Introduction to RF Radiation Safety / 8 / 8.5

8.5 NEAR FIELDS AND FAR FIELDS

Chapter 5 defined the reactive near field, radiating near field, and far field and described their approximate boundaries and electromagnetic characteristics. The purpose of this section is to apply those concepts to RF safety assessment. The field region influences which quantities can be used, how measurements must be made, and how the results may be compared with exposure limits.

Field-region boundaries are engineering approximations rather than abrupt physical divisions. They depend on wavelength, antenna dimensions and geometry, observation position, and the accuracy required for the assessment. A calculated boundary is therefore a planning aid; the applicable assessment standard and the actual electromagnetic conditions determine whether a particular measurement or conversion method is valid.

8.5.1 Near Field

The near field comprises the reactive near field and the radiating near field. Both may exhibit substantial spatial variation, but their electromagnetic characteristics and assessment implications differ.

In the reactive near field, stored electric and magnetic energy is significant. Electric-field strength and magnetic-field strength may differ greatly in magnitude and spatial distribution, and either component may dominate depending on the source. Field strength can change rapidly over short distances, while nearby conductors, the measuring probe, its cable, or the person making the measurement may perturb the field.

Reactive near-field assessment therefore commonly requires separate electric-field and magnetic-field measurements, small probes, closely spaced measurement locations, and careful control of probe orientation and environmental disturbance. Incident power density must not be inferred from only one field component using a plane-wave conversion.

In the radiating near field, outward energy flow dominates and reactive components are less significant, but the wavefront may remain curved and the field pattern can vary with distance. Large aperture antennas, phased arrays, radar antennas, and cellular antenna systems can place accessible locations within this region. Plane-wave relationships may be usable where the applicable assessment method permits them, but spatial nonuniformity, beam shape, and the projected space occupied by the body must still be considered.

The applicable exposure standard determines which reference-level quantities may be used in each near-field condition. Depending on frequency and averaging interval, reactive near-field assessment may require both electric-field and magnetic-field measurements or may require assessment against the basic restrictions. A plane-wave-derived power density must not be used where the standard does not permit it.