There are many different designs for antennas that Wi-Fi cards and access points can use.
Generally, antennas are either omnidirectional or directional.
Omnidirectional antennas are most commonly used with Wi-Fi cards and access points. These antennas send out radio waves in all directions equally on the horizontal plane but don't send out much in the way of signals vertically. The radiation pattern of an omnidirectional antenna looks like a doughnut, with the antenna in the center of the doughnut, as depicted in Figure 17.1.
Figure 17.2 shows a fairly standard omnidirectional antenna made by Maxrad, Inc., which is intended to work with Wi-Fi and the 2.4GHz spectrum, and is mounted externally on a roof.
In contrast to omnidirectional antennas, a directional antenna, which is also called a yagi, transmits radio signals in a focused beam, like a flashlight, or spotlight. Generally, the manufacturer's specifications give you some idea of the width of the radiation pattern of a yagi. For example, Figure 17.3 shows the radiation pattern for a yagi antenna from Cisco's Aironet division.
In general, directional antennas have much higher gain than omnidirectional antennas. Furthermore, the higher the gain of the directional antenna, the narrower is its beam.
In many cases, Wi-Fi cards and access points come with built-in antennas that cannot be changed. If you buy equipment with a built-in antenna and no way to add an external antenna, you are stuck with what you bought. For example, the Linksys Wireless Broadband unit discussed in Chapter 14 comes with two small parallel omnidirectional antennas, and no way to add an additional antenna.
High-gain directional antennas work best to facilitate point-to-point communications, for example, between two wireless bridges, each of which are located on top of a building on a campus. They also can be used to cover a long, but narrow area.
Intelligent use of directional antennas can cut down greatly on the number of access points required to cover an area.
Figure 17.4 shows a yagi (directional antenna) intended for use with 2.4GHz and 5GHz Wi-Fi broadcasts.
Antenna development for Wi-Fi (and other wireless technologies) is a hot area right now, with many new developments. For example, a company named WiFi-Plus, Inc., has developed multipolarized antennas. According to the company's chief technology officer, Jack Nilsson, these antennas have the ability to propagate and receive signals that are both horizontal and vertical. These models are better than conventional models for going around obstructions. WiFi-Plus's multipolarized antennas can also be used in situations where Wi-Fi is being broadcast using a directional antenna to a deep valley. A conventional directional antenna might broadcast signals that would overshoot the valley, but a multipolarized antenna is capable of broadcasting signals that travel horizontally following the direction of the RF beam, but also can be received down in the valley.
For more information about multipolarized antennas, see the company Web site, www.wifi-plus.com.