8.8 MEASURING DIFFERENT RF SOURCES

The measurement plan must be matched to the source and to the purpose of the assessment. Before measurements begin, the assessor should identify the applicable exposure standard and exposure category; obtain the source frequencies, powers, operating modes, duty cycles, antenna or applicator characteristics, and accessible locations; and determine the operating condition that the result is intended to represent.

Calculations, equipment records, and a site inspection can be used to predict likely areas of maximum exposure and to decide whether a screening survey will be sufficient. Source behavior then determines the quantities to be measured, the instruments and settings required, the temporal and spatial sampling, and any extrapolation needed to represent the specified operating condition. The following examples illustrate those source-specific choices.

8.8.1 Broadcasting Transmitters

AM, FM, and television broadcasting transmitters commonly operate for long periods at relatively stable power. Initial calculations based on transmitter power, antenna gain or pattern, feed losses, and accessible geometry can identify locations requiring measurement. Ground-level public areas and elevated work locations should be considered separately because their positions relative to the antenna pattern may be very different.

A broadband field meter can provide an efficient screening survey where its frequency response and the applicable limits allow a conservative comparison. At multi-transmitter sites, frequency-selective measurements may be needed to identify each service and apply the required multiple-frequency summation. Far-field measurements can normally use an appropriate electric-field, magnetic-field, or power-density quantity; near antennas and energized structures, the field region and possible contact or induced-current pathways must also be considered.