Nahfeld

Nahfeld, or near field, is a region surrounding a radiating object such as an antenna or optical source in which the electromagnetic field cannot be described as propagating plane waves. In radio frequency engineering the near field extends roughly to within a distance on the order of one wavelength from the source, though it is commonly subdivided into reactive near field and radiating (Fresnel) near field, followed by the far field.

The reactive near field is the innermost zone where electric and magnetic fields store energy rather than radiate it, and the field strength generally falls off rapidly with distance (often approximated as 1/r^3 for the dominant components). The radiating near field extends further out, up to roughly 0.62 times the square root of D cubed over λ, where D is the largest dimension of the source and λ is the wavelength. Beyond this region, up to the distance where the far field begins (approximately 2D^2/λ), the field starts to resemble outward-propagating waves with a more stable beam pattern. The far field, where radiation dominates and the field decays approximately as 1/r, lies beyond this.

Near-field behavior is characterized by strong coupling to nearby objects, rapid spatial variation, and the presence of evanescent components. In optics and nanophotonics, near-field effects enable phenomena such as evanescent coupling and subwavelength imaging in near-field microscopy. Applications include near-field communication and RFID, wireless power transfer, antenna testing, and scanning near-field optical microscopy (NSOM).

Terminology: Nahfeld is the German term for near field and is used in German-speaking engineering literature. It is usually contrasted with the far field, where radiation patterns assume plane waves at large distances.