Can a small ring of metal cut radiation from hands-free kits? GIVE US A RING

Paul Marks / New Scientist 18nov00

USING a hands-free earpiece with a mobile phone may channel more microwave radiation into your head than holding the phone up to your ear, according to tests announced last week by Britain's Consumers' Association. But the tests also found there might be a cheap and simple solution.

The CA's tests found, however, that fitting a small ring of an iron-based compound called ferrite to the headset wire eradicated the extra radiation. "The ferrite choke acts as a high impedance to the wave and reflects it back down the cable," says Les Barclay, a member of the government's Independent Expert Group on Mobile Phones.

"It would not be an expensive solution," says Ian Davis at Ora Telecom of Aylesbury, which makes hands-free kits. Ferrite chokes already have many applications. "They are used a lot in computers to stop things like digital pulses getting into, say, the mains," says Barclay.

Most tests have found that hands-free kits cut the amount of radiation entering the user's head. But the CA reported that tests on five types of cellphone and 10 different hands-free earpiece kits showed the latter could transmit up to three and a half times as much radiation to users' heads as the cellphone alone.

The levels of radiation recorded in the CA's tests on a dummy head depended on the position of the hands-free cable. In the tests, by ERA Technology in Surrey, the earphone cable dangled from the test rig--just as it would from the head in normal use.

When ERA moved the phone up and down, they found that the distance between the earpiece and the tip of the phone's antenna determined how much radiation would reach a user's ear. At a distance of about 60 centimetres, the electric field at the ear was at a minimum--less than that experienced from using a cellphone alone.

The phone antenna induces a microwave-frequency wave in the cable (see Diagram) says Roy Brooker, principal scientist at the CA. If the distance from the antenna to the earpiece is a whole number of wavelengths, a standing wave is set up in the cable. At the earpiece, the wave's amplitude is zero, minimising radiation into the ear.

But external influences such as other radio sources can affect the standing wave, making it impossible to work out where radiation is minimised. "Unfortunately, there is no way that consumers can work out the best [phone] position to reduce radiation," says a CA spokeswoman.

Uncertainty over radiation from cellphones has been compounded by the lack of a standard test. In August, Britain's Department of Trade and Industry used a common method known as the specific absorption rate test, and concluded that hands-free kits "offer substantial reductions in exposure compared to the normal use of a mobile phone". The Australian Consumers' Association came to the same conclusion.

The DTI is not convinced by the new findings, which used a different measurement method. "We have reservations about how the [radiation] probe the CA used works in the liquids and gels in their phantom head," a spokeswoman says. "Their tests need reviewing in a proper scientific way."

All radiation levels found by the CA were well within safety guidelines. No research has yet shown that mobile phones are harmful, but the biological effects of cellphone radiation--including cancer risks-- are being explored.