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Faraday Cage
Name: Sophia
Status: student
Grade: 6-8
Country: USA
Date: Winter 2011-2012
Question:
I am making an experiment about what needs to happen for my cell phone not to ring inside a box or enclosure. I want to use it for a science project. My mom helped me research that aluminum foil works if I cover the phone with no air spaces left (it rings if I open the foil a little bit). I looked on the Internet and I found out that metal wire mesh also works. So I am realizing that air is not an issue. Does it have something to do with the air hole not being bigger than the electromagnetic wave height? Do different air holes (round openings) let different waves through? How small do round air holes or air squares (if a mesh) have to be to cover all the different waves from radio to gamma? What are the most a hole can be by wave type?
Replies:
Sophia,
Wavelength is the greatest concern. Even with solid foil, a tiny bit of signal might get through, but not enough to be noticed at all. As the holes get bigger, just a little more signal gets through. As the holes get close to the wavelength, the signal becomes noticeable. When the holes are larger than the wavelength, most of the signal gets through. Find the wavelength of your cellphone. Keep your holes somewhat smaller, perhaps a diameter of ten or twenty percent of that wavelength.
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
Metals do block electromagnetic radiation. How effective the metal
is depends the metal, the wavelength of the radiation, and the direction of
the radiation.
Look at your microwave oven. Notice the window has a wire mesh screen. These
fairly large holes are possible because the wavelength of microwaves are
Very long compared to the size of the holes. The shorter the wavelength of
the radiation, the smaller the holes have to be. You can observe a similar
effect if you are in a car that goes under a metal overpass.
Vince Calder
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Update: June 2012
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