Winter 2003 Issue

Photo A. Pringles®-can antenna and laptop.

By Kent Britain,* WA5VJB

According to BBC News Online (see:< http://news.bbc.co.uk/2/hi/science/nature/1860241.stm >), the British security firm i-sec successfully hacked its way into several computers throughout London by cruising around with a laptop and a wireless LAN card looking for open networks. They supposedly found lots of open networks! For example, according the BBC article, “In one 30-minute journey using the Pringles®-can antenna, witnessed by BBC News Online, i-sec managed to find almost 60 wireless networks.”

I have no trouble with i-sec’s James Bond style story of hacking computers while on the streets of London. What confuses me, however, is the firm’s use of the Pringles® can for an antenna (photo A)!

I’ve built a lot of horn antennas, and the diameter of the Pringles® can is just too small for a 2.4-GHz radio wave to fit inside it. It’s simply impossible for that antenna to work on that frequency! Nevertheless, in order to see if this idea really worked (or if pigs can fly), Lloyd Ellsworth, NE8I, brought several 2.4-GHz horn antennas made from Pringles® cans to the Central States VHF Society antenna contest in Milwaukee last year.

One antenna was per the BBC article (Lloyd even used a sour cream and onion Pringles® can so as to be just like the BBC antenna.), and on the other antenna the length and position of the probe had been optimized. At 2.4 GHz, gain on both antennas was less than –20 dBi, my antenna range noise floor. The Pringles® antenna in the photo was swept with my network analyzer, and it simply doesn’t work below 3.0 GHz. Even so, supposedly these guys are picking up signals!

For the tube to become a properly working horn antenna, you must have a good ground connection between the coax connector and the horn (figure 1). In this case you need a good ground connection to cardboard and aluminum foil. If you don’t get the connection tight enough, or twist the connector after you’ve tightened it, you break loose the aluminum foil. Now you have this sort of fat longwire antenna hanging out there—not a horn antenna, but at least it can pick up 2.4-GHz signals.

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