Francis Bryden of St Cuthbert's College asks :-
In order to show students that radio waves wont travel through a metal mesh I wrapped a walkie-talkie in mesh, but it picked up a signal. I then tried wrapping it in two layers of aluminium foil, but it still received a signal. Why was this?
Bob Bennett, a retired radiophysicist from the University of Canterbury, responded.
There are several reasons for the waves getting through. First, the receiver will respond to as little as 1 microvolt picked up by its antenna (aerial) while the transmitter antenna will have a few volts applied. If the transmitter is close we need to reduce the waves by a factor of up to a million.
Metal stops radio waves by reflecting them from its surface. This happens because the waves make electric currents flow in the surface of the metal which act as an antenna which radiates a wave back. A mesh of wires does the same thing and can be a quite good reflector if the wires are close together (ie the spacing is much smaller than the wavelength of the radio wave), but some small part of the wave does bulge through the holes and a little part does get away on the other side. Anything close to the holes gets a stronger signal.
How effective is aluminium foil? The current in the metal surface actually flows in a thin layer falling off away from the surface. Assuming that the walkie-talkies are UHF ones the current falls off by a factor of 10 for every 0.009 millimetres (mm) thickness. But domestic foil is only 0.016 mm thick so that the current will be reduced to one per cent by one layer, which may be insufficient because the current flowing in the other surface will send a wave on .
Another possibility is that the wrapping only makes good contact at a few places and some signal leaks through even if the foils are quite close. (If the distance between the contacts is more than half a wavelength (300 mm) the waves will shoot straight through.) When the voltage is very small, pieces of metal touching do not always make a conducting contact without some pressure being applied.
If you want to try again I would suggest that the wrapping is done with a sheet of foil with the edges not laid down but brought up together and folded over twice or thrice and pressed tight to ensure good contacts, with the same done at the ends. Use thick foil otherwise wrap a layer of paper to insulate the next foil from contact. One would hope that 3 layers would be enough.