8.15.2 Why Are Near-Field Measurements More Difficult Than Far-Field Measurements?
Near-field assessment is not difficult merely because the probe is close to an antenna. The field structure, relationship between electric and magnetic components, spatial variation, and interaction with the probe can differ from the simpler conditions often assumed in the far field.
Why Are Near-Field Measurements More Difficult Than Far-Field Measurements?
The boundary between field regions depends on wavelength and antenna size, not on one universal distance. The near field also contains two distinct regions—the reactive near field and the radiating near field—whose characteristics should not be treated as identical.
The Near Field Contains Two Regions
In the reactive near field, stored electric and magnetic energy close to the source can be important, and one component may dominate. In the radiating near field, also called the Fresnel region, radiation is established but the angular field distribution can still change with distance. For electrically large antennas, this region may extend well beyond the immediate vicinity of the antenna.
Plane-Wave Conversion May Not Be Valid
Far from an antenna under suitable conditions, electric-field strength, magnetic-field strength, and power density have a predictable relationship. Closer to the source, using one measured component to calculate the others may be invalid. The applicable method may require separate electric- and magnetic-field measurements or a more detailed calculation.
Spatial Variation and Measurement Perturbation
Near fields can vary rapidly over short distances. A probe must therefore be positioned carefully and sampled over the locations or volume required by the assessment method. The probe, supporting equipment, cable, and operator can also disturb the field, particularly when they are close to the source.
Large Antennas Can Have Extended Near Fields
A person can be many wavelengths from one part of a large aperture and still remain in its radiating near field. Distance alone is therefore not enough; antenna dimensions, frequency, geometry, and manufacturer or site information should be considered.
Far-Field Measurements Are Usually Simpler
In the far field, the wave impedance and directional pattern are more stable, so one field component may support calculation of the others. That physical relationship does not override the quantities required by the applicable exposure standard. For example, RPS S-1 requires both incident electric-field strength and incident magnetic-field strength from 100 kHz to 30 MHz regardless of the field region. Reflections, multiple sources, polarization, spatial variation, and time variation can still require careful measurement.
The Assessment Method Controls
The instrument, sampling grid, averaging procedure, and calculation assumptions should follow the applicable standard or validated method. Calling a location 'near field' does not prescribe one procedure; identifying the region helps determine which assumptions are defensible.
Summary
Near-field assessment must distinguish the reactive and radiating near fields and use a method appropriate to the field structure, spatial variation, and potential measurement perturbation.
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