In Chapter 9, "Wi-Fi on Your Desktop," I showed you an example of a Wi-Fi network interface card (NIC) that comes with an external antenna (the D-Link DWL-520). And, in Chapter 14, "Setting Up Your Access Point," I showed you an example of an external antenna attached to an access point (the Apple Extreme Base Station).
Antennas are used with Wi-Fi cards to give them a better connection to the access point broadcasting a signal.
Antennas are used with access points to add power (and range) to the access point's radio broadcast. They are also an integral part of a Wi-Fi network that covers a substantial area. The use of antennas with your access points can greatly reduce costs in deploying a wireless local area network because you can use fewer access points than you would need without the antennas.
If you are planning to use antennas as part of your Wi-Fi network deployment, it's important to understand the different kinds of antennas that are available. (You also need to make sure to buy access points that come with plugs for external antennas.)
An important part of deploying a Wi-Fi network is doing a site survey, which takes into account the topography and obstacles of the area that needs to be "unwired" and comes up with the best plan and layout for access points and antennas.