Field Regions

The fields surrounding an antenna are divided into 3 principle regions: Reactive Near Field Radiating Near Field or Fresnel Region Far Field or Fraunhofer Region The far field region is the most important, as this determines the antenna's radiation pattern. Also, antennas are used to communicate wirelessly from long distances, so this is the region of operation for most antennas. We will start with this region. Far Field (Fraunhofer) Region The far field is the region far from the antenna, as you might suspect. In this region, the radiation pattern does not change shape with distance (although the fields still die off with 1/R^2). Also, this region is dominated by radiated fields, with the E- and H-fields orthogonal to each other and the direction of propagation as with plane waves. If the maximum linear dimension of an antenna is D, then the far field region is commonly given as: This region is sometimes referred to as the Fraunhofer region, a carryover term from optics. Reactive Near Field Region In the immediate vicinity of the antenna, we have the reactive near field. In this region, the fields are predominately reactive fields, which means the E- and H- fields are out of phase by 90 degrees to each other (recall that for propagating or radiating fields, the fields are orthogonal (perpendicular) but are in phase). The boundary of this region is commonly given as: Radiating Near Field (Fresnel) Region The radiating near field or Fresnel region is the region between the near and far fields. In this region, the reactive fields are not dominate; the radiating fields begin to emerge. However, unlike the Far Field region, here the shape of the radiation pattern may vary appreciably with distance. The region is commonly given by: Note that depending on the values of R and the wavelength, this field may or may not exist. Finally, the above can be summarized via the following diagram: Next we'll look at numerically describing the directionality of an antenna's radiation pattern.