far field
The region sufficiently distant from a radio antenna for angular field distribution to be independent of distance.
far field: where antenna radiation patterns finally stabilize
The far field is the region around a radiating antenna where the electric and magnetic fields have decoupled from each other and propagate together as a plane wave. At these distances, the field strength drops predictably with distance (proportional to 1/r), and the radiation pattern becomes independent of how far away you measure it. Closer to the antenna sits the near field, where the relationship between E and H fields is messy, and patterns shift dramatically with distance.
The boundary between near and far field depends on wavelength and antenna size. For most practical antennas, the far field begins at a distance of roughly 2D2/λ, where D is the largest dimension of the antenna and λ is the wavelength. A 1 GHz antenna with D = 0.3 m sits in far field beyond about 0.6 m; a 60 GHz array with D = 0.05 m may not reach true far field until tens of centimeters away. In some systems, notably phased arrays and directive antennas, the transition is gradual rather than sharp.
Far field measurements are standard in antenna characterization. Anechoic chambers, rooftop ranges, and compact antenna test ranges all work by ensuring the antenna under test is in far field when measurements begin. At these distances, you can measure gain, radiation pattern, polarization, and side lobe levels without worrying that the test distance itself is distorting your results. Near field probing, by contrast, requires careful modeling and is reserved for near-field imaging and diagnosis of antenna coupling problems.
The distinction matters for system design. A base station antenna performs according to its far field pattern when serving distant mobile devices; a reading head on an RFID portal operates partly in near field and partly in far field, so its actual behavior may deviate from catalogue radiation plots. Likewise, two antennas close together may interact via near field coupling even if their far field patterns suggest they should be decoupled. Wave propagation models and link budgets assume far field radiation, which is why they break down at very short ranges.