Fundamentals of Electromagnetics - RF/Microwave Interaction with Biological Tissues - page 23

Near field

Far field

Antenna

Fresnel

Fraunhofer

D 2 / l 2 D 2 / l

0 Distance

FIGURE 1.4 Angular distribution of fields around an antenna in the zone of near
fields (Fresnel zone) and far fields (Fraunhofer zone).

The radiating near-field (Fresnel) region extends from the reactive near-field
limit to a distance 2 D 2 /l, where D is the largest dimension of the antenna. For
this expression to be valid, D must also be large compared to the wavelength.
If the antenna has a maximum overall dimension which is very small com-
pared to the wavelength, this field region may not exist. For an antenna
focused at infinity, the radiating near field is sometimes referred to as the
Fresnel region on the basis of analogy to optical terminology. In this region
the field pattern is, in general, a function of the radial distance and the radial
field component may be appreciable.
The far-field (Fraunhofer) region is commonly taken to exist at distances
greater than 2 D 2 /l from the antenna. This criterion is based on a maximum
phase error of p/8. The reference to Fraunhofer is due to analogy to optical
terminology, when the antenna is focused at infinity. In this region the fields
are essentially transverse and the angular distribution is independent of the
radial distance where the measurements are made.
In RF and microwave communications, far-field situations are usually the
ones of most practical concern. This is very convenient because approxima-
tions can then be made to obtain closed-form solutions for the fields. This is
not always the case when evaluating biological effects, and it is very important
to clearly distinguish between near-field and far-field exposure. The evaluation
of hazards due to RF/microwave exposure on human beings or animals is
usually made in far-field conditions. Transmitting stations are normally far
enough from living and working situations on the one hand while the antenna
of a mobile telephone is so small with respect to the wavelength that the head
of an end user is in the far field of the antenna on the other hand. The evalu-
ation of specific biological effects , however, in particular in medical applica-
tions, is usually made in near-field conditions, as will be obvious later in this