Far field and near field
Beyond roughly a wavelength from an antenna the field behaves as a propagating wave and inverse-square estimates apply; closer than that they do not.
physics
Last verified 12 August 2026.
- Category
- physics
Definition
In the far field, electric and magnetic components are in a fixed ratio, power density falls with the square of distance, and one measured quantity determines the others. In the near field — within roughly a wavelength, or 12.5 cm at 2.4 GHz — the components vary independently, inverse-square estimation is invalid, and exposure is assessed dosimetrically as SAR. This is why phone exposure is expressed as SAR and tower exposure as power density.
Sources
- itu-fs-relations Free-space path loss and power density relations (ITU-R P.525) — International Telecommunication Union. standard, link checked 2026-08-12.
- fcc-oet65 OET Bulletin 65: Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields — Federal Communications Commission, Office of Engineering and Technology, 1997. government, link checked 2026-08-12.
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