AN4190

Antenna theory

Doc ID 023812 Rev 1

7/29

1.1.4

Radiation pattern lobes

The space surrounding an antenna is usually sub-divided into three regions:

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Reactive near field

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Radiating near field, also called Fresnel region

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Far field, also called Fraunhofer region.

In the reactive near field region of an antenna, the non-radiating field components dominate.

The term reactive near field arises from the fact that for a non-resonant antenna such as an

electrically small dipole, reactive power circulates between the reactive near field and the

source, an external matching network, or both. The strict IEEE definition is “that portion of

the near field region immediately surrounding the antenna, wherein the reactive field

dominates”. Therefore, for dipole-like antennas, the energy in this region is predominantly

either electric or magnetic. For electrically small antennas, the reactive near field is taken to

extend to a distance of approximately R ~

λ / 2π from the antenna.

In the radiating near field, the radiation fields predominate but the angular field distribution is

dependent on the distance from the antenna. The strict IEEE definition is “that portion of the

near field region of an antenna between the far field and the reactive portion of the near field

region, wherein the angular field distribution is dependent upon distance from the antenna”.

If the antenna is large, compared to a wavelength, the outer boundary of the radiating near

λ.

The far field region of an antenna is the region surrounding an antenna which is sufficiently

far from the antenna such that only the radiating field components are significant. In other

words, the far field is that region of the field of an antenna where the angular field

distribution is essentially independent of the distance from a specified point in the antenna

region. In the far field, the field components are orthogonal and an equipartition of energy

between electric and magnetic stored energy exists.

In Figure 5 the separation between the three different regions is shown.

Figure 5.

Near and far field regions

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