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BAE's anti-Wi-Fi wallpaper is made from a 0.1-millimetre-thick sheet of kapton, the same plastic used to make flexible printed circuit boards in lightweight portable gadgets like camcorders. The kapton is coated on each side with a thin film of copper.
On one side most of the copper is removed, leaving a grid of copper crosses. On the other side, matching crosses, turned through 45 degrees, are etched away leaving a film of copper with a grid of cross-shaped holes. BAE says that by carefully changing the size of the crosses and their spacing, the sheet can pass precisely defined frequencies, while blocking all others.
Ofcom engineers have confirmed to New Scientist that the wallpaper can block Wi-Fi at 2.4, 5 and 6 gigahertz, while letting through GSM and 3G cellphone signals, plus emergency service calls
When a current is fed through the diodes, all frequencies are blocked. Switching them off "opens" the panel to let mobile and emergency signals through.