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Microwave radiation hazards based on radiation fields
Short answer
RF fields: reactive near field r<0.62√(D³/λ) stores energy, E/H not 377; radiating near field to 2D²/λ forms pattern; far field is plane wave, E/H=377Ω. Reactive near field causes high SAR, burns and shock; near-field main beam heats eyes and testes; far-field S=EIRP/4πr² usually falls below limits.
Compare RF radiation fields and explain how they are hazardous to living body (microwave radiation hazards based on the radiation fields).
RF radiation fields around an antenna are classified by distance r from the antenna (D = largest dimension, λ = wavelength):
Antenna |<-- R1 -->|<------- R2 ------->| far field
[D] | reactive | radiating near | (Fraunhofer)
| near | (Fresnel) |
R1 = 0.62√(D³/λ) R2 = 2D²/λ
| Region | Range | Features |
|---|---|---|
| Reactive near field | r < 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/λ | Pattern changes with distance, fields not simple |
| Far field (Fraunhofer) | r > 2D²/λ | Plane wave, E/H = 377 Ω, S = EIRP/4πr² |
Comparison
| Point | Reactive near | Radiating near | Far field |
|---|---|---|---|
| Energy | Stored | Radiated, pattern forming | Radiated |
| E/H ratio | Not 377 Ω | Varies | 377 Ω |
| Field decay | 1/r² or 1/r³ | Irregular | 1/r |
| Measurement | E and H separately | Complex | Power density only |
| Exposure | Highest | High | Calculable, usually lowest |
How they are hazardous
- Reactive near field: strong E and H fields close to the antenna can deliver very high local SAR, causing burns and RF shock; workers on a live tower are at risk. Even a small antenna (phone) gives high local SAR to the head.
- Radiating near field: in the main beam of a BTS or radar dish, power density can exceed the limit; tissues heat, the eye lens and testes are most at risk (cataract, sterility).
- Far field: power density S = PtGt/(4πr²) falls with distance; for the public it is usually below ICNIRP reference levels, but long exposure to high-power radars or broadcast stations can still cause heating.
- In all zones, the body absorbs energy as heat (SAR = σE²/ρ). Above about 4 W/kg whole-body SAR, core temperature rises about 1 °C; limits are 0.4 W/kg (workers) and 0.08 W/kg (public).
Hence exclusion zones are set within the near field and the main beam, and exposure in the far field is checked against standards.
✓ Answer checked by Claude Opus 5.5 (high effort)
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