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Magnetic wood could be a major
plank in the battle against noisy cell phone users. The high-tech material
absorbs microwave radio signals, making it impossible to use a mobile
phone in any room lined with it. Or a radio for that matter. So theatres
and restaurants, for example, can stop people using cell phones on their
premises without resorting to signal jammers.
The
Anti-Cellphone Sandwich
These are illegal in some countries,
including the US, Britain and Australia. Jammers also cause wider problems
because their signals can spill out of the building they are covering,
interfering with other people's calls.
The magnetic wood - so called
because it is packed with minute magnetic particles - is the brainchild
of Hideo Oka and a team of electronics engineers at Iwate University in
Morioka, northern Japan. They chose wood as their preferred blocking material
because it offers more natural, aesthetic options for interior design.
Oka hopes that it will soon be possible to buy the novel wood panelling
by the metre at your local hardware store.
While normal wood is transparent
to radio waves, Oka's blocks them because it contains fine particles of
a magnetic material called nickel-zinc ferrite. When an electromagnetic
wave hits the ferrite particles, the magnetic part of the wave is absorbed.
Bluetooth
Frequencies
The team looked at four different
ways of making wood absorb radio waves before hitting on the best one.
The first was simply wood coated with a ferrite powder. The others were
made by mixing ferrite powder with cider wood powder and pressing it into
boards, or impregnating the wood with particles, or sandwiching wood pulp
containing ferrite powder between two thin wooden panels.
Oka tested each wood in turn
by putting collars of each material over a short antenna that broadcasts
radio waves at the typical GSM mobile phone frequencies of 900 megahertz
and 1.8 gigahertz.
The antenna can also broadcast
at frequencies up to 2.5 gigahertz, which covers the range commonly used
for wireless networks like Bluetooth and the emerging IEEE 802.11 standard,
better known as Wi-Fi. A receiver measured the strength of the radio waves
transmitted through the material.
Ferrite
Sandwich
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The
anti-cellphone sandwich
(Click to enlarge)
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In the end, Oka found that
ferrite sandwiched between thin sheets of wood performed best. Further
tests showed that a 4-millimetre-thick sandwich absorbed the most microwave
radiation, cutting the wave's power by 97 per cent. Increasing the thickness
of the outer wooden sheets of the sandwich increased the frequency of
radio waves that the shield would absorb.
The wood-based shields could
be used to make doors and walls for rooms or even entire buildings where
mobile phones simply won't work. While the prospect of being forcefully
cut off might horrify some cell phone addicts, Oka says theatre goers
and restaurant customers might appreciate the silence.
Panels that absorb radio waves
could also help with a problem emerging in Japanese cities, where many
homes are being fitted with wireless computing networks. If several networks
are set up close together, they can interfere with each other. The new
panels could divide up the house into different areas, allowing several
networks can operate close by.
Oka believes he can make the
wood cheap enough for it to be viable. And he now hopes to cut the cost
still further by making the panels from recycled magnetic materials and
waste wood.
New
Scientist June 27, 2002
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