Antenna Communications

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 signals 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 deployments. Usually other factors degrade your wireless signal propagation on indoor deployments.


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



Here is how it works: The two antennas are placed one wavelength apart. When the AP hears a preamble of a frame, it switches between the two antennas and uses an algorithm to determine which antenna has the better signal. After an antenna is chosen, it is used for the rest of that frame. You can switch antennas and listen to the preamble because it has no real data. As soon as the real data gets there, it uses only one of the antennas.

Most of the time this happens with a single radio in the AP and two antennas connected to it. This is important because the two antennas cover the same area. You wouldn’t try to cover two different areas with the same radio. Additionally, the antennas need to be the same. If you used a weaker antenna on one side versus the other, the coverage area would not be the same.