The space surrounding an antenna is usually subdivided into
three regions:
- reactive near-field
- radiating near-field (Fresnel) and
- far-field (Fraunhofer) regions
Although no abrupt changes in the field configurations are
noted as the boundaries are crossed, there are distinct differences among them.
The boundaries separating these regions are not unique, although various
criteria have been established and are commonly used to identify the regions.
Reactive near-field
region:
Reactive near-field region is defined as “that portion of
the near-field region immediately surrounding the antenna wherein the reactive field
predominates.” For most
antennas, the outer boundary of this region is
commonly taken to exist at a distance R < 0.62(D3/ƛ)1/2 from the antenna surface, where λ is the wavelength and D is
the largest dimension of the antenna. “For a very short dipole, or
equivalent radiator, the outer boundary is commonly taken to exist at a distance λ/2π from
the antenna surface.”
Radiating near-field (Fresnel) region:
Radiating near-field (Fresnel) region is defined as “that
region of the field of an antenna between the reactive near-field region and the
far-field region wherein radiation fields predominate and wherein the angular field
distribution is dependent upon the distance from the antenna. If the antenna has a maximum dimension
that is not large compared to the wavelength, this region may not exist. For an antenna
focused at infinity, the radiating near-field region is sometimes referred to as
the Fresnel region on the basis of analogy to optical terminology. If the antenna has a
maximum overall dimension which is very small compared to the wavelength, this field
region may not exist.” The inner boundary is
taken to be the distance R < 0.62(D3/ƛ)1/2 and the outer boundary the distance
R < 2D2/λ where D is the
largest∗ dimension of the antenna. This criterion is based
on a maximum phase error of π/8. In this region the field patterns, in general,
a function of the radial distance and the radial field component may be
appreciable.
Far-field (Fraunhofer) region:
Far-field (Fraunhofer) region is defined as “that region of the field of an antenna where the angular field distribution is essentially independent of the distance from the antenna. If the antenna has a maximum∗ overall dimension D, the far-field regions commonly taken to exist at distances greater than 2D2/λ from the antenna, λ being the wavelength. For an antenna focused at infinity, the far-field region is
sometimes referred to as the Fraunhofer region on the basis of analogy to optical
terminology.” In this region, the field components are essentially transverse
and the angular distribution is independent of the radial distance where the
measurements are made.
It is observed that the patterns are almost identical,
except for some differences in the pattern structure around the first null and
at a level below 25 dB. Because infinite distances are not realizable in
practice, the most commonly used criterion for minimum distance of far-field
observations is 2D2/λ.
