Wireless LAN's are fabulous things. You put a box next to your cable or ADSL modem with a little aerial on the back and your broadband internet connection miraculously becomes available all over your house to any device fitted with the appropriate wireless card. Or does it...?

In some cases the position of the modem and wireless box are such that coverage does not extend to all points of your house. For example, most incoming telephone (and cable) connections tend to be on an external wall, which by default, is at one end of a property and not in the middle of it. And often they are at floor level, which is where all the modem/wireless equipment also ends up. Now although radio can at times seem magic, putting a wireless box on the floor at one end of your property probably means that upstairs at the other end, the signal is very weak. So what can be done to extend the coverage of wireless LANs?

The first, and easiest thing, is to put the wireless box in a more favourable position. Many can be wall mounted (though this still puts the device at one end of a building), but even raising the position from on the floor to head-height helps. If you have a phone socket or cable connection somewhere else in your house, i.e. in a lounge that might be more central, move the wireless box here and put it on a shelf so it's not at floor level. Putting the box centrally within a property helps even out the signal throughout the building.

If neither of these are possible and the only place where the box can be located is next to the phone socket, is there anything else that can be done to make the signal stronger? The answer is a resounding YES. By far the easiest way to do this is to replace the antenna that comes with your wireless box for one with higher gain. Standard antennas have a gain of 2 dBi - this means that the effective radiated power (erp) of your wireless box is the same as the transmitter power, so if the transmitter is 10 milliWatts (mW), the radiated power will also be 10 mW. Replacing the aerial with one with higher gain increases the radiated power for the same transmitter power and has the added benefit that it improves reception by the wireless box too extending the overall range. Without going into the maths, to double the range of a transmitter, the radiated power has to go up by a factor of 4 (which can also be expressed as 6 dB). So an antenna with a gain of 8 dBi will double the range (or quadruple the coverage) of the system compared to one with 2 dBi gain.

And such antennas are available, and they're not expensive. Visit eBay and do a search for 9dBi and you'll find that for about GBP15 (including postage) there are lots of 9 dBi gain antennas available (remember to make sure that the connector on the bottom of the antenna is the right one for your wireless box - R-SMC are the most common, followed by R-TNC). Replace your standard antenna with one of these high-gainers (which are about 40cm long) and you should find that your coverage has gone up significantly.

Is it legal to do this? Good question! In the UK, Ofcom allows powers up to 100 mW erp . The simple answer is 'it depends'. Most wireless LAN access points have a transmitter power of 100 mW already, so installing a high-gain antenna means the erp will exceed the UK limits. What are the chances of getting caught? That's a different matter...