Near and far 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.

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.

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.

Another intermediate region, called the transition zone, is defined on a somewhat different basis, namely antenna geometry and excitation wavelength.

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.

[a] Definitions of the regions attempt to characterize locations where the activity of the associated field components are the strongest.

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 (also known as the near field or near zone) has the following meanings with respect to different telecommunications technologies: The most convenient practice is to define the size of the regions or zones in terms of fixed numbers (fractions) of wavelengths distant from the center of the radiating part of the antenna, with the clear understanding that the values chosen are only approximate and will be somewhat inappropriate for different antennas in different surroundings.

The choice of the cut-off numbers is based on the relative strengths of the field component amplitudes typically seen in ordinary practice.

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.

Either of the following two relations are equivalent, emphasizing the size of the region in terms of wavelengths λ or diameters D: This distance provides the limit between the near and far field.

[2][clarification needed] where D is the largest physical linear dimension of the antenna and dF is the far-field distance.

In this region, near-field behavior dies out and ceases to be important, leaving far-field effects as dominant interactions.

[4] The near field is a region in which there are strong inductive and capacitive effects from the currents and charges in the antenna that cause electromagnetic components that do not behave like far-field radiation.

Non-propagating (or evanescent) fields extinguish very rapidly with distance, which makes their effects almost exclusively felt in the near-field region.

[4] In this reactive region, not only is an electromagnetic wave being radiated outward into far space but there is a reactive component to the electromagnetic field, meaning that the strength, direction, and phase of the electric and magnetic fields around the antenna are sensitive to EM absorption and re-emission in this region, and respond to it.

This energy is carried back and forth from the antenna to the reactive near field by electromagnetic radiation of the type that slowly changes electrostatic and magnetostatic effects.

The reactive component of the near field can give ambiguous or undetermined results when attempting measurements in this region.

When a secondary radiating antenna surface is thus activated, it then creates its own near-field regions, but the same conditions apply to them.

The amplitude of other components (non-radiative/non-dipole) of the electromagnetic field close to the antenna may be quite powerful, but, because of more rapid fall-off with distance than

Thus, the near fields only transfer energy to very nearby receivers, and, when they do, the result is felt as an extra power draw in the transmitter.

In general, the fields of a source in a homogeneous isotropic medium can be written as a multipole expansion.

[6] It can be thought of as the primarily magnetic energy stored in the field, and returned to the antenna in every half-cycle, through self-induction.

become significant; this is sometimes called the electrostatic field term and can be thought of as stemming from the electrical charge in the antenna element.

Very close to the source, the multipole expansion is less useful (too many terms are required for an accurate description of the fields).

Typically one finds simple relations describing the antenna far-field patterns, often involving trigonometric functions or at worst Fourier or Hankel transform relationships between the antenna current distributions and the observed far-field patterns.

An examination of how the near fields form about an antenna structure can give great insight into the operations of such devices.

Thus, the far field "impedance of free space" is resistive and is given by: With the usual approximation for the speed of light in free space c0 ≈ 2.9979 × 108 m/s, this gives the frequently used expression: The electromagnetic field in the near-field region of an electrically small coil antenna is predominantly magnetic.

This article incorporates public domain material from websites or documents of the United States Government.