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QuickStudy: Faraday cages

How to restrain a wild Wi-Fi signal

By Geeta Dayal
August 23, 2006 12:00 PM ET

Computerworld - Say you wanted to protect your Wi-Fi network from surrounding buildings. The most obvious way to do this would be to secure the devices on your network using the wireless security protocol of choice. A very effective, but more extreme, way to do this would be to secure the building itself by making it act as a Faraday cage, shielding the radio frequency waves used by Wi-Fi.

Making a large building into a Faraday cage involves encasing the building in a thin layer of conductive material or metal mesh. In physics, a Faraday cage or Faraday shield -- named for the British physicist Michael Faraday, who discovered the phenomenon in the 19th century and built the first iteration in 1836 -- is an ingenious application of Gauss' law.

Gauss' Law establishes the relation between electric flux flowing out of a closed surface and the electrical charge enclosed inside the surface.  Basically, in a hollow object that can conduct electricity, such as an aluminum sphere, charge will (ideally) distribute itself evenly over the surface of the sphere, and there won't be an electric field inside the sphere. This has the effect of blocking EMFs (elecromagnetic fields) and shielding radio frequency waves.

Faraday cages are far more common entities than you might realize. For instance, cars and airplanes can act like Faraday cages. That's why a lightning bolt striking an airplane is unlikely to take the airplane down; the aircraft will distribute the electric charge over its entire surface, protecting what's inside. You've probably experienced the phenomenon in even more mundane circumstances -- losing your cell-phone coverage inside an office building, or radio reception as you pass through a tunnel. And you're likely to "assemble" a Faraday cage on a regular basis: The metal screen built into the glass of your microwave oven lets the microwave act as a Faraday cage when the door is closed.

But constructing a functional, building-size Faraday cage might be another matter. It would likely be prohibitively expensive (though that didn't stop the Pentagon from doing so some years ago).  Not only would it be costly, but other equipment is likely to be hindered by the effect. For instance, cell phones will get poor reception or most likely not work at all in a building tricked out to be a Faraday cage. Resale values might also be affected for obvious reasons.

For a small space, though, such a structure might make perfect sense. Other expensive and goofy techniques to achieve similar ends, such as "Wi-Fi-proof wallpaper" and "Wi-Fi paint" with metallic ingredients, have been rolled out over the past few years, but unless you live in a cave, you'd probably also need to replace your windows with special radio-absorbing glass.



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