Why can’t I get decent reception on my mobile phone in the lifts of the building where I work?
• This is because the lift is a metal box which acts as a Faraday cage. Like all electromagnetic waves, the radio waves trying to reach your mobile phone are alternating electric and magnetic fields. This makes electrons inside metals, which are generally good conductors, move around.
Ideally it would make the electrons move up and down along the aerial inside your phone and you would receive the signal. However, the metal box of the lift waylays the radio signal so that little or none of it gets as far as your phone.
Advertisement
Mike Follows, Willenhall, West Midlands, UK
• Mobile phones communicate using radio waves, which are a form of electromagnetic wave. As its name suggests, an EM wave involves an electric field. Most modern lifts are made out of metal, which shorts out the electric field of the wave and reflects it. This means that radio waves will not get through.
Mobile phones often work badly or not at all inside buildings, especially interior rooms, partly because of metal in the building’s structure and partly because other building materials also absorb radio waves. However, they do work in external glass elevators, with no metal to stop the signal.
Personally I’m only too pleased they don’t work in most lifts.
Harvey Rutt, School of electronics and computer science, University of Southampton, UK
• There are ways to work around the isolating effect of standing in a conducting box. One is to install a radio repeater to interface between the outside and inside of the box. Radio repeaters are widely used in tunnels which have walls that are reinforced with a dense grid of steel, shielding them from the outside world.
Another method is to make a hole in the metal box. To ensure sufficient signal can leak through, the hole should be at least as big as the radio signal’s wavelength. GSM mobile phones use radio frequencies at 800, 900, 1800 and 1900 megahertz, corresponding to wavelengths of between 16 and 37.5 centimetres. The direction of the hole matters too: it is best to point it at the nearest mobile phone base station. The higher the frequency of a radio wave, the more directional it is.
Yichao Fan, Leamington Spa, Warwickshire, UK