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10/ 19/83 3910.3A (4) When surveying radar antenna systems, the area between the feedhorn and the reflector should always be considered hazardous and carefully avoided. (5) When surveying in the main beam of a radar, the beam size, shape and character, and the limit of the PEL should be determined prior to the survey. The latter can be calculated or obtained from Figure 3-3, paragraph 36.a. 35. CONTROL OF HAZARDS. The three basic methods of controlling exposure to ionizing radiation are good guidelines to be used in controlling exposure to virtually all forms of RF radiation. They include: a. Limit Exposure Time. Although the effects of exposure to RF radiation are not considered to be cumulative, as in the case of ionizing radiation, the duration of exposure is an element of the PELs. They were selected to limit the specific absorption rate (SAR) to 0.4 W/kg in any 0.1 hour period implying that SARs in excess of that limit could cause a disruption in biological tissue or function. b. Increase Distance. The inverse square relationship of intensity to distance described in paragraph 24.a(2) for ionizing radiation is also applicable for RF emissions in the far field provided that: (1) The transmitting antenna (source) is isotropic; i.e. , it transmits energy equally in all directions, and (2) The transmission is through free space; i.e. , the energy is neither absorbed, reflected, refracted, nor scattered. Such ideal conditions seldom exist, but the inverse square relationship is a valuable "estimator" for determining approximate safe distances from RF sources. It should not be used as a substitute for distances determined by field measurement. Mathematical models are available for calculating safe distances from directional emitters such as radars and RML's. The values obtained are theoretical and should always be substantiated by field measurement if possible. c. Shielding. RF radiation can be reflected, refracted, scattered, and absorbed. It is these properties that enable it to be directed, conducted, and attenuated. In many systems, the very devices that enclose and direct RF energy for operational purposes also provide the required shielding to protect against personnel exposure; radar waveguides are an example. In most FAA systems that generate RF radiation it is properly confined where necessary and no further shielding is required. In those unusual instances where special shielding is needed, reference can be made to the information provided in paragraph 36.b. Chap 3 Par 34 Page 27