This generates an oscillating (or reversing) electrical dipole, which affects both the near field and the far field. In a normally-operating antenna, positive and negative charges have no way of leaving the metal surface, and are separated from each other by the excitation "signal" voltage (a transmitter or other EM exciting potential). Far field: The radiation pattern can extend into the far field, where the reactive stored energy has no significant presence. The potential energy momentarily stored in this magnetic field is indicative of the reactive near field. Summary of regions and their interactions Near field: This dipole pattern shows a magnetic field B in red. The rapid drop in power contained in the near-field ensures that effects due to the near-field essentially vanish a few wavelengths away from the radiating part of the antenna, and conversely ensure that at distances a small fraction of a wavelength from the antenna, the near-field effects overwhelm the radiating far-field. By contrast, the near-field 's E and B strengths decrease more rapidly with distance: The radiative field decreases by the inverse-distance squared, the reactive field by an inverse- cube law, resulting in a diminished power in the parts of the electric field by an inverse fourth-power and sixth-power, respectively. Non-radiative near-field behaviors dominate close to the antenna or scatterer, while electromagnetic radiation far-field behaviors predominate at greater distances.įar-field E (electric) and B (magnetic) radiation field strengths decrease as the distance from the source increases, resulting in an inverse-square law for the power intensity of electromagnetic radiation in the transmitted signal. ![]() The near field and far field are regions of the electromagnetic (EM) field around an object, such as a transmitting antenna, or the result of radiation scattering off an object.
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