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Near and far field
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====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.]] {{Clear}} 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|Ξ»}} of the radiation. For such an antenna, the near field is the region within a radius {{math|''r'' βͺ ''Ξ»''}}, while the far-field is the region for which {{math|''r'' β« 2 ''Ξ»''}}. The transition zone is the region between {{math|''r'' {{=}} ''Ξ»''}} and {{math|''r'' {{=}} 2 ''Ξ»''β}}. 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.
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