Introduction to the Radiating field regions of an antenna

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.