Antenna Theory - Near and Far Fields

After the antenna parameters discussed in the previous chapter, another important topic of consideration is the near field and the far field regions of the antenna.

The radiation intensity when measured nearer to the antenna, differs from what is away from the antenna. Though the area is away from the antenna, it is considered effective, as the radiation intensity is still high there.

Near Field

The field, which is nearer to the antenna, is called as near-field. It has an inductive effect and hence it is also known as inductive field, though it has some radiation components.

Far field

The field, which is far from the antenna, is called as far-field. It is also called as radiation field, as the radiation effect is high in this area. Many of the antenna parameters along with the antenna directivity and the radiation pattern of the antenna are considered in this region only.

Field Pattern

The field distribution can be quantifying in terms of field intensity is referred to as field pattern. That means, the radiated power from the antenna when plotted, is expressed in terms of electric field, E (v/m). Hence, it is known as field pattern. If it is quantified in terms of power (W), then it is known as power pattern.

The graphical distribution of radiated field or power will be as a function of

• spatial angles (θ, Ø) for far-field.

• spatial angles (θ, Ø) and radial distance(r) for near-field.

The distribution of near and far field regions can be well understood with the help of a diagram.

The field pattern can be classified as −

• Reactive near-field region and Radiating near-field region – both termed as nearfield.

• Radiating far-field region – simply called as far-field.

The field, which is very near to the antenna is reactive near field or non-radiative field where the radiation is not pre-dominant. The region next to it can be termed as radiating near field or Fresnel’s field as the radiation predominates and the angular field distribution, depends on the physical distance from the antenna.

The region next to it is radiating far-field region. In this region, field distribution is independent of the distance from antenna. The effective radiation pattern is observed in this region.