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How can I reduce the GSM signal coming from outside into my bedroom, knowing that it's impossible to create a perfect Faraday cage because of the windows & door.

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You just need to connect the door mesh to the wall mesh when the door is closed. A lot of electrical equipment has flexible metallic strips on the cases to make reliable electrical contact of the shielding when the case is closed. –  RedGrittyBrick May 16 '13 at 15:51
    
Just use a GSM Jammer bombard all frequencies with junk and .. Ohh - You scared your brain will fry from all the "radiation"? - Then you also need to block AM/FM, TV, WiFi, Microwave, etc etc. I would be worried about all the quarks and protons bombarding us from space... Even the Suns radiation is more dangerous than GSM waves :) –  ppumkin May 16 '13 at 16:23
    
The GSM jammer also has the minor problem of being illegal to use in most jurisdictions. –  Dan Neely May 16 '13 at 18:28
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@ppumkin : You surely have to create an account on skeptics.stackexchange.com. You'll earn a lot of rep' there too :-D –  Pierre W May 17 '13 at 9:24
    
Awesome :) omg what kind of questions are on there... +1 :) –  ppumkin May 18 '13 at 10:09
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2 Answers

All you need is a Faraday cage, with the proper size holes.

The Global System for Mobile communication (GSM) operates at 2100, 1900, 1800, 900, 850, or less commonly 450 or 400MHz.

The diameter of the openings in a Faraday cage, should be 1/10th the wavelength of the wave you want to block.

Wavelength (λ) is equal to the speed of travel, divided by the frequency of the wave.

Electromagnetic radiation travels at the speed of light (c).

So the formula would be.

Diameter mesh size (mm) = (c / f) * 10,000

So to block a 2100MHz GSM signal, you'll need a Faraday cage with ~1/2" (12.7 mm) diameter openings.

D = (299,792,458 m/s / 2100 MHz ) * 10,000
D = 14.2758313333 mm

As you've mentioned, you'll have to integrate the mesh into the windows and doors of the room.

This colander worn as a hat, should effectively block GSM signals from melting your brain.

colander

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+1 for .. quote. "block GSM signals from melting your brain." Hehehe :] –  ppumkin May 16 '13 at 16:26
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That's not a colander, that's the chic $2500 wifi-b-gone protective cap in double-refined unidirectional oxygen-free copper effect with deluxe strengthening ribs. –  RedGrittyBrick May 16 '13 at 16:27
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@RedGrittyBrick : You're right, the point is to find the right fancy name to be able to charge it as much as it deserve :-D –  Pierre W May 17 '13 at 9:46
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How much of a reduction do you need (e.g. 20 dB)? What is the budget? For windows (and possibly doors) you can use a metal screen (it should block most of the RF radiation).

Something as simple and cheap as aluminum foil should get you ~40+ dB of shielding. Copper tape has a nice property of being self-adhesive which can be convenient for small areas.

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I have tested the "kitchen" aluminium foil : but if you wrap your phone with it, it still receive the signal. I've deduced that there is not much metal in there or the aluminium does not block the signal. –  Pierre W May 17 '13 at 9:48
    
The issue is the strength of the incoming signal. Handset receivers are getting better and better so they can probably receive down to ~110 dBm, so it is likely the foil is attenuating but not enough to kill the signal completely if the tower is nearby –  Yuriy May 27 '13 at 23:05
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