nearfield

Nearfield refers to the region around an electromagnetic source where the field does not yet resemble a propagating plane wave. This region is typically within about one wavelength of the source, though the exact boundary depends on frequency and the size and shape of the source.

In antenna theory, near-field is commonly divided into two subregions: the reactive near-field and the radiating near-field, also called the Fresnel region. The far-field, or Fraunhofer region, lies beyond this area where the fields are predominantly radiative, plane-wave-like, and decrease as 1/r. A commonly cited set of approximate boundaries uses the largest dimension of the source, D, and the wavelength, λ: the reactive near-field is roughly r < 0.62√(D³/λ); the radiating near-field extends from that boundary to about r ≈ 2D²/λ. When D is much smaller than λ, the reactive region can extend to about a wavelength.

Field behavior differs across these zones. In the reactive near-field, energy is stored in the magnetic and electric fields and E and H are largely out of phase by 90 degrees, with little net radiation. In the radiating near-field, some radiation begins to occur and complex field patterns emerge while still exhibiting nonuniform wavefronts. In the far-field, the fields are transverse, E and H are in phase and perpendicular to the direction of travel, and the amplitude decays as 1/r.

Near-field concepts underpin technologies such as wireless power transfer, near-field communication (NFC) and RFID, indutive sensing, and certain forms of near-field optical microscopy.