BTS radiation fields at 900 and 1800 MHz

Radiation fields for λ/4 antennas

λ = c/f
900 MHz:  λ = 3×10⁸/9×10⁸  = 33.33 cm,  D = λ/4 = 8.33 cm
1800 MHz: λ = 3×10⁸/1.8×10⁹ = 16.67 cm, D = λ/4 = 4.17 cm

For a small antenna (D < λ), the formula 2D²/λ gives a distance smaller than λ/2π, so the reactive near field ends at about λ/2π and the far field starts beyond about one wavelength (~33 cm at 900 MHz, ~17 cm at 1800 MHz).

Comparison:

• All field regions at 1800 MHz are half as large as at 900 MHz, because λ and D are both halved.
• At 1800 MHz the energy is absorbed nearer the skin; at 900 MHz it penetrates deeper.
• Real BTS panel antennas are much larger (≈ 1–2 m), so their far field begins tens of metres away.

How microwave EMR becomes hazardous

The body absorbs RF energy, which turns into heat (SAR = σE²/ρ). Above about 4 W/kg, core temperature rises by about 1 °C. Organs with low blood flow, such as the eyes and testes, are damaged first (cataract, sterility); near-field exposure can cause burns. Possible non-thermal effects (headache, sleep disturbance) are still being studied.

IEEE safety standard

IEEE C95.1-2019 (limits for human exposure, 0 Hz–300 GHz):

• Whole-body SAR: 0.08 W/kg (unrestricted/public), 0.4 W/kg (restricted/occupational).
• Localised SAR (10 g): 2 W/kg (public), 10 W/kg (occupational).
• Public power density for 400–2000 MHz is S = f/200 W/m² (f in MHz), the same as ICNIRP: 4.5 W/m² at 900 MHz and 9 W/m² at 1800 MHz.