Editing Near and far field (section)

===Regions according to electromagnetic length===
The most convenient practice is to define the size of the regions or zones in terms of fixed numbers (fractions) of wavelengths distant from the center of the radiating part of the antenna, with the clear understanding that the values chosen are only approximate and will be somewhat inappropriate for different antennas in different surroundings. The choice of the cut-off numbers is based on the relative strengths of the field component amplitudes typically seen in ordinary practice.

====Electromagnetically short antennas====
[[File:Field regions for typical antennas vector.svg|thumb|left|500px|alt=Antenna field regions for antennas that are equal to, or shorter than, one-half wavelength of the radiation they emit, such as the whip antenna of a citizen's band radio, or the antenna in an AM radio broadcast tower.|Field regions for antennas equal to, or shorter than, one-half wavelength of the radiation they emit, such as the whip antenna of a citizen's band radio, or an AM radio broadcast tower.]]
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For antennas shorter than half of the wavelength of the radiation they emit (i.e., electromagnetically "short" antennas), the far and near regional boundaries are measured in terms of a simple ratio of the distance {{mvar|r}} from the [[radio frequency|radiating source]] to the [[wavelength]] {{mvar|λ}} of the radiation. For such an antenna, the near field is the region within a radius {{math|''r'' ≪ ''λ''}}, while the far-field is the region for which {{math|''r'' ≫ 2 ''λ''}}. The transition zone is the region between {{math|''r'' {{=}} ''λ''}} and {{math|''r'' {{=}} 2 ''λ'' }}.

The length of the antenna, {{mvar|''D''}}, is not important, and the approximation is the same for all shorter antennas (sometimes idealized as so-called ''point antennas''). In all such antennas, the short length means that charges and currents in each sub-section of the antenna are the same at any given time, since the antenna is too short for the RF transmitter voltage to reverse before its effects on charges and currents are felt over the entire antenna length.

====Electromagnetically long antennas====
For antennas physically larger than a half-wavelength of the radiation they emit, the near and far fields are defined in terms of the '''[[Fraunhofer distance]]'''. Named after [[Joseph von Fraunhofer]], the following formula gives the [[Fraunhofer distance]]:
:<math display=block>d_\text{F} \; = \; \frac{2 D^2}{\lambda} \, ,</math>

where {{mvar|D}} is the largest dimension of the radiator (or the [[diameter]] of the [[Antenna (radio)|antenna]]) and {{mvar|λ}} is the [[wavelength]] of the radio [[wave]]. Either of the following two relations are equivalent, emphasizing the size of the region in terms of wavelengths {{math|λ}} or diameters {{math|D}}:
:<math display=block>d_\text{F} \; = \; 2 { \left( { D \over \lambda } \right) }^2 \lambda \; = \; 2 { \left( { D \over \lambda } \right) } D</math>

This distance provides the limit between the near and far field. The parameter {{mvar|D}} corresponds to the physical length of an antenna, or the diameter of a reflector ("dish") antenna.

Having an antenna electromagnetically longer than one-half the dominated wavelength emitted considerably extends the near-field effects, especially that of focused antennas. Conversely, when a given antenna emits high frequency radiation, it will have a near-field region larger than what would be implied by a lower frequency (i.e. longer wavelength).

Additionally, a far-field region distance {{math|''d''<sub>F</sub>}} must satisfy these two conditions.<ref>{{cite book |author=Rappaport, Theodore S. |title=Wireless Communications Principles and Practice |edition=19th printing, 2nd |publisher=Prentice-Hall |year=2010 |page=108}}</ref>{{Clarify|reason=These are imprecise, dumbed-down versions of what has been said more clearly above.|date=May 2015}}
:<math display=block>d_\text{F} \gg D\,</math>
:<math display=block>d_\text{F} \gg \lambda\,</math>

where {{mvar|D}} is the largest physical linear dimension of the antenna and {{math|''d''<sub>F</sub>}} is the far-field distance. The far-field distance is the distance from the transmitting antenna to the beginning of the Fraunhofer region, or far field.

====Transition zone====
The ''transition zone'' between these near and far field regions, extending over the distance from one to two wavelengths from the antenna,{{citation needed|date=December 2011}} is the intermediate region in which both near-field and far-field effects are important. In this region, near-field behavior dies out and ceases to be important, leaving far-field effects as dominant interactions. (See the "Far Field" image above.)