farfält

Farfält, or the far field, refers to the region around a radiating source where the emitted field becomes essentially independent of distance in its angular pattern and the radiated wave behaves as a propagating, radiative field. In this region the field strength decreases approximately as 1/r, and the angular distribution of the radiation is fixed by the source geometry rather than by proximity to the source.

In electromagnetism, the far-field (radiation field) is characterized by electric and magnetic fields that are transverse to the direction of propagation and to each other. The fields vary as e^{-j k r}/r, where k is the wavenumber and r is the distance from the source. The reactive, nonradiative components dominate only in the near field; far-field waves carry energy away from the source, and the Poynting vector points largely in the radial direction.

Distance criteria commonly used to define the onset of the far field depend on the source size D and the wavelength λ. A standard rough criterion is r > 2 D^2 / λ, known as the Fraunhofer distance. For very small sources, the far field is reached at relatively short distances; for large apertures or antennas, the far field begins farther away. Between near-field and far-field regimes there are radiating near-field (Fresnel) and reactive near-field regions.

The concept applies across disciplines. In optics, the far field corresponds to Fraunhofer diffraction, where far-field patterns are the Fourier transform of the aperture. In acoustics and radar, the far field is where the field behaves like outward-propagating spherical or locally planar waves, enabling simplified analysis and measurement.