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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. Non-radiative near-field behaviors dominate close to the antenna or scatterer, while electromagnetic radiation far-field behaviors predominate at greater distances.
The far-field pattern of an antenna may be determined experimentally at an antenna range, or alternatively, the near-field pattern may be found using a near-field scanner, and the radiation pattern deduced from it by computation. [1] The far-field radiation pattern can also be calculated from the antenna shape by computer programs such as NEC.
The first technique developed was the far-field range, where the antenna under test (AUT) is placed in the far-field of a range antenna. Due to the size required to create a far-field range for large antennas, near-field techniques were developed, which allow the measurement of the field on a distance close to the antenna (typically 3 to 10 ...
Determination of a far-field radiation pattern constitutes the primary application of antenna near-field scanners. This novel technique offered an attractive alternative to conventional open area test sites for measurements of high gain, electrically large antennas or antenna arrays (gain > 20 dBi, diameter > 5λ) in an indoor, controlled and ...
In addition to the usual derivation from antenna theory, the basic equation also can be derived from principles of radiometry and scalar diffraction in a manner that emphasizes physical understanding. [6] Another derivation is to take the far-field limit of the near-field transmission integral. [7]
The far-field region is far enough from the antenna to ignore its size and shape: It can be assumed that the electromagnetic wave is purely a radiating plane wave (electric and magnetic fields are in phase and perpendicular to each other and to the direction of propagation). This simplifies the mathematical analysis of the radiated field.
Some so-called "antennas" look very much like genuine loop antennas, but are designed to couple with the inductive near-field, over distances of 1–2 meters (3–7 feet), rather than to transmit or receive long-distance electromagnetic waves in the radiative far-field. Because of this difference the near-field "antennas" are not radio antennas ...
Dipole antenna used by the radar altimeter in an airplane. Animated diagram of a half-wave dipole antenna receiving a radio wave. The antenna consists of two metal rods connected to a receiver R. The electric field (E, green arrows) of the incoming wave pushes the electrons in the rods back and forth, charging the ends alternately positive ...