As they say, some guys have all the luck. If your access point, like the Linksys Wireless Broadband Router does not have a connection for an external antenna, you can't add an external antenna to it (and you are out of luck!).
Although some Wi-Fi access points intended for home use don't have an external antenna connection, almost all Wi-Fi access point and bridge units that are intended for commercial applications have connections for (and are used with) antennas. Of course, these industrial-strength units do tend to cost more.
On the other hand, if your access point, like 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!).
Adding an external omnidirectional antenna to an access point is essentially a no-brainer. It will improve the range of your access point, and there are no trade-offs (you don't lose anything, except your out-of-pocket cost to buy the antenna).
More complex, directional antennas are a trickier story. If you are thinking of fitting your access point with one of these, you need to think carefully about how radio signals will propagate given the terrain you need to cover.
You should be able to buy an external Wi-Fi omnidirectional antenna for your NIC card for between $20 and $50. Believe it or not, the radio pigtail, if you need one for connecting to the antenna, may set you back as much (or more than) the card. This connector will probably cost between $20 and $30.
As you can imagine, there's a great range in price for antennas designed to work with access points. There's probably not much point in buying an omnidirectional external antenna for less than $50 because the antenna that ships as part of your Wi-Fi access point is just about as good.
For $50 to $100, you can get a good omnidirectional antenna intended for internal use.
Omnidirectional antennas intended for outdoor use start at about $50 and can run you $200 and up.
Directional antennas intended for external use run a little more than omnidirectional antennas. Highly specialized directional antennas can be very expensive indeed.
If you are interested in the multipolarized antennas mentioned earlier in this chapter, you can probably expect to pay a bit more for this feature. An omnidirectional multipolarized antenna intended for external use runs about $150. Directional multipolarized antennas, often used for point-to-point back haul traffic, cost between $200 and $900.
Of course, like everything else that is related to technology, many antennas can be found for sale on eBay. If you've read this chapter, and understand Wi-Fi antennas, you should be able to buy one safely on eBay, and save some cash.
If springing for a commercial antenna is too expensive for you, you can always consider building a "Cantenna." The total cost of a Cantenna, which uses an old tin can as its primary part, is under $5.
If you build a Cantenna, and the FCC comes looking for you, please don't tell the Feds that you heard about it from me! Technically, using a Cantenna with your Wi-Fi equipment is probably illegal under FCC regulations.
Besides the tin can, which should be between three and four inches in diameter, the only things you need to build the Cantenna are
About 1.25 inches of 12-gauge copper wire
Four nuts and bolts
An N-type female connector
The N-type connector is the standard connector for antennas, and will be used to connect it to your Wi-Fi equipment. The cost for this connector is $3?$5, which is where I got the original figure of less than $5. You can buy this connector from any of the sources I mentioned that sell radio pigtails (or go to your local Frye's or electronic hobby store such as Radio Shack).
You drill holes in the can for the connector and the nuts and bolts. Their placement is important, because it determines the propagation characteristics of the antenna.
The copper wire is attached to the back of the connector and soldered to the tin can.
The best how-to article that I've seen that describes creating a Cantenna, "How to Build a Tin Can Waveguide Antenna," is by Gregory Rehm and can be found at www.turnpoint.net/wireless/cantennahowto.html. The article includes a calculator to help you determine where to place the connector and nuts and bolts.
You can also purchase a premade Cantenna for $20 (this one meets FCC regulations) from Super Cantenna at www.cantenna.com.