Near and far field
The near field and far field are regions of the electromagnetic field around an object, such as a transmitting antenna, or the result of radiation scattering off an object. Non-radiative near-field behaviors dominate close to the antenna or scatterer, while electromagnetic radiation far-field behaviors predominate at greater distances.
Far-field and 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. By contrast, the near-fields and 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. 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.
Summary of regions and their interactions
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. This generates an oscillating 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 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 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.The near field is governed by multipole type fields, which can be considered as collections of dipoles with a fixed phase relationship.
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.
Reactive near field
The interaction with the medium can cause energy to deflect back to the source feeding the antenna, as occurs in the reactive near field. This zone is roughly within 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. 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 RFID, and emission coils for wireless charging and 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 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 or magnetic fields with electric dipole characteristics.In the far-field region of an antenna, radiated power decreases as the square of distance, and 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" 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 This means that the total energy per unit area at a distance is proportional to The area of the sphere is proportional to, so the total energy passing through the sphere is constant. This means that the far-field energy actually escapes to infinite distance.
Definitions
The separation of the electric and magnetic fields into components is mathematical, rather than clearly physical, and is based on the relative rates at which the amplitude of different terms of the electric and magnetic field equations diminish as distance from the radiating element increases. The amplitudes of the far-field components fall off as, the radiative near-field amplitudes fall off as, and the reactive near-field amplitudes fall off as. Definitions of the regions attempt to characterize locations where the activity of the associated field components are the strongest. Mathematically, the distinction between field components is very clear, but the demarcation of the spatial field regions is subjective. All of the field components overlap everywhere, so for example, there are always substantial far-field and radiative near-field components in the closest-in near-field reactive region.The regions defined below categorize field behaviors that are variable, even within the region of interest. Thus, the boundaries for these regions are approximate rules of thumb, as there are no precise cutoffs between them: All behavioral changes with distance are smooth changes. Even when precise boundaries can be defined in some cases, based primarily on antenna type and antenna size, experts may differ in their use of nomenclature to describe the regions. Because of these nuances, special care must be taken when interpreting technical literature that discusses far-field and near-field regions.
The term near-field region has the following meanings with respect to different telecommunications technologies:
- The close-in region of an antenna where the angular field distribution is dependent upon the distance from the antenna.
- In the study of diffraction and antenna design, the near field is that part of the radiated field that is below distances shorter than the Fraunhofer distance, which is given by from the source of the diffracting edge or antenna of longitude or diameter.
- In fiber-optic communication, the region near a source or aperture that is closer than the Rayleigh length.
Regions according to electromagnetic length
Electromagnetically short antennas
For antennas shorter than half of the wavelength of the radiation they emit, the far and near regional boundaries are measured in terms of a simple ratio of the distance from the radiating source to the wavelength of the radiation. For such an antenna, the near field is the region within a radius, while the far-field is the region for which. The transition zone is the region between and.The length of the antenna,, is not important, and the approximation is the same for all shorter 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.
Electromagnetically long antennas
For antennas physically larger than a half-wavelength of the radiation they emit, the near and far fields are defined in terms of the Fraunhofer distance. Named after Joseph von Fraunhofer, the following formula gives the Fraunhofer distance:where is the largest dimension of the radiator and is the wavelength of the radio wave. Either of the following two relations are equivalent, emphasizing the size of the region in terms of wavelengths or diameters :
This distance provides the limit between the near and far field. The parameter corresponds to the physical length of an antenna, or the diameter of a reflector antenna.
Having an antenna electromagnetically longer than one-half the dominated wavelength emitted considerably extends the near-field effects, especially that of focused antennas. Conversely, when a given antenna emits high frequency radiation, it will have a near-field region larger than what would be implied by a lower frequency.
Additionally, a far-field region distance must satisfy these two conditions.
where is the largest physical linear dimension of the antenna and is the far-field distance. The far-field distance is the distance from the transmitting antenna to the beginning of the Fraunhofer region, or far field.