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Chapter 5: Antenna Communications 71

Foundation Topics

Principles of Antennas

If someone asked you what the most important part of a wireless network is, what would

you say? I’d have to say the antenna. Why? Without it, you have a nice little AP that can

offer network services for anyone within about 3 feet. But that’s not what you want. You

want to make sure that your space is properly covered. You need antennas to do this. In

fact, you need the right antennas to do this. In this section you will learn about the factors

involved in dealing with antennas, which include polarity and diversity.

Polarization

The goal of an antenna is to emit electromagnetic waves. The electro portion of the term

electromagnetic describes the wave and that it can move in different ways. The way that it

moves is its polarization. There are three types of polarization:

Vertical

Horizontal

Circular

As shown in Figure 5-1, vertical polarization means that the wave moves up and down in

a linear way. Horizontal polarization means that the wave moves left and right in a linear

way.

The third type of polarization, circular polarization, indicates that the wave circles as

it moves forward, as illustrated in Figure 5-2.

The electric field is generated by stationary charges, or current. There is also a magnetic

field—hence the term electromagnetic. The magnetic field is found perpendicular (at a 90-

degree angle) to the electric field. This magnetic field is generated at the same time as the

electric field; however, the magnetic field is generated by moving charges. Cisco antennas

are always vertically polarized in wireless networks. This makes the electric field vertical.

Why is this important? The importance is that the antenna is designed to propagate sig-

nals in a certain direction. Here is where installation errors can hurt you. For example, if

you have a long tube-like antenna, it would face up/down. If you placed it flat instead, the

signal would propagate in a different direction and would end up degraded.

Although this is not a huge factor in indoor deployments, it can be in outdoor deploy-

ments. Usually other factors degrade your wireless signal propagation on indoor deploy-

ments.

Diversity

By now you should understand what the multipath issue is. Traffic takes different paths

because of the obstacles in the wireless path. One way to deal with multipath issues is to

use two antennas on one AP. Diversity is the use of two antennas for each radio to in-

crease the odds that you receive a better signal on either of the antennas.