Page 150 - Satellite Communications, Fourth Edition

130 Chapter Five

When the transmitted signals have the same magnitudes (E E )
2
1
and where the receiving system introduces negligible depolarization,
then I and XPD give identical results.
For clarity, linear polarization is shown in Fig. 5.11, but the same defi-
nitions for XPD and I apply for any other system of orthogonal polarization.

5.5 Ionospheric Depolarization
The ionosphere is the upper region of the earth’s atmosphere that has
been ionized, mainly by solar radiation. The free electrons in the iono-
sphere are not uniformly distributed but form layers. Furthermore,
clouds of electrons (known as traveling ionospheric disturbances) may
travel through the ionosphere and give rise to fluctuations in the signal.
One of the effects of the ionosphere is to produce a rotation of the polar-
ization of a signal, an effect known as Faraday rotation.
When a linearly polarized wave traverses the ionosphere, it sets in
motion the free electrons in the ionized layers. These electrons move in
the earth’s magnetic field, and therefore, they experience a force (sim-
ilar to that which a current-carrying conductor experiences in the mag-
netic field of a motor). The direction of electron motion is no longer
parallel to the electric field of the wave, and as the electrons react back
on the wave, the net effect is to shift the polarization. The angular shift
in polarization (the Faraday rotation) is dependent on the length of the
path in the ionosphere, the strength of the earth’s magnetic field in the
ionized region, and the electron density in the region. Faraday rotation
is inversely proportional to frequency squared and is not considered to
be a serious problem for frequencies above about 10 GHz.
Suppose a linearly polarized wave produces an electric field E at the
receiver antenna when no Faraday rotation is present. The received
2
power is proportional to E . A Faraday rotation of q degrees will result
F
in the copolarized component (the desired component) of the received
signal being reduced to E E cos q , the received power in this case
F
co
2
being proportional to E . The polarization loss (PL) in decibels is
co
E co
PL 20 log
E (5.19)
20log(cos
)
F
At the same time, a cross-polar component E E sinq is created,
x
F
and hence the XPD is
E co
XPD 20 log
E x (5.20)
20log(cot
)
F