8.5.2 Far Field

In the far field, the wavefront is locally planar and electric-field strength, magnetic-field strength, and incident power density are related through the free-space impedance. Once far-field conditions have been established, one suitable field quantity may therefore be sufficient to characterize the propagating wave. That physical relationship does not, by itself, determine which quantities an exposure standard requires for demonstrating compliance.

The far-field boundary criteria and their limitations are described in Chapter 5. Those criteria should not be applied mechanically: the appropriate boundary depends on antenna dimensions and geometry, and an applicable product, site, or measurement standard may prescribe a more conservative distance or additional validity conditions.

Far-field measurement is simpler than near-field measurement, but it is not automatically straightforward. Reflections, multipath propagation, multiple transmitters, polarization, modulation, duty cycle, traffic loading, beam steering, and spatial variation can still affect the measured field. The measurement plan must account for these factors and for the spatial and temporal averaging requirements of the applicable exposure standard.

Table 8.1 summarizes the properties and assessment implications of the three field regions. The boundaries shown are approximate planning aids, not exposure limits; the applicable assessment standard and antenna-specific information remain controlling.

Table 8.1. Summary of the three field regions and their assessment implications.

Property	Reactive Near Field	Radiating Near Field (Fresnel Region)	Far Field (Fraunhofer Region)
Indicative Boundary	Electrically large antenna: r < R₁, where R₁ ≈ 0.62√(D³/λ). Electrically small antenna: reactive behavior often remains important to about λ/2π.	For an electrically large antenna: approximately R₁ < r < R₂. This region may be small or absent for an electrically small antenna.	Electrically large antenna: r > R₂, where R₂ ≈ 2D²/λ. For electrically small antennas, use a source-specific or standard-defined separation.
Energy and Field Structure	Stored electric and magnetic energy is significant; one field component may dominate.	Radiated energy dominates, but field distribution remains distance-dependent.	Radiated energy dominates and the field structure is comparatively stable.
Wavefront and Angular Pattern	Strongly source-dependent and highly curved.	Curved; the angular field pattern changes with distance.	Locally planar; the angular field pattern is essentially independent of distance.
E/H Relationship	Magnitude and phase vary; there is no fixed plane-wave impedance.	Approaches the propagating-wave relationship, but plane-wave assumptions remain method-dependent.	Approximately orthogonal and in phase; E/H ≈ 377 Ω in free space.
Assessment Implication	Assess E and H separately; detailed spatial sampling or dosimetry may be required.	Use source-specific measurement or calculation with adequate spatial sampling.	One suitable quantity (E, H, or S) may suffice when far-field conditions are established and the method permits.