Editing Near and far field (section)

==Summary of regions and their interactions==
[[File:Felder um Dipol.svg|thumb|200px|'''Near field''': This dipole pattern shows a magnetic field {{math|'''B'''}} in red. The potential energy momentarily stored in this magnetic field is indicative of the reactive near field.]]
[[File:Sidelobes en.svg|thumb|200px|'''Far field:''' The radiation pattern can extend into the far field, where the reactive stored energy has no significant presence.]]

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). This generates an oscillating (or reversing) electrical dipole, which affects both the near field and the far field.

The boundary between the ''near field'' and ''far field'' regions is only vaguely defined, and it depends on the dominant [[wavelength]] ({{mvar|λ}}) emitted by the source and the size of the radiating element.

===Near field===
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 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.

:An easy-to-observe example of a near-field effect is the change of noise levels picked up by a set of [[dipole antenna#"Rabbit ears" TV antenna|rabbit ear TV antennas]] when a human body part is moved in close to the "ears". 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 near field is governed by [[multipole radiation |multipole type fields]], which can be considered as collections of dipoles with a fixed [[phase (waves)|phase relationship]].
The general purpose of [[antenna (radio)|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.

====Reactive near field====
The interaction with the medium (e.g. body capacitance) can cause energy to deflect back to the source feeding the antenna, as occurs in the ''reactive'' near field. This zone is roughly within {{sfrac|1|6}} of a wavelength of the nearest antenna surface.

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. 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. [[Electromagnetic induction|Magnetic induction]] as seen in a [[transformer]] can be seen as a very simple example of this type of near-field electromagnetic interaction.
For example send / receive coils for [[Radio-frequency identification|RFID]], and emission coils for [[inductive charging|wireless charging]] and [[induction heating|inductive heating]]; however their technical classification as "antennas" is contentious.

====Radiative near field====
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. Passive reflecting elements can be placed in this zone for the purpose of beam forming, such as the case with the [[Yagi–Uda antenna]]. Alternatively, multiple active elements can also be combined to form an antenna array, with lobe shape becoming a factor of element distances and excitation phasing.

===Transition zone===
Another intermediate region, called the ''transition zone'', is defined on a somewhat different basis, namely antenna geometry and excitation wavelength. It is approximately one wavelength from the antenna, and is where the electric and magnetic parts of the radiated waves first balance out: The electric field of a [[dipole antenna|linear antenna]] gains its corresponding magnetic field, and the magnetic field of a [[loop antenna]] gains its electric field. It can either be considered the furthest part of the near field, or the nearest part of the far field. It is from beyond this point that the electromagnetic wave becomes self-propagating. The electric and magnetic field portions of the wave are proportional to each other at a ratio defined by the characteristic impedance of the medium through which the wave is propagating.

===Far field===
In contrast, the ''far field'' is the region in which the field has settled into "normal" [[electromagnetic radiation]]. In this region, it is dominated by transverse [[electric field|electric]] or [[magnetic field]]s with [[dipole|electric dipole]] characteristics.
In the far-field region of an antenna, radiated power decreases as the [[inverse-square law|square of distance]], and [[absorption (electromagnetic radiation)|absorption of the radiation]] does not feed back to the transmitter.

In the far-field region, each of the electric and magnetic parts of the EM field is "produced by" (or associated with) a change in the other part, and the ratio of electric and magnetic field intensities is simply the [[wave impedance]] in the medium.

Also known as the ''radiation-zone'', the far field carries a relatively uniform wave pattern. The radiation zone is important because far fields in general fall off in amplitude by <math>\ \tfrac{1}{r}\ .</math> This means that the total energy per unit area at a distance {{mvar|r}} is proportional to <math>\ \tfrac{1}{r^2}\ .</math> The area of the sphere is proportional to <math>r^2</math>, so the total energy passing through the sphere is constant. This means that the far-field energy actually escapes to infinite distance (it ''radiates'').