What are the near field and far field of an antenna?
The space around an antenna is divided into three regions: the reactive near field, where stored energy dominates (r < 0.62√(D³/λ)); the radiating near field or Fresnel region, where the pattern still changes with distance; and the far field or Fraunhofer region (r > 2D²/λ), where the angular pattern is independent of distance and fields fall off as 1/r.[1][2] Here D is the largest antenna dimension.
How are the boundaries defined?
| Region | Approximate extent | Character |
|---|---|---|
| Reactive near field | r < 0.62√(D³/λ) | E and H out of phase; energy stored and returned |
| Radiating near field (Fresnel) | 0.62√(D³/λ) ≤ r < 2D²/λ | Pattern depends on r |
| Far field (Fraunhofer) | r ≥ 2D²/λ | Transverse plane-like wave, E/H = η0, 1/r decay |
The 2D²/λ criterion corresponds to a maximum phase error of π/8 (22.5°) across the aperture. It assumes D is large compared with λ; for electrically small antennas the reactive boundary is usually taken as λ/2π.[1]
Why does the far-field distance matter in practice?
Gain and patterns are defined in the far field, so far-field ranges must be at least 2D²/λ long. A 1.2 m dish at 12 GHz (λ = 2.5 cm) needs about 115 m, which is why near-field scanning and compact ranges exist.[3] The Friis equation is also valid only in the far field.
What happens in the reactive near field?
Objects within the reactive near field couple strongly to the antenna and change its impedance, resonant frequency and efficiency. This is why a hand, a metal enclosure or a battery close to a phone antenna detunes it; see detuning and hand and body effects. Inductive links such as NFC deliberately operate in this region.[4]
Frequently asked questions
What is the far-field distance for a 10 cm antenna at 2.4 GHz?
2D²/λ = 2(0.1)²/0.125 ≈ 0.16 m, but it should also be several wavelengths away, so ≥ about 1 m is used in practice.
Is the far field always 2D²/λ?
It is the conventional criterion; stricter phase limits, low-sidelobe antennas and small antennas need larger or different criteria.
Do E and H fall off as 1/r in the far field?
Yes, and power density falls as 1/r².
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
- “IEEE Standard for Definitions of Terms for Antennas (IEEE Std 145-2013)”. IEEE Antennas and Propagation Society. 2014.
- “IEEE Recommended Practice for Antenna Measurements (IEEE Std 149-2021)”. IEEE Antennas and Propagation Society. 2022.
- “RFID Handbook, 3rd ed.”. K. Finkenzeller, Wiley. 2010.