Wireless LANs are great in theory, but what happens when you find holes in the coverage?

In its tests, the Wi-Fi Alliance determined that a typical wireless card can talk to a gateway that's situated 40 to 60 feet away within a home, and 60 to 80 feet away in an office. That range is smaller than the theoretical 150-foot radius promised by some Wi-Fi manufacturers.

You can't do much about the main problem, which is interference. That gets caused by interior walls, floors, or even those walking bags of signal-absorbing water we call human beings. But the antennas on your gateway can cause trouble, too. Even the best antenna can't radiate a perfect sphere of signal strength.

In these situations, you can move your access point, add new access points, or you can add one or more new antennas. Before deciding on a course of action, however, you need to measure the strength of the Wi-Fi signal around the dead spot - a place where you want to use a computer, but can't. For example, a quick survey might show that you can get a signal at one end of an office table, but not at the other, and that you could solve the problem just by moving your chair.

The utilities that accompany many Wi-Fi-enabled notebooks or cards include a rudimentary signal strength meter, but they rarely give enough data to perform real troubleshooting. With the help of NetStumbler, a free application, you can temporarily turn your laptop (or Wi-Fi-enabled Pocket PC) into a slick Wi-Fi signal analysis device that scans the frequencies used by Wi-Fi devices. The program lists all the nearby Wi-Fi devices it detects, and the precise strength of the signal you can receive.

NetStumbler is a little fidgety, however: It doesn't work with all models of Wi-Fi cards, and there's a steep learning curve to using it. The WINc client from Cirond does a lot of what you need.

A cheap way to scan for Wi-Fi signal without turning on the laptop is Smart ID's WFS-1 WiFi Detector (£24.95), a handheld sensor that displays signal strength in the 2.4 GHz band. The WFS-1 is about the size of a deck of playing cards, and it contains four LEDs that display signal strength while you hold down a button. Because it's directional, it can even help you figure out if a non-Wi-Fi device (like your microwave oven) is the source of your problems -- something that NetStumbler can't do.

Once you've ruled out obvious potential sources of interference, your next step is to move your gateway around, if at all possible -- and ideally, closer to the dead spot. Obviously, you can't do this easily in a large installation with multiple APs, as moving it may create overlaps with other APs or gaps in different places. With a single AP, you have more leeway, but can still open up holes in coverage elsewhere.

Pay special attention to the gateway's spatial orientation; even so-called omnidirectional antennas can be highly directional, so that a gateway hung on a wall might send its signal into the ceiling and floor. Each time you reposition the gateway or antenna, recheck the signal strength where you want to receive a signal.

If moving or reorienting the gateway or its antennas doesn't solve your problem, you may be able to use a different antenna. Apple, for instance, sells two antennas for its AirPort Extreme Base Station. Some companies make high-quality antennas - one example available in the UK is the cAntenna. The legality of using different antennas on Wi-Fi gear may be unclear however, although we are not aware of any legal problems created by sensible changes of antenna.