Page 34 - Antennas for Base Stations in Wireless Communications

Fundamentals of Antennas 7

determined by the physical structure and orientation of the antenna. It
has nothing in common with the antenna directionality terms: horizon-
tal, vertical, and circular. Thus, a simple straight wire antenna will have
one polarization when mounted vertically and a different polarization
when mounted horizontally. Electromagnetic wave polarization filters
are structures that can be employed to act directly on the electromag-
netic wave to filter out wave energy of an undesired polarization and to
pass wave energy of a desired polarization.
Reflections generally affect polarization. For radio waves, the most
important reflector is the ionosphere—the polarization of signals
reflected from it will change unpredictably. For signals reflected by
the ionosphere, polarization cannot be relied upon. For line-of-sight
communications, for which polarization can be relied upon, having the
transmitter and receiver use the same polarization can make a huge
difference in signal quality; many tens of dB difference is commonly
seen, and this is more than enough to make up the difference between
reasonable communication and a broken link.
Polarization is largely predictable from antenna construction, but
especially in directional antennas, the polarization of sidelobes can be
quite different from that of the main propagation lobe. For radio anten-
nas, polarization corresponds to the orientation of the radiating ele-
ment in an antenna. A vertical omnidirectional WiFi antenna will have
vertical polarization (the most common type). One exception is a class
of elongated waveguide antennas in which a vertically placed antenna
is horizontally polarized. Many commercial antennas are marked as to
the polarization of their emitted signals.
Polarization is the sum of the E-plane orientations over time projected
onto an imaginary plane perpendicular to the direction of motion of
the radio wave. In the most general case, polarization is elliptical (the
projection is oblong), meaning that the polarization of the radio waves
emitting from the antenna is varying over time. Two special cases are
linear polarization (the ellipse collapses into a line) and circular polar-
ization (in which the ellipse varies maximally). In linear polarization,
the antenna compels the electric field of the emitted radio wave to a par-
ticular orientation. Depending on the orientation of the antenna mount-
ing, the usual linear cases are horizontal and vertical polarization. In
circular polarization, the antenna continuously varies the electric field
of the radio wave through all possible values of its orientation with
regard to the Earth’s surface. Circular polarizations (CP), like elliptical
ones, are classified as right-hand polarized or left-hand polarized using
a “thumb in the direction of the propagation” rule. Optical researchers
use the same rule of thumb, but point it in the direction of the emitter,
not in the direction of propagation, and so their use is opposite to that
of radio engineers. Some antennas, such the helical antenna, produce