# 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.

- Canonical URL: https://emfbase.com/glossary/far-field
- Collection: Glossary
- Record id: `glossary:far-field`
- Status: verified
- Last verified: 2026-08-12
- JSON: https://emfbase.com/api/glossary/far-field

## Key values

- **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` International Telecommunication Union. *Free-space path loss and power density relations (ITU-R P.525)*. https://www.itu.int/rec/R-REC-P.525/en — standard, link checked 2026-08-12
- `fcc-oet65` Federal Communications Commission, Office of Engineering and Technology (1997). *OET Bulletin 65: Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields*. https://www.fcc.gov/general/oet-bulletins-line — government, link checked 2026-08-12

Data licence: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Dataset version 2026.08.1. Retrieved from EMFBase, https://emfbase.com.

This record is reference data, not medical or legal advice. Regulatory limits, scientific guidelines and precautionary values are labelled separately and answer different questions.