Figure 92 Antenna representations in diagrams

To function at all, an antenna must be made of conducting material. Radio waves hitting an antenna cause electrons to flow in the conductor and create a current. Likewise, applying a current to an antenna creates an electric field around the antenna. As the current to the antenna changes, so does the electric field. A changing electric field causes a magnetic field, and the wave is off.

The size of the antenna you need depends on the frequency: the higher the frequency, the smaller the antenna. The shortest simple antenna you can make at any frequency is 1/2 wavelength long (though antenna engineers can play tricks to reduce antenna size further). This rule of thumb accounts for the huge size of radio broadcast antennas and the small size of mobile phones. An AM station broadcasting at 830 kHz has a wavelength of about 360 meters and a correspondingly large antenna, but an 802.11b network interface operating in the 2.4-GHz band has a wavelength of just 12.5 centimeters. With some engineering tricks, an antenna can be incorporated into a PC Card, and a more effective external antenna can easily be carried in a backpack.

Antennas can also be designed with directional preference. Many antennas are omnidirectional, which means they send and receive signals from any direction. Some applications may benefit from directional antennas, which radiate and receive on a narrower portion of the field. Figure 9-3 compares the radiated power of omnidirectional and directional antennas.

Figure 9-3. Radiated power for omnidirectional and directional antennas

For a given amount of input power, a directional antenna can reach farther with a clearer signal. They also have much higher sensitivity to radio signals in the dominant direction. When wireless links are used to replace wireline networks, directional antennas are often used. Mobile telephone network operators also use directional antennas when cells are subdivided. 802.11 networks typically use omnidirectional antennas for both ends of the connection, though there are exceptions—particularly if you want the network to span a longer distance. Also, keep in mind that there is no such thing as a truly omnidirectional antenna. We're accustomed to thinking of vertically mounted antennas as omnidirectional because the signal doesn't vary significantly as you travel around the antenna in a horizontal plane. But if you look at the signal radiated vertically (i.e., up or down) from the antenna, you'll find that it's a different story. And that part of the story can become important if you're building a network for a college or corporate campus and want to locate antennas on the top floors of your buildings.

Of all the components presented in this section, antennas are the most likely to be separated from the rest of the electronics. In this case, you need a transmission line (some kind of cable) between the antenna and the transceiver. Transmission lines usually have an impedance of 50 ohms.

In terms of practical antennas for 802.11 devices in the 2.4-GHz band, the typical wireless PC Card has an antenna built in. As you can probably guess, that antenna will do the job, but it's mediocre. Most vendors, if not all, sell an optional external antenna that plugs into the card. These antennas are decent but not great, and they will significantly increase the range of a roaming laptop. You can usually buy some cable to separate the antenna from the PC Card, which can be useful for a base station. However, be careful— coaxial cable (especially small coaxial cable) is very lossy at these frequencies, so it's

Omnidirectional

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Figure 9-3. Radiated power for omnidirectional and directional antennas

Omnidirectional easy to imagine that anything you gain by better antenna placement will be lost in the cable. People have experimented with building high-gain antennas, some for portable use, some for base station use. And commercial antennas are available—some designed for 802.11 service, some adaptable if you know what you're doing.

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