8.8.8 Multiple RF Sources
A multiple-source assessment begins with an inventory of the relevant transmitters, frequencies, operating conditions, locations, and field regions. A broadband measurement can provide a conservative screening result when it is compared with the most restrictive applicable reference level across the probe band, or when the probe incorporates an appropriate shaped response. If the screening result is not decisive, frequency-selective measurement or source-specific calculation is required.
Contributions at different frequencies must be combined using the exposure-ratio equations in the applicable standard, with the required temporal and spatial averaging. Electric-field strength, magnetic-field strength, and power density should not simply be added as raw values. Thermal and electrostimulation effects may require separate summations, and transitions between the applicable quantities and frequency ranges must be handled as specified by the standard.
The measurement system should also be checked for overload, out-of-band response, antenna coupling, and intermodulation products that could be mistaken for real sources. The report should identify the principal contributors and explain how the combined exposure result was calculated.
8.8.9 Source-Specific Measurement Plan
The source review should culminate in a written measurement plan. At minimum, the plan should define the assessment objective, exposure category and limits, source inventory and operating states, field region, measurement locations, quantities and instruments, temporal and spatial sampling, required corrections or extrapolation, uncertainty treatment, decision rule, and records to be retained.
A staged investigation is often efficient: use source information and calculations to plan the survey; perform a conservative screening assessment; undertake detailed frequency-selective, current, laboratory, or computational assessment only where needed; and verify any controls by repeat measurement. Section 8.9 addresses the practical quality controls that apply while this plan is carried out.