As they say, some guys have all the luck. If your access point does not have a connection for an external antenna, you can't add an external antenna to it. (And you are out of luck!)
On the other hand, if your access point, such as the Apple Extreme Base Station, comes with a connector for an external antenna, it is usually a simple matter to plug an antenna in. (And, you've got all the luck!)
Other access point units fall somewhere in between. For example, the D-Link AirPlus Xtreme Wireless Router that I've discussed earlier in this book, comes with a single, rather small antenna (see Figure 16.5).
This antenna can be unscrewed, as shown in ...