8.5.3 Transition Between Regions

The transition from one field region to another is gradual. A location slightly beyond a calculated boundary does not necessarily exhibit ideal plane-wave behavior, particularly near complex antennas, arrays, nearby structures, or reflective surfaces.

The field region should therefore be assigned using the antenna geometry, wavelength, applicable assessment standard, and the purpose of the measurement. Where the validity of a plane-wave assumption is important to the compliance conclusion, that assumption should be justified through the prescribed assessment method, supporting calculations, measurements of both field components, or validated computational modeling.

8.5.4 Measurement Implications

Near-field measurements generally require more detailed spatial sampling and greater attention to probe dimensions, orientation, isotropy, cable effects, body perturbation, and the independent behavior of the electric and magnetic fields. The measurement locations must be sufficient to identify localized maxima and to support any spatial averaging required by the applicable standard.

Far-field measurements above 30 MHz may often be performed using one appropriate electric-field, magnetic-field, or power-density measurement system where the applicable RPS S-1 table permits. From 100 kHz to 30 MHz, RPS S-1 requires both incident electric-field strength and incident magnetic-field strength to be assessed regardless of the field region. In every case, the selected quantities must be valid for the exposure duration and spatial averaging condition being assessed.

Determining the field region is therefore an early step in planning an RF safety assessment. It does not by itself establish compliance, but it determines which quantities, instruments, sampling procedures, conversions, and compliance pathways can be used. The following sections examine the measurement instruments and practical methods used for those tasks.