Field Guide · term

Also known as: near field, far field, Fraunhofer region, Fresnel region

The near field and far field are the two regions of space around a transmitting antenna, distinguished by how the electromagnetic field behaves with distance.1 Close to the antenna, in the near field, energy is partly stored and sloshes back and forth between the antenna and the surrounding space, and the field structure is complex and distance-dependent. Far away, in the far field, the wave has settled into a clean, outward-travelling spherical wave whose radiation pattern no longer changes shape with distance — only its amplitude falls off. Almost all communication and scanning happens in the far field.

reactive radiating near field 2D²/λ far field (planar)
Field regions from the antenna outward: reactive near field, radiating near field, and — past the Fraunhofer distance — the far field, where wavefronts are essentially planar.

How it works

Every antenna surrounds itself with two kinds of field. Reactive (stored) fields dominate very close in; they represent energy trapped in the antenna’s immediate vicinity, falling off steeply (as 1/r² and 1/r³) and carrying no net power away. Radiating fields fall off more gently (as 1/r) and carry power to infinity. The two coexist near the antenna and compete; only far away does the radiating field win decisively. This gives three nested regions:

  • Reactive near field — the innermost zone, extending to roughly 0.62·√(D³/λ) for an antenna of largest dimension D. Stored energy dominates; probing here disturbs the antenna.
  • Radiating near field (Fresnel region) — beyond that, out to the Fraunhofer distance. Power radiates, but the pattern is still evolving: the relative phase of contributions from different parts of the antenna changes with distance, so the “beam” is not yet fully formed.

  • Far field (Fraunhofer region) — beyond the Fraunhofer distance, conventionally

    d_F = 2D² / λ.

    Here the wavefront is locally planar, the electric and magnetic fields are in phase and perpendicular, their ratio is the impedance of free space (377 Ω), and the pattern is fixed.

The 2D²/λ boundary comes from requiring that path-length differences across the aperture stay within λ/16, so the wave looks flat. For a small antenna (D comparable to λ) the far field begins within a wavelength or two; for a large dish it can be hundreds of metres away.

In practice

The far-field assumption underpins nearly every everyday RF formula. A quoted antenna gain or beamwidth is a far-field property; the Friis transmission equation and field-strength calculations assume a plane wave and hold only beyond d_F. Antenna ranges must place the test source at least a Fraunhofer distance away (or use a compact/near-field range that measures the near field and mathematically transforms it to the far field). Near-field effects also explain coupling: two antennas placed within each other’s near field interact strongly, which matters for co-sited scanner antennas and for stacked arrays.

Relevance to SDR

For virtually all scanning and trunking work the transmitter is many wavelengths away, so the receiver sits comfortably in the far field and the tidy plane-wave, fixed-pattern picture applies — which is why a published antenna gain and a simple path-loss estimate are meaningful. The near field matters in two practical situations: when antennas are mounted close together (a scanner antenna beside a transmitting antenna can couple strongly, causing desense or damage), and when a handheld or nearby object is within a wavelength of the antenna and detunes it. GopherTrunk operates purely on far-field signals as delivered by the SDR and has no near-field awareness; the concept matters at the antenna-installation stage, not in the decode chain.

Sources

  1. Near and far field — Wikipedia, for the region definitions and the Fraunhofer distance 2D²/λ. ↩

See also