Near Field vs Far Field Antenna Radiation Pattern
This page covers antenna radiation pattern basics and also describes difference between Near field and Far field radiation patterns.
Figure depicts radiation pattern of directional antenna. As shown in the figure, radiation pattern will have main lobe,side lobes and back lobe. Main lobe is the direction where in antenna radiates maximum energy into the air. Side lobes are the directions away from the max. energy directions. Radiation pattern help determine gain and directivity of the antenna. There are two fields of the antenna based on distance from it.
The directivity of the antenna is referred as antenna's ability to radiate EM-wave in particular direction. Beamwidth is used to mention directivity. Angular distance between two points on the main lobe. These points are 3dB less than the maximum power on the radiation pattern.
Antenna Gain (G) = (4 x Pi x Ae)/λ2= (4 x Pi x f2 x Ae)/c2
The region upto the distance of 2 D2/λ from the antenna is referred as near field region.
Where, D is the antenna diameter and Lambda is wavelength of free space.
The region above the distance of 2 D2/λ is referred as far field region. This region is also called as Fraunhofer region. In these region power radiated from antenna decays inverse of square of distance(1/R2).
Near field of the antenna will have more energy compare to the far field. This is because of proximity of EM wave to the antenna radiator part. Power in near field region follow 1/R4 and hence power intensity falls off very rapidly.
EXAMPLE: Near field distance calculation
Let us assume that an antenna is having length of about 7cm and it is radiating 14cm wavelength signal.
What will be the near field distance?
Answer: 7cm is the near field region
Antenna Measurement Test Setup
Refer Antenna measurement setup➤ which depicts test setup used for various antenna parameter measurements in near field and far field.