Page 141 - Satellite Communications, Fourth Edition

Polarization 121

is parallel to the electric field E, and hence the induced voltage V will
be a maximum, denoted by V max . In Fig. 5.5b the dipole is at right angles
to the electric field, and the induced voltage is zero. In Fig. 5.5c the dipole
lies in the plane of polarization (the wavefront) but is at some angle a
to the electric field. The induced voltage will be given by

cos (5.7)
V V max
Note that for Eq. (5.7) to apply, the dipole has to lie in the same plane
as E (the wavefront). If the dipole is inclined at some angle q to the wave-
front, the received signal is reduced further by the radiation pattern of
the antenna. This is described more fully in Sec. 6.6. The reciprocity the-
orem for antennas (see Sec. 6.2) ensures that an antenna designed to
transmit in a given polarization will receive maximum power from a
wave with that polarization. An antenna designed for a given sense of
polarization will receive no energy from a wave with the orthogonal
polarization. Figures 5.5a and b illustrate the specific case where the
desired signal is vertically polarized and the orthogonal signal is hori-
zontally polarized. However, as mentioned above, certain impairments
can result in a loss of polarization discrimination, discussed in later
sections.
The combined power received by the two crossed dipoles will be max-
imum when the incoming wave is circularly polarized. The average
power received from a sinusoidal wave is proportional to the square of
the amplitude. Thus, for a circularly polarized wave given by either of
Eq. (5.4) or Eq. (5.5), the power received from each component is pro-
2
portional to E , and the total power is twice that of one component
alone. The crossed dipoles would receive this total. A single dipole will
always receive a signal from a circularly polarized wave, but at a loss
of 3 dB. This is so because the single dipole will respond only to one of
the linear components, and hence the received power will be half that
of the crossed dipoles. Again, because of the symmetry of the circularly
polarized wave, the dipole need only lie in the plane of polarization; its
orientation with respect to the xy-axes is not a factor.
Agrid of parallel wires will reflect a linear polarized wave when the elec-
tric field is parallel to the wires, and it will transmit the orthogonal wave.
This is illustrated in Fig. 5.6. This is used in one type of dual polarized
antenna, illustrated in Fig. 5.7. Here, the grid allows the wave, the elec-
tric field of which is transverse to the wires to pass through, whereas it
reflects the parallel (E ) wave. The reflector behind the grid reflects the
v
wave that passes through. Thus two orthogonal, linearly polarized waves,
having high polarization isolation (see Sec. 5.4) are transmitted from the
antenna system. Some details of the construction of this type of antenna
will be found in Maral and Bousquet (1998).