Page 161 - Satellite Communications, Fourth Edition

Antennas 141

Radiating
near-field
(Fresnel) region

Reactive Far-field
near-field (Fraunhofer)
region region

R 2 =2D 2 D 3
λ R 1 = .62 λ

(a)

E

k

H
(b)
Figure 6.4 (a) The electromagnetic-field regions surrounding an antenna. (b) Vector
diagrams in the far-field region.

Here, D is the largest dimension of the antenna (e.g., the diameter of a
parabolic dish reflector), and l is the wavelength. Only the far-field region
2
is of interest here, which applies for distances greater than about 2D /l.
In the far-field region, the radiated fields form a transverse electro-
magnetic (TEM) wave in which the electric field is at right angles to the
magnetic field, and both are at right angles (transverse) to the direction