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ANTI-SPY PAINTS AND FINISHES

Theme: paints and finishes

Last year, the US press got excited about stealth paint, the brainchild of a Silicon Valley company, Force Field Wireless (www.

forcefieldwireless. com). One of its new paints is DefendAir Radio Shield. This is an acryliclatex paint containing copper particles and an aluminium compound, which reflects signals with frequencies from 100MHz to 5GHz.

The firm also sells copper and aluminium powder that painters can add to their own paint, and a 30inch by 25ft roll of window film. The paint costs $69 (£36) per gallon, the powder for mixing with a gallon of paint is $35 (£18), and the film is $45 (£23) per roll.

The particles of metal in the paint matrix are joined up electrically to form a Faraday cage. This blocks electromagnetic radiation, including Wireless Fidelity (WiFi), Bluetooth, mobile phone and radio transmissions.

Aluminium kitchen foil is an effective, though probably less acceptable, alternative to paint. Other cage materials include plaster on wire mesh and composite steel/concrete floors. Foil-backed plasterboard works the same way, as long as the foil backing on each board is joined up to the others with flexible copper or aluminium strips. The shield has to be as complete as possible: walls, floors, ceilings and windows. But once the paint is in place that's it - no signals will get through the DefendAir frequency range.

There are obvious disadvantages. Occupants will want to use mobile phones, listen to the radio and use WiFi between floors. The ideal is a Faraday cage that can be controlled.

This is what BAE Systems has developed, after a £145,000 research programme commissioned by the Radio Communications Agency, predecessor to Ofcom.

Last June, BAE Systems announced its switchable stealth wallpaper and film for office buildings. Project manager Kevin Mitchell says: 'The main concept has been frequency-selective surfaces used to reduce the radar profile of military equipment. So we had a strong track record.' Mitchell prefers 'long, close-coupled frequency-selective surfaces'to 'stealth'wallpaper or film, but it's easy to predict which terminology will win out.

BAE Systems is developing the system, and has had more than 50 enquiries - a number of them from overseas government agencies, which Mitchell is not inclined to name. And you can buy one-off panels from BAE Systems, although the company is in fairly serious negotiations with potential manufacturers.

'The difference from paint, ' Mitchell says, 'is that we have something that can be tailored to specific frequencies and that you can switch on and off. If, for example, there were a threat to a building of a mobilephone-triggered bomb, you could block all mobile-phone frequencies. A more run-ofthe-mill example would be if you leased two floors of a building and wanted to run a WiFi network, you could switch off the electronic barrier between floors.' The BAE Systems panels are quite expensive - around a couple of hundred pounds for a small panel. But Mitchell doesn't envisage coating all the surfaces of a building with the foil. He says: 'Although you do have to cover all the surfaces, this can be with Baco-foil or paint that cover all the frequencies. But you can make windows [of BAE Systems wallpaper] and switch them on and off as needed.' The process of making it work involves starting off with a very thin, perhaps 50100µm, polyimide dielectric plastic material, such as Dupont's Kapton. It needs to have a conductive coating on both sides and is rather like the flexible circuit-board material you find joining cartridge heads to the electronics in dot matrix printers.

Mitchell says: 'You can go along to a circuitboard maker and ask them to etch particular patterns on each side, which gives the filtering performance. We can make a transparent film in the same way, but not, so far, transparent and switchable.'

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