Antennas
Polarization

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.