Desperately seeking signals

Technology originally developed for military stealth applications could soon spell the
end of that frantic search for decent mobile phone reception. Jon Excell reports.

Wireless communication, whether by mobile phone or wireless local area network (LAN), is increasingly important today. It’s therefore something of an irony then that modern building design, which relies heavily on metal structures, is distinctly unfriendly when it comes to radio waves.

With the aid of a £500,000 contract from the government’s Radiocommunications Agency, Surrey-based ERA Technology is developing and studying technologies that could get round this problem.

While mobile phones usually work well in brick buildings, modern offices generally have a steel framework. Metal-backed wall cladding, floors supported by solid metal trays and even heat-retaining metal-coated windows are also common. Although conventional building materials are by no means transparent to radio signals, these newer materials are, by comparison, almost impenetrable.

ERA project leader Martin Shelley explained how radio signals are weakened by two basic principles: material losses (whereby the building material absorbs the energy of the radio waves) and reflections. While little can be done about material loss, Shelley said that techniques developed by ERA can be used to drastically reduce levels of reflection.

Chief among these techniques is a technology originally developed for stealth defence applications. This relies on special coatings known as Frequency Selective Surfaces (FSS).

Inexpensive FSS panels can be applied to new or old buildings in the form of paper,window coatings or floor tiles. They generally consist of either one or more substrates on to which a lattice of metallic elements is deposited, or metallic sheets containing a lattice of carefully designed apertures.

By ‘matching’ structures such as walls and windows across a limited frequency band, FSS can be used to cut reflection and improve radio propagation throughout the building.

To illustrate the operation of FSS, Shelley drew an analogy with an electrical circuit: ‘If you put a capacitor across a transmission line you create a mismatch which causes a reflection. You can regard a dielectric material in the same way; you are putting a capacitor across a transmission line, which in this case is air. The FSS acts like an inductor in parallel with the capacitor creating a tuned circuit that, at a particular frequency, removes the reflection.’

Using Qinetiq Metal Printing (QMP), a manufacturing method developed at Qinetiq for fabricating ‘wallpaper’ and glass films with conducting patterns, a number of preliminary FSS designs have been completed. These will be evaluated over the coming months.

The ERA study has also been looking into using FSS structures to encourage radio signals to turn corners and travel up and down stairs. Shelley’s team has identified a type of FSS that suppresses the direct reflection that occurs at any interface between air and a wall, instead reflecting the energy in a different direction. A full-scale demonstration of this technique is planned for the near future.

As well as improving communications, making buildings more radio-friendly could have a secondary environmental advantage claimed Shelley. Current radio systems – including mobile phone networks – are designed to counter all but the most serious blocking effects. By developing systems that make buildings more transparent, it may eventually be possible to scale down base stations and reduce ‘radio pollution’.

While the commercialisation of ERA’s findings is a few years away, Shelley said he has a gut feeling that the most pressing, the easiest, and therefore the first, application will be in relation to metal-coated glass.

The technology could, for instance, be used on some of the latest trains which, with their metal-coated windows, prevent legions of conversation-starved travellers from making any number of pointless phone calls.

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