GSM BTS propagation fields and non-ionizing SAR

Propagation fields of a GSM BTS

A GSM BTS antenna (900/1800 MHz panel, length D ≈ 1–2 m) produces three field regions:

 Antenna  |<-- R1 -->|<------- R2 ------->|  far field
   [D]    | reactive |  radiating near    |  (Fraunhofer)
          | near     |  (Fresnel)         |
R1 = 0.62√(D³/λ)       R2 = 2D²/λ

Example: 900 MHz (λ = 0.333 m), D = 2 m → R1 = 0.62√(8/0.333) = 3.04 m, R2 = 2×4/0.333 = 24 m.

Radiation parameters used to describe BTS exposure:

• EIRP = Pt·Gt (W or dBm): effective radiated power in the main beam.
• Power density S = EIRP/(4πr²) (W/m²) in the far field.
• Electric field E = √(377·S) (V/m), magnetic field H = E/377 (A/m).
• Antenna gain, beamwidth and tilt decide where the main beam reaches the ground.
• SAR (W/kg) for absorbed power in the body.

Non-ionising SAR and its hazard

Microwave radiation is non-ionising: each photon carries too little energy to break chemical bonds. Its effect on the body is measured by the Specific Absorption Rate:

SAR = σ|E|²/ρ   (W/kg)

where σ is tissue conductivity, E the internal rms field and ρ the tissue density.

How it becomes hazardous:

1. Absorbed power becomes heat. Whole-body SAR above about 4 W/kg raises core temperature by about 1 °C, causing heat stress.
2. High local SAR heats organs with poor blood flow: eye lens (cataract), testes (temporary sterility); very high levels cause burns.
3. Limits: ICNIRP/IEEE whole-body 0.08 W/kg (public), 0.4 W/kg (occupational); local 2 W/kg (public, 10 g).
4. Near a BTS, workers in the near field or main beam can exceed limits; the public at ground level is usually far below them.