Polarization is the physical orientation of the element on the antenna that
actually emits the RF energy. Polarization is a physical phenomenon of radio
signal propagation. Normally, any two antennas that form a link with each other
must be set for the same polarization. The polarization is usually adjustable
during or after the time of antenna installation. Basic information about
antenna polarization is shown in Figure
.
There are
two categories, or types, of polarization. They are linear and circular, as
shown in Figure
. Each
type has two sub-categories. The sub-categories for linear polarization are
vertical or horizontal, as illustrated in Figure
. The
sub-categories for circular polarization are right-handed or left-handed.
An omnidirectional antenna is usually a vertically polarized antenna. All
Cisco antennas are set for vertical polarization.
The antennas for both
ends of a link do not need to be the same size or type. In some cases, the
antenna mounting arrangements at one end of a link may only be able to
physically support a relatively small antenna. The link may require a larger
antenna at the other end to provide the needed antenna gain for the path
length. On the other hand, a high-gain, narrow-pattern antenna may be needed at
one end in order to avert an interference problem, which may not be a concern
at the other end.
If two antennas have different gains, it does not
matter which antenna is at which end, except in consideration of mounting or
interference issues. Remember that even though the two antennas for a link may
look very different from each other, they must have the same polarization for
the link to work properly.
Cross-Polarization
When two antennas do not have the same
polarization, the condition is called cross-polarization. For example, if two
antennas both had linear polarization, but one had vertical polarization and
the other had horizontal polarization, they would be cross polarized. The term
cross-polarization is also used to describe any two antennas with opposite
polarization.
Cross-polarization is sometimes beneficial. For example,
suppose the antennas of link A are cross-polarized to the antennas of link B.
In this example, links A and B are two different links, which are located near
each other, but are not intended to communicate with each other. In this case,
the fact that links A and B are cross-polarized is beneficial, because the
cross-polarization will prevent or reduce any possible interference between the
links. Figure
summarizes this
relationship.