reflected from the conducting walls and floor of the shielded room. At

RF frequencies, the human body absorbs energy from an incident electromagnetic field in much

the same way that a simple dipole antenna does. That is, the incident electromagnetic field

causes currents to flow in the body. Because the human body is a lossy conductor, these

currents heat the tissues. The amount of heat deposited in a particular part of the body is

proportional to the square of the current density in that part of the body. Therefore, maximum

heat is produced where a maximum current density exists.

Figure D1 depicts three workplace exposure situations that may occur when using RF

heaters and may affect worker exposures. In figure D-1a, an RF heater and an operator are in an

unshielded room with no metallic objects and no metal floor or buried pipes. Figure D-1b

depicts a room with a metal floor that acts as a ground plane. Both the heater and the operator's

feet are grounded to the floor; the effect of a metal floor may also be created by the steel

reinforcement used in concrete floors. Figure D-1c shows a heater and operator in a room with

a metallic wall that is not in contact with the operator or the heater. In some situations, such as

in a shielded room, the operator is exposed to the combined effects of a metallic floor and wall.

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