Passive reflecting elements can be placed in this zone for the purpose of beam forming, such as the case with the Yagi–Uda antenna. The interaction with the medium can fail to return energy back to the source, but cause a distortion in the electromagnetic wave that deviates significantly from that found in free space, and this indicates the radiative near-field region, which is somewhat further away. Magnetic induction as seen in a transformer can be seen as a very simple example of this type of near-field electromagnetic interaction.įor example send / receive coils for RFID, and emission coils for wireless charging and inductive heating however their technical classification as "antennas" is contentious. Although the far field is the usual region of antenna function, certain devices that are called antennas but are specialized for near-field communication do exist. The near field has been of increasing interest, particularly in the development of capacitive sensing technologies such as those used in the touchscreens of smart phones and tablet computers. This zone is roughly within 1 / 6 of a wavelength of the nearest antenna surface. body capacitance) can cause energy to deflect back to the source feeding the antenna, as occurs in the reactive near field. The general purpose of conventional antennas is to communicate wirelessly over long distances, well into their far fields, and for calculations of radiation and reception for many simple antennas, most of the complicated effects in the near field can be conveniently ignored. The near field is governed by multipole type fields, which can be considered as collections of dipoles with a fixed phase relationship. Likewise the change in sound quality of an FM radio tuned to a distant station when a person walks about in the area within an arm's length of its antenna. The electric and magnetic fields can exist independently of each other in the near field, and one type of field can be disproportionately larger than the other, in different subregions.Īn easy-to-observe example of a near-field effect is the change of noise levels picked up by a set of rabbit ear TV antennas when a human body part is moved in close to the "ears". The near field refers to places nearby the antenna conductors, or inside any polarizable media surrounding it, where the generation and emission of electromagnetic waves can be interfered with while the field lines remain electrically attached to the antenna, hence absorption of radiation in the near field by adjacent conducting objects detectably affects the loading on the signal generator (the transmitter). The boundary between the near field and far field regions is only vaguely defined, and it depends on the dominant wavelength ( λ) emitted by the source and the size of the radiating element. 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). Summary of regions and their interactions įar field: The radiation pattern can extend into the far field, where the reactive stored energy has no significant presence. 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. By contrast, near-field E and B strength 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 scattering object, while electromagnetic radiation far-field behaviors dominate at greater distances.įar-field E (electric) and B (magnetic) field strength decreases as the distance from the source increases, resulting in an inverse-square law for the radiated power intensity of electromagnetic radiation. 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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