Near and far field

The near field and far field are regions of the electromagnetic field around an object, such as a transmitting antenna, or the result of radiation scattering off an object. Non-radiative 'near-field' behaviors of electromagnetic fields dominate close to the antenna or scattering object, while electromagnetic radiation 'far-field' behaviors dominate at greater distances.

Far-field E and B field strength decreases inversely with distance from the source, resulting in an inverse-square law for the radiated power intensity of electromagnetic radiation. By contrast, near-field E and B strength decrease more rapidly with distance (with inverse-distance squared or cubed), resulting in relative lack of near-field effects within a few wavelengths of the radiator.

The far field is the region in which the field acts as "normal" electromagnetic radiation. In this region, it is dominated by electric or magnetic fields with electric dipole characteristics. The near field is governed by multipole type fields, which can be considered as collections of dipoles with a fixed phase relationship. The boundary between the two regions is only vaguely defined, and it depends on the dominant wavelength (λ) emitted by the source.

In the far-field region of an antenna, radiation decreases as the square of distance, and absorption of the radiation does not feed back to the transmitter. However, in the near-field region, absorption of radiation does affect the load on the transmitter. Magnetic induction (for example, in a transformer) can be seen as a very simple model of this type of near-field electromagnetic interaction.

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