Editing Near and far field (section)

===Regions according to diffraction behavior===
[[File:FarNearFields-USP-4998112.svg|thumb|left|500px |alt=Near- and far-field regions for an antenna larger (diameter or length {{mvar|D}}) than the wavelength of the radiation it emits, so that {{math|{{frac|''D''|λ}} ≫ 1}}. Examples are radar dishes and other highly directional antennas. |Near- and far-field regions for an antenna larger (diameter or length {{mvar|D}}) than the wavelength of the radiation it emits, so that {{math|{{frac|''D''|λ}} ≫ 1}}. Examples are radar dishes, satellite dish antennas, radio telescopes, and other highly directional antennas.]]
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====Far-field diffraction====
{{Main|Fraunhofer diffraction}}
As far as acoustic wave sources are concerned, if the source has a maximum overall dimension or aperture width ({{mvar|D}}) that is large compared to the wavelength {{mvar|λ}}, the far-field region is commonly taken to exist at distances, when the Fresnel parameter <math>S</math> is larger than 1:<ref>{{cite book |title=Acoustic Waves: Devices, imaging, and analog signal processing |editor=Kino, G. |publisher=Prentice Hall |year=2000 |at=Chapter&nbsp;3, page&nbsp;165}}</ref>
:<math display=block>S = {4\lambda \over D^2} r > 1, \text{ for } r > r_\text{F} = {D^2 \over 4\lambda}.</math>

For a [[light beam|beam]] focused at infinity, the far-field region is sometimes referred to as the ''Fraunhofer region''. Other synonyms are ''far field'', ''far zone'', and ''radiation field''. Any [[electromagnetic radiation]] consists of an [[electric field]] component {{math|'''E'''}} and a [[magnetic field]] component {{math|'''H'''}}. In the far field, the relationship between the electric field component {{math|'''E'''}} and the magnetic component {{math|'''H'''}} is that characteristic of any freely propagating wave, where {{math|'''E'''}} and {{math|'''H'''}} have equal [[Euclidean vector#Length|magnitudes]] at any point in space (where measured in units where [[speed of light|{{math|''c'']] {{=}} 1}}).

====Near-field diffraction====
{{Main|Fresnel diffraction}}
In contrast to the far field, the [[diffraction]] pattern in the near field typically differs significantly from that observed at infinity and varies with distance from the source. In the near field, the relationship between {{math|'''E'''}} and {{math|'''H'''}} becomes very complex. Also, unlike the far field where [[electromagnetic wave]]s are usually characterized by a single [[polarization (waves)|polarization]] type (horizontal, vertical, circular, or elliptical), all four polarization types can be present in the near field.<ref name=OSHA-EM-rad/>

The near field is a region in which there are strong inductive and capacitive effects from the currents and charges in the antenna that cause electromagnetic components that do not behave like far-field radiation. These effects decrease in power far more quickly with distance than do the far-field radiation effects. Non-propagating (or evanescent) fields extinguish very rapidly with distance, which makes their effects almost exclusively felt in the near-field region.

Also, in the part of the near field closest to the antenna (called the ''reactive near field'', [[#Reactive near field, or the nearest part of the near field|see below]]), absorption of electromagnetic power in the region by a second device has effects that feed back to the transmitter, increasing the load on the transmitter that feeds the antenna by decreasing the antenna impedance that the transmitter "sees". Thus, the transmitter can sense when power is being absorbed in the closest near-field zone (by a second antenna or some other object) and is forced to supply extra power to its antenna, and to draw extra power from its own power supply, whereas if no power is being absorbed there, the transmitter does not have to supply extra power.