A Better Tin-Foil Hat ? [LWN.net]

A Better Tin-Foil Hat ?

Posted Nov 21, 2005 19:03 UTC (Mon) by Max.Hyre (subscriber, #1054)
In reply to: Richard Stallman's Tin-Foil Hat (Bruce Perens' Journal) by hamjudo
Parent article: Richard Stallman's Tin-Foil Hat (Bruce Perens' Journal)

For a faraday cage to block radio waves, it has to act as a completely sealed box at the frequencies of interest.

A couple of questions from a SW guy:

What would it cost to make some sort of package rated RF-proof, as a function of frequency?
Could a transparent-ish material be used in places with visible information (your photo, your authorization level, whatnot), allowing people to look at the badge while keeping your RF data inside? If the holes in the microwave work, would (metal) mesh screening of the type found on old window screens do well enough up (down?) to the wavelength at which its holes are permeable?

Tinfoil is probably the best response on the spur of the moment, but there should be a better answer, given more time. Might even be a niche market there.

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A Better Tin-Foil Hat ?

Posted Nov 21, 2005 19:43 UTC (Mon) by MathFox (guest, #6104) [Link]

What would it cost to make some sort of package rated RF-proof, as a function of frequency?

A tin, as you can find in the supermarket filled with canned meat, fish, beans, etc. is as good as you can get it: keeping all electromagnetic waves out upto X-ray frequencies. We're talking about cent prices in quantity. Any reasonable metal box (without holes) will shield RFID chips.

Light is a kind of electromagnetic wave, so you'll need to find a transparant material that reflects the (low frequency) radio waves, but allows (high frequency) light to pass through. These materials are available, but more expensive than tinfoil. Getting the window solidly connected to the rest of the material could be a problem too.

water is a good transparent absorber for RF

Posted Nov 21, 2005 21:25 UTC (Mon) by stevenj (guest, #421) [Link]

Light is a kind of electromagnetic wave, so you'll need to find a transparant material that reflects the (low frequency) radio waves, but allows (high frequency) light to pass through. These materials are available, but more expensive than tinfoil.

At least for microwave (2.45GHz) RFID, which is used for the longest-range applications from what I'm reading, there is at least one very common and cheap material available that absorbs the RF but is optically transparent: water.

At 2.45GHz, 3cm of water should be sufficient to attenuate the reflected RF power by over 98% according to the numbers here. If you use salt water then the absorption is even greater.

Unfortunately, for longer wavelengths water may be impractical.