An antenna is a device that propagates radio frequency (RF) signals through the air. The transmitter (the Wi-Fi card or access point) sends an RF signal to the antenna, which amplifies the signal and sends it along through the medium of air.
When you are thinking about antennas, you need to think about the following characteristics:
Antennas used with Wi-Fi need to be tuned to 2.4GHz (802.11b or 802.11g) or 5GHz (802.11a). The frequency of the antenna needs to match the frequency of the radio transmitter.
Antennas are rated to handle a specific amount of power put out by a radio transmitter. In the case of Wi-Fi, this is not a great issue because most antennas are capable of handling the one-watt maximum transmission allowed by the FCC (see "Antennas and the FCC" later in this chapter for more information).
The radiation pattern of an antenna defines the shape of the radio wave that the antenna propagates, or sends into the air. The radiation pattern that all other radiation patterns are compared to is called isotropic. In an isotropic radiation pattern, the antenna transmits radio waves in all three dimensions equally, so that the pattern represents a ball, or globe, with the antenna at its center. Figure 17.1 is a depiction of the isotropic radiation pattern.
Figure 17.1. An omnidirectional antenna sends out signals in all directions on the horizontal plane.
The amount of gain an antenna provides means how much it increases the power of signals passed to it by the radio transmitter. The amount of gain is measured in decibels (dB), and bears a logarithmic relationship to the power input to the antenna. What you should keep in mind is that an antenna with a 3dB gain outputs double the power input to it, and an antenna with 6dB gain quadruples the power.
If you look at the specifications provided by antenna manufacturers, you will find gain measured in dBi, or gain in decibels relative to an isotropic radiation pattern. So dBi measures how much "better" a particular antenna is than if using a fictitious antenna with an isotropic radiation pattern, and it is a good measure of how effective an antenna is.