Page 140 - Satellite Communications, Fourth Edition

120 Chapter Five

The axial ratio of an elliptical polarized wave is the ratio of major
axis to minor axis of the ellipse. Orthogonal elliptical polarization
occurs when a wave has the same value of axial ratio but opposite
sense of rotation.
Satellite communications links use linear polarization and circular
polarization, but transmission impairments can change the polarization
to elliptical in each case. Some of these impairments, relating to the
transmission medium, are described in Secs. 5.5, 5.6, and 5.7, and the influ-
ence of the antenna structure on polarization is described in Chap. 6.
Antennas are covered in detail in Chap. 6, but at this stage the relation-
ship of the antenna to the polarization type will be defined.

5.2 Antenna Polarization

The polarization of a transmitting antenna is defined by the polariza-
tion of the wave it transmits. Thus a horizontal dipole would produce a
horizontally polarized wave. Two dipoles mounted close together sym-
metrically and at right angles to each other would produce a circularly
polarized wave if fed with currents equal in amplitude but differing in
phase by 90°. This is shown by Eqs. (5.4) and (5.5). Note that because
of the symmetry of the circular polarization, the dipoles need not lie
along the horizontal and vertical axes; they just need to be spatially at
right angles to each other. The terms horizontal and vertical are used
for convenience.
The polarization of a receiving antenna has to be aligned to that of
the wave for maximum power transfer. Taking again the simple dipole
as an example, a vertical dipole will receive maximum signal from a ver-
tically polarized wave. Figure 5.5 illustrates this. In Fig. 5.5a the dipole

E
E

E
V = V max V = 0 V = V max cos

Direction of wave
Direction of wave propagation Direction of wave propagation propagation into page
(a) (b) (c)
Figure 5.5 Linear polarization relative to a receiving dipole.