Page 135 - Satellite Communications, Fourth Edition

Chapter
5

Polarization

5.1 Introduction
In the far field zone of a transmitting antenna, the radiated wave takes
on the characteristics of a transverse electromagnetic (TEM) wave. Far
2
field zone refers to distances greater than 2D /l from the antenna,
where D is the largest linear dimension of the antenna and l is the wave-
length. For a parabolic antenna of 3 m diameter transmitting a 6-GHz
wave (l 5 cm), the far field zone begins at approximately 360 m. The
TEM designation is illustrated in Fig. 5.1, where it can be seen that both
the magnetic field H and the electric field E are transverse to the direc-
tion of propagation, denoted by the propagation vector k.
E, H, and k represent vector quantities, and it is important to note their
relative directions. When one looks along the direction of propagation, the
rotation from E to H is in the direction of rotation of a right-hand-threaded
screw, and the vectors are said to form a right-hand set. The wave always
retains the directional properties of the right-hand set, even when
reflected, for example. One way of remembering how the right-hand set
appears is to note that the letter E comes before H in the alphabet and
rotation is from E to H when looking along the direction of propagation.
At great distances from the transmitting antenna, such as are nor-
mally encountered in radio systems, the TEM wave can be considered
to be plane. This means that the E and H vectors lie in a plane, which
is at right angles to the vector k. The vector k is said to be normal to
the plane. The magnitudes are related by E HZ , where Z 0 120pΩ.
0
The direction of the line traced out by the tip of the electric field
vector determines the polarization of the wave. Keep in mind that the
electric and magnetic fields are varying as functions of time. The mag-
netic field varies exactly in phase with the electric field, and its ampli-
tude is proportional to the electric field amplitude, so it is only necessary
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