Antenna Field Zones :
The space surrounding the antenna is divided into three regions according to the predominant field behaviour. The boundaries between the regions are not distinct and the field behaviour changes gradually as these boundaries are crossed. In this course, we are mostly concerned with the far-field characteristics of the antennas.
1. Reactive near-field region : This is the region immediately surrounding the antenna, where the reactive field dominates. For most antennas, it is assumed that this region is a sphere with the antenna at its centre
2. Radiating near-field (Fresnel) region : This is an intermediate region between the reactive near-field region and the far-field region, where the radiation field is more significant but the angular field distribution is still dependent on the distance from the antenna.
3. Far-field (Fraunhofer) region : Here r >> D and r >> λ
The angular field distribution does not depend on the distance from the source any more, i.e., the far-field pattern is already well established.