nærfelt

Nærfelt is the region around a radiating object, such as an antenna or a light source, where the electromagnetic field does not yet form a propagating plane wave. In this region, the fields can be reactive, meaning energy is stored rather than radiated, and their spatial distribution is complex and dependent on the device geometry.

The near field is commonly divided into two subregions: the reactive near-field and the radiating near-field (Fresnel region). The reactive near-field typically extends up to a distance on the order of 0.62 sqrt(D^3 / lambda), where D is the largest dimension of the radiator and lambda is the wavelength. In this zone, the electric and magnetic fields are out of phase and energy storage dominates. The radiating near-field extends further, up to about 2 D^2 / lambda, where the field patterns are still developing but begin to resemble the far-field behavior. Beyond this region lies the far field (Fraunhofer region), where the radiated fields behave like propagating plane waves, E and H are in phase, and the field strength falls roughly as 1/r with distance.

Characteristics of the near field have practical implications for coupling to nearby objects, such as RFID tags, NFC devices, wireless power transfer, and near-field sensors. Measuring near-field distributions requires specialized probes capable of mapping local E and H fields rather than relying on far-field assumptions. In optics, near-field concepts relate to evanescent waves and sub-wavelength imaging techniques like near-field scanning optical microscopy.

In summary, nærfelt denotes the close-in region around a source where energy storage and complex, non-propagating field structures dominate, in contrast to the radiating far field that governs long-range propagation.