{
  "id": "glossary:far-field",
  "collection": "glossary",
  "slug": "far-field",
  "name": "Far field and near field",
  "summary": "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.",
  "status": "verified",
  "lastVerified": "2026-08-12",
  "data": {
    "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.",
    "category": "physics",
    "seeAlso": [
      "quantities:power-density",
      "glossary:sar"
    ]
  },
  "sources": [
    {
      "id": "itu-fs-relations",
      "title": "Free-space path loss and power density relations (ITU-R P.525)",
      "publisher": "International Telecommunication Union",
      "url": "https://www.itu.int/rec/R-REC-P.525/en",
      "kind": "standard",
      "retrieved": "2026-08-12"
    },
    {
      "id": "fcc-oet65",
      "title": "OET Bulletin 65: Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields",
      "publisher": "Federal Communications Commission, Office of Engineering and Technology",
      "url": "https://www.fcc.gov/general/oet-bulletins-line",
      "year": 1997,
      "kind": "government",
      "retrieved": "2026-08-12"
    }
  ],
  "links": {
    "self": "https://emfbase.com/api/glossary/far-field",
    "html": "https://emfbase.com/glossary/far-field",
    "markdown": "https://emfbase.com/glossary/far-field.md",
    "collection": "https://emfbase.com/api/glossary"
  }
}