Near and far fields in RF safety

Before converting an E-field reading into power density, decide whether a travelling-plane-wave model represents the local field. This lesson uses antenna aperture and wavelength to estimate the field region, then tests the assumptions against the actual geometry. Region estimates help choose an assessment method; they are not exposure limits or exclusion distances.

Next action: If your point is in the radiating near field, plan sampling with Uncertainty and spatial averaging.

At a glance (practitioner takeaways)

• Far field (plane-wave region): E and H are related by η₀ ≈ 377 Ω, and conversions like S = E²/377 are often valid (with environmental caveats).
• Near field: do not assume the plane-wave relationship. Reactive storage is important closest to the source; farther out, the radiating near field still has position-dependent structure.
• Region boundaries are approximations, not sharp lines—use them to choose methods and justify assumptions.
• Close-to-body or contact-adjacent sources often require SAR evaluation (and sometimes induced/contact current metrics) rather than power density.
• In the radiating near field, expect spatial variation; use scanning, multiple heights, and conservative interpretation.
• Defensibility comes from matching the method to the region and documenting assumptions, distances, and uncertainty.

Key definitions (regions and what they mean)

Wavelength

Antenna size parameter

Let D be the largest physical dimension of the radiating structure/aperture (arrays: overall aperture size, not one element).

Reactive near field

The region closest to the antenna where reactive (stored) energy can dominate, coupling is strong, and the field can be highly non-uniform.

Radiating near field (Fresnel region)

A transition region where radiation occurs but the wavefront is not yet well approximated as a plane wave; E/H may not equal 377 Ω, and field structure varies with position.

Far field (Fraunhofer region)

The region where the field behaves approximately like a locally plane wave; angular field distribution stabilizes; E and H ratios approach η₀ in free space.

Region boundary rules-of-thumb (use for method selection)

These aperture estimates are useful for electrically large antennas in free space, including the panel example below. They are not universal boundaries for electrically small antennas or complex coupled structures; use antenna-specific guidance there.

Reactive near-field extent (common approximation)

Far-field distance (Fraunhofer)

How to use these in practice

• If R is clearly greater than R_far, far-field assumptions are often reasonable (still check reflections and environment).
• If R is clearly less than R_reactive, treat it as strongly near-field and avoid plane-wave conversions.
• If R_reactive < R < R_far, treat it as radiating near field: expect spatial variation and don’t assume E/H = 377 Ω without validation.

The factor D²/λ reflects wavefront variation across the aperture: a wider aperture needs more distance before a far-field approximation is appropriate. Doubling D multiplies this estimate by four; doubling frequency at fixed D doubles it.

Multi-band antennas: for a fixed active aperture D, the highest relevant frequency gives the largest Fraunhofer estimate. If active aperture size differs by band, calculate 2D²/λ for each band and use the largest applicable distance.

What changes physically from near field to far field

In the far field

Plane-wave relationship (free space):

Angular pattern is stable; along a given direction, amplitude decays roughly with 1/R (power density with 1/R²), absent major reflections.

In the near field

• E and H may be out of phase and not in the 377 Ω ratio.
• Strong gradients: small moves can change readings materially.
• Coupling to nearby conductors (rails, ladders, enclosures, cabling) can reshape the field.
• Instruments can perturb the field or saturate; probe placement and cable routing can matter.

Quick decision workflow (field-region aware)

1. Define the source and bands (frequency range(s), max credible power, duty cycle, operating modes).
2. Estimate λ and D (use full aperture size for arrays/panels).
3. Compute the applicable region estimates for each active aperture and band; compare the resulting distances rather than selecting a band by frequency alone.
4. Classify accessible points by distance R (use a consistent distance definition; document antenna reference point).
5. Select method by region:
   • Far field: far-field E (or S) measurements and/or conservative calculations.
   • Radiating near field: scanning measurements and/or validated modeling; measure both E and H where required/appropriate.
   • Reactive near field / close-to-body: use a validated method appropriate to the frequency and the adopted basic restriction; be cautious with probe coupling.
6. Apply standard-specific averaging and combining (time/spatial, multi-source).
7. Document uncertainty and sensitivity (what would increase exposure).

Measurement approach by region (practical guidance)

Far field (typical site-accessible areas)

• Prefer calibrated broadband probes (or frequency-selective approaches when multiple sources exist).
• Plane-wave conversions ( S = E²/377) are often appropriate if the environment isn’t strongly reflective.
• Still scan enough points to confirm the field isn’t dominated by reflections/hotspots.

Radiating near field

• Plan for spatial variability: define an accessible volume and scan.
• Don’t assume E/H = 377 Ω; if the standard requires a particular quantity, measure or compute it appropriately.
• Be cautious about probe placement near metal and about operator presence altering the field.

Reactive near field / very close

• Expect strong coupling and local maxima.
• For close-proximity human exposure (handsets, wearables, industrial applicators), compliance often hinges on SAR methods and standardized setups rather than simple field probes.
• If you must measure fields, use appropriate near-field probes and protocols; interpret results carefully in light of perturbation and calibration limits.

Near-field classification does not automatically choose SAR. The relevant metric also depends on frequency, the body region and the adopted framework. ICNIRP 2020 distinguishes basic restrictions from reference levels and specifies field-region applicability in its reference-level conditions.

Classify a panel antenna measurement region

Common errors

• Selecting the highest frequency without checking whether another band has a larger active aperture.
• Using the wrong D (element size instead of full aperture for arrays/panels).
• Treating boundary distances as exact “lines” rather than approximate transitions.
• Applying plane-wave conversions in the radiating near field without checking whether E/H behaves like 377 Ω.
• Taking a single reading in a region with strong gradients and assuming it represents an accessible volume.
• Letting setup bias results: probe too close to conductors, cable coupling, operator body effects, or instrument saturation.

Further reading

• IEEE Std C95.1 and ICNIRP RF guidelines (exposure framework and quantities).
• IEC 62232 (assessment methods near radiocommunication installations).
• IEC 62311 (equipment assessment methodology).
• IEC 62209 series / IEEE Std 1528 (SAR measurement methods for devices, as applicable).
• IEC 62704 series (computational SAR methods, where applicable).
• ITU-T K.52 and ITU-T K.70 (practical guidance and mitigation).

Knowledge check

Exercises