When a signal from a transmitter is applied to an antenna, it sends out electromagnetic waves in to free space. The EM field characteristics vary as a function of distance from the antenna. They are broadly divided into two regions, the near-field region, and the far field region.

The Near Field Region is the region right next to the antenna. It is defined by the following equation:

In this region, the fields are sort of unpredictable and therefore no measurements are usually made in this region.

This region is further divided into two parts:

Reactive Near Field: This is the region that is adjacent to the antenna. In this region, the E-Field and H-Field are 90 degrees out of phase with each other and are therefore reactive. To radiate or propagate the E/H fields need to be orthogonal (perpendicular) and in phase with each other.

Radiative Near Field: This region is also known as the Fresnel Region. It is the region between the reactive near field and the far field. This is the region where the EM fields start to transition from reactive to radiating fields. However, since they have not completely transitioned, the shape of the radiation pattern still varies with distance.

The Far Field Region is the region that comes after the near radiative near field. In this region, the EM fields are dominated by radiating fields. The E and H-fields are orthogonal to each other and to the direction of propagation as with plane waves. The far-field region is represented by the following equation:

Antennas are usually used to transfer signals at large distances which are considered to be in the far-field region. One condition that must be met when making measurements in the far field region is that the distance from the antenna must be much greater than the size of the antenna and the wavelength.

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