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1/29/98 108 C95.3-1991 Revision — 2 nd Draft 10/98 Draft Electromagnetic Radiation Hazards Technical Manual [B3] provides the results of such computations in the form of normalized curves. However, a high-order of precision is not warranted when computing the expected power density because of the many physical parameters in the environment that create significant variations in the values predicted by idealized computations. B4. Scanning Correction In the case of scanning antennas, the average power density at a fixed point will be reduced by the value of the effective antenna-pattern beamwidth divided by the scanning angle (the number of degrees of antenna rotation during a scan). This assumes that a constant rotational velocity is used, and that the antenna rotates in one direction, rather than stopping after a scan, and reversing direction. Accordingly, the potentially-hazardous distance is decreased by at least the square root of this ratio (if the period of rotation is less than the averaging time specified in the MPE ). The antenna's effective beamwidth in the far field will, in general, be somewhat different from the 3 dB beamwidth. The exact value depends upon the form factor of the radiation pattern and associated sidelobes. In the Fresnel Zone, the effective angle of the beamwidth will vary with distance. Here, the average power density W of the scanning antenna is given approximately by the following relationship; W = aP / A a / 2πr 360 / θ θ > 360(a/2πr) (Eq B5) and, ( )( )( ) W = 4P/A θ > 360(a/2πr) (Eq B6) where: θ = the scanned angle, in degrees P = the average power transmitted A = the effective area of the antenna a = antenna diameter or width r = distance from the antenna B5. Bibliography [B1] Bickmore, R. W. and R. C. Hansen, ''Antenna Power Densities in the Fresnel Region,'' IRE Proceedings, no. 47, pp. 2119 [E] 2120, Dec. 1959. [B2] Mumford, W. W., ''Some Technical Aspects of Microwave Radiation Hazards,'' IRE Proceedings, no. 49, pp. 427 [E] 447, Feb. 1961. [B3] USAF, Electromagnetic Radiation Hazards Technical Manual, TO 31Z-10-4, Aug. 1966, US Dept. of the Air Force, 1966. Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft, subject to change.
1/29/98 109 C95.3-1991 Revision — 2 nd Draft 10/98 Draft Appendix C Additional Techniques for SAR Measurement C1 . Thermography A method for rapid evaluation of the distribution of SAR throughout an entire planar surface of a biological object or a model that is composed of heterogeneous dielectric materials is described in [C3, C10 ]. The method involves the use of a thermographic camera for recording the rate of rise of temperature in a plane that bisects an entire model (phantom) or a cadaver of a biological subject under study. The temperature distribution before, and immediately after brief, high-power irradiation, is observed on the Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft, subject to change.
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