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Short note: Microwave radiation fields

Short answer

Space around an antenna is divided into reactive near field (r < 0.62 sqrt(D^3/lambda)), radiating near field (Fresnel) from that to 2D^2/lambda, and far field beyond 2D^2/lambda. Far field is a plane wave with E/H = 377 ohms and S = EIRP/4 pi r^2. Example D=2 m, lambda=0.333 m gives R1=3.04 m, R2=24 m.

Write a short note on microwave radiation fields.

The space around a radiating antenna is divided into three field regions, depending on distance r, antenna size D and wavelength λ:

 Antenna  |<-- R1 -->|<------- R2 ------->|  far field
   [D]    | reactive |  radiating near    |  (Fraunhofer)
          | near     |  (Fresnel)         |
R1 = 0.62√(D³/λ)       R2 = 2D²/λ
RegionRangeFeatures
Reactive near fieldr < 0.62√(D³/λ)Stored energy, E and H not in phase, very high fields
Radiating near field (Fresnel)0.62√(D³/λ) to 2D²/λPattern changes with distance, fields not simple
Far field (Fraunhofer)r > 2D²/λPlane wave, E/H = 377 Ω, S = EIRP/4πr²
  • Reactive near field: closest to the antenna; energy is stored and exchanged between antenna and space. E and H are not in phase, so both must be measured. Fields can be very strong, giving high local SAR.
  • Radiating near field (Fresnel region): radiation dominates, but the angular pattern still depends on distance; power density varies irregularly.
  • Far field (Fraunhofer region): the wave is nearly plane, E ⊥ H, E/H = 377 Ω, pattern is fixed, and S = PtGt/(4πr²) = E²/377.

Example: GSM 900 MHz panel, D = 2 m, λ = 0.333 m: R1 = 3.04 m, R2 = 24 m.

For safety, exposure limits are checked with power density in the far field, while near-field zones are usually made exclusion zones for workers only.

✓ Answer checked by Claude Opus 5.5 (high effort)

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