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Radiation pattern
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==Reciprocity== [[Image:Omnidirectional antenna - horizontal and vertical radiation patterns.png|thumb|The radiation patterns of a vertical [[half-wave dipole]], an omnidirectional antenna. The horizontal and vertical polar patterns are projections of the 3 dimensional pattern onto horizontal and vertical planes, respectively. An omnidirectional antenna radiates equal signal strength in all horizontal directions, so its horizontal pattern is just a circle.]] It is a fundamental property of antennas that the '''receiving pattern''' (sensitivity as a function of direction) of an antenna when used for [[Radio receiver|receiving]] is identical to the far-field radiation pattern of the antenna when used for [[Transmitter|transmitting]]. This is a consequence of the [[reciprocity (electromagnetism)|reciprocity theorem]] of electromagnetics and is proved below. Therefore, in discussions of radiation patterns the antenna can be viewed as either transmitting or receiving, whichever is more convenient. There are limits to reciprocity: It applies only to ''[[passive component|passive]]'' antenna elements β ''active'' antennas that incorporate amplifiers or other individually powered components are ''not'' reciprocal. And even when the antenna is made of exclusively of passive elements, reciprocity only applies to the waves emitted and intercepted by the antenna. Reciprocity does ''not'' apply to the distribution of current in the various parts of the antenna generated by the intercepted waves nor currents that create emitted waves: Antenna current profiles typically differ for receiving and transmitting, despite the waves in the [[near and far field|far field]] radiating inward and outward along the same path, with the same overall pattern, just with reversed direction.
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