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Near and Far Field

The near field and far field regions of an isolated source of electromagnetic radiation are generally used terms in antenna measurements and describe regions around the source where different parts of the field are more or less important. The boundary between these two regions depends on the geometric dimensions of the source and the emitted by the source dominant wavelength λ. In the region of near field of an antenna the angular field distribution is dependent upon the distance from the antenna. The different parts of energy emitted by different geometric regions of the antenna have got a different running time and the resultant field cannot be constructively interfered to an evenly wave front.

A point like isotropic source cannot have a nearfield. This near field occur, if the geometric dimension of the source lies near the wavelength λ at least. The two regions are defined simply for mathematical convenience, enabling certain simplifying approximations of the Maxwell's equations. In the near field region there is a region, into an antenna collect a part of the just emitted energy too.

Figure 1: Near field region of an antenna array

Figure 1: Near field region of an antenna array

Figure 1: Near field region of an antenna array

The figure shows an antenna array of four in phase fed elements. Every element emits an electromagnetic field. These partially fields are combined to a common field. Although the figure is drawn large, all shown distances are near field region. To show the farfield, the width of this figure must be fourfold, to show the far field region, to show, that the magnitudes of the electromagnetic fields are added to a coherent wave front. In the far field, the shape of the antenna pattern is independent of distance from the source.

For small antennas (radiators width is smaller than the wavelength) the near field is the region within a radius r << λ, while the far field is the region for which r >> λ.

Al larger antennas (antenna arrays or using a big reflector, like parabolic dish antenna) the boundary between the two regions can be roughly calculated as:

rfar = 2 · D2 D = geometric dimension
λ = wavelength
(1)

λ

The far-field region is sometimes referred to as the Fraunhofer region, and the near-field region is sometimes referred to as the Fresnel region.

A part of the Fresnel region is an interactive region, emitting and collecting the energy. The boundary of this region is near the distance of D²/(2·λ) and it is sometimes referred to as the Rayleigh- region.

An example can be realised by analysing of the THALES- MSSR antenna, used at the ATC-radar ASR–909, consisting of 35 vertical elements and having an aperture of 8.5 meters. If the SSR uplink frequency of 1030 MHz is used as an example, it is possible to calculate the beam forming distance (far field). The SSR uplink frequency has a wavelength of λ = 0,291 m at 1030 MHz.

  • The far field begins at 497 m.
  • The Rayleigh- region is up to 124 m

The measuring tool for the antenna pattern must be located 497 meters far away the antenna at least. It is recommended, to choose a measuring site in a larger distance even.