Near and far field
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In the study of diffraction and antenna design, the near field is that part of the radiated field nearest to the antenna, where the radiation pattern depends on the distance from the antenna. Beyond the near field is the far field.
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[edit] Overview
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Solving Maxwell's equations for the electric and magnetic fields for a localized oscillating source, such as an antenna, surrounded by a homogeneous material (typically vacuum or air), yields fields that, far away, decay proportional to 1/r where r is the distance from the source. These are the radiating fields, and the region where r is large enough for these fields to dominate is the far field.
More generally, the fields of a source in a homogeneous isotropic medium can be written as a multipole expansion.[1] The terms in this expansion are spherical harmonics (which give the angular dependence) multiplied by spherical Bessel functions (which give the radial dependence). For large r, the spherical Bessel functions decay as 1/r, giving the radiated field above. As one gets closer and closer to the source (smaller r), approaching the near field, other powers of r become significant.
The next term that becomes significant is proportional to 1/r2 and is sometimes called the induction term.[2] [3] It can be thought of as the energy stored in the field and returned to the antenna in every half-cycle. For even smaller r, terms proportional to 1/r3 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). Rather, in the near field, it is sometimes useful to express the contributions as a sum of radiating fields combined with evanescent fields, where the latter are exponentially decaying with r. And in the source itself, or as soon as one enters a region of inhomogeneous materials, the multipole expansion is no longer valid and the full solution of Maxwell's equations is generally required.
[edit] Near field
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 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 within a small number of wavelengths of the diffracting edge or antenna.
- In optical fiber communications, the region close to a source or aperture.
The diffraction pattern in the near field typically differs significantly from that observed at infinity and varies with distance from the source.
[edit] Far field
The far-field region is the region outside the near-field region, where the angular field distribution is essentially independent of distance from the source. If the source has a maximum overall dimension D that is large compared to the wavelength, the far-field region is commonly taken to exist at distances greater than (2D)²/λ from the source, λ being the wavelength.
For a beam focused at infinity, the far-field region is sometimes referred to as the Fraunhofer region. Other synonyms are far field, far zone, and radiation field..
[edit] See also
- Local effects
- Fresnel diffraction for more on the near field
- Fraunhofer diffraction for more on the far field
- Near Field Communication for more on near field communication technology
- Other
- Ground waves is a mode of propagation.
- Sky waves is a mode of propagation.
[edit] Patents
- Leydorf, G. F., U.S. Patent 3278937 , Antenna near field coupling system. 1966.
[edit] References
- ^ John David Jackson, Classical Electrodynamics, 3rd edition (Wiley: New York, 1998)
- ^ Johansson, J. and Lundgren, U., EMC of Telecommunication Lines
- ^ Capps, C., Near field or far field?, EDN, 16 August 2001
- This article contains material from the Federal Standard 1037C (in support of MIL-STD-188), which, as a work of the United States Government, is in the public domain.
