Why Does Aluminum Foil Block Cell Phone Signals?

Radio waves, such as those used to carry cell phone signals, are an example of electromagnetic radiation, moving electric fields that travel at the speed of light. An aluminum-foil barrier cancels the fields, so a radio wave cannot pass through it. A cell phone surrounded on all sides by foil receives no radio waves.

Scientists use a structure called a Faraday Cage to provide an area free of radio signals in order to perform sensitive experiments or to test electronic equipment. The cage is simply an enclosed box made of an electrical conductor such as aluminum. The conductor's thickness has little effect on signal blocking; thin foils or heavy plates work equally well. The cage can have holes, gaps or windows, provided they are much smaller than the shortest wavelength of radio signal you want to block. The door on a microwave oven, for example, has a wire mesh; the holes in the mesh let you see inside the oven while keeping the microwaves safely inside.

While the Faraday Cage formed by aluminum foil surrounding a cell phone keeps signals from reaching it, it also blocks signals coming from the cell phone. If you sat inside a Faraday Cage the size of a small room with a cell phone, you would not be able to make any calls because the cell tower would not receive your signal. You could communicate with a partner using walkie-talkies inside the cage, but not to anyone outside it.

Although aluminum foil is easy to obtain, you can use any good conductor to block cell phone signals. Copper and zinc are both excellent conductors that make effective Faraday Cages. Iron and steel are also good conductors; steel-framed buildings often have many cell signal dead zones because the beams form unintended Faraday Cages, blocking radio waves.