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Richard Stallman's Tin-Foil Hat (Bruce Perens' Journal)Richard Stallman's Tin-Foil Hat (Bruce Perens' Journal)Posted Nov 21, 2005 18:36 UTC (Mon) by hamjudo (subscriber, #363)Parent article: Richard Stallman's Tin-Foil Hat (Bruce Perens' Journal) The tinfoil pouch theory is good, but we need to continuously reverify it in practice. For a faraday cage to block radio waves, it has to act as a completely sealed box at the frequencies of interest. The classic counter-intuitive experiment: put a wireless device inside your refrigerator and see how well it works with the door closed. At least my cell phone works in my fridge. Google for "slot antenna" and note how the slot around the door might work as well as the slot in the antenna or might not, the slot antenna is designed for a particular range of wavelengths, whereas the rubber seal around a refrigerator door is just designed to make an air tight gap, the thickness is controlled for ideal RF properties. Microwave ovens are designed to contain RF, the seal around the door on a microwave oven might let air through, but it better not let RF through, at least not RF at or below the oven's operating frequency. As expected, my cell phone did not work from inside my microwave oven. I don't have any RF equipment that operates at significantly higher frequencies, so I can't try that experiment. At a sufficiently high frequency, the little holes on the RF shield in the window will become little RF radiators. One of the problems with the RFid tags, is they don't usually tell you what frequency(ies) they operate at. The lower the frequency, the sloppier you can be when you wrap the card in foil. For any RF technology, you can improve the range by using a higher gain antenna, a more sensitive reciever and/or a more powerfull transmitter. The range is even adjustable for some RFID tag readers. So, a tinfoil pouch might be sufficient to block a reader set for close range, but not sufficient for the same reader set on long range. Then it might not work at all, if they upgrade to higher frequency tags and readers. After Stallman's little training exercise, the security folks are probably researching higher gain antennas. As you can imagine, I have no faith in tinfoil hats, because, there is no way to make an RF tight enclosure out of aluminum foil, that isn't also air tight. Breathing is even more important to me, than privacy.
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A Better Tin-Foil Hat ? Posted Nov 21, 2005 19:03 UTC (Mon) by Max.Hyre (subscriber, #1054) [Link] 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:
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.
Richard Stallman's Tin-Foil Hat (Bruce Perens' Journal) Posted Nov 21, 2005 19:09 UTC (Mon) by allesfresser (subscriber, #216) [Link] I think that in this case the aluminum foil was more important from a visibility (i.e., protest) standpoint than an actual RF shield. At least it seemed to act that way...
I only wish I had as much ganas...or to use a more widely-known term, 'chutzpah'. :-)
Richard Stallman's Tin-Foil Hat (Bruce Perens' Journal) Posted Nov 21, 2005 19:29 UTC (Mon) by madscientist (subscriber, #16861) [Link] What're they gonna do? Arrest you for tinfoil? Right in the middle of a UN meeting?!?! Not likely. Go Richard! I do think the comment about killing Bob Kramer was not well-considered though. I'm sure it was meant as a joke but, well, it's not very funny and the people involved in these issues seem to take that kind of talk far more seriously than they should. Best not to give them any ammo, as it were... ridiculous as it may seem.
RF shields do not need to be "air-tight" Posted Nov 21, 2005 20:41 UTC (Mon) by stevenj (guest, #421) [Link] As you can imagine, I have no faith in tinfoil hats, because, there is no way to make an RF tight enclosure out of aluminum foil, that isn't also air tight. Breathing is even more important to me, than privacy. It should be sufficient for any holes to be much smaller than the wavelength. I don't know much about RFID, but I just Googled it and it seems that the highest frequency for current RFID is 2.45GHz, which corresponds to about 12cm. Coincidentally, 2.45GHz is, I believe, the same as the frequency used in a typical microwave oven—just as the mm-scale holes in the grille on the oven door don't let a significant fraction of the microwave energy out, the much smaller air gaps in an aluminum-foil wrapping shouldn't be penetrable by an RFID detector at 12cm and greater wavelengths.
RF shields do not need to be "air-tight" Posted Nov 22, 2005 1:20 UTC (Tue) by hamjudo (subscriber, #363) [Link] AM pocket radios don't work inside cars and steel frame buildings unless they are "close enough" to a window. The shortest wavelength in the AM broadcast band (in the US) is about 175 meters, yet the radios can sometimes get a signal through a hole that is less than half a meter across. So a mesh with millimeter scale holes will only work reliably, when the mesh is kept a safe distance away from the antenna. (A distance that I don't know how to calculate.)I've got a couple other reservations and another experiment.
RF shields do not need to be "air-tight" Posted Nov 22, 2005 2:39 UTC (Tue) by stevenj (guest, #421) [Link]
The real problem you may have is for low frequency RFID, where the thickness of the foil (0.03mm according to Wikipedia) is less than the skin depth (the distance for the power to decay by exp(-2) ~ 90%). At microwave frequencies (GHz), the foil is many skin-depths thick and the transmission through it should be negligible. At the lowest frequency RFID, around 130KHz according to Wikipedia, the foil is only 1/10 of the skin depth in thickness, which is not enough to attenuate the power by much. On the other hand, you can simply wrap your object in 20-30 layers of the foil.
RF shields do not need to be "air-tight" Posted Nov 22, 2005 2:57 UTC (Tue) by stevenj (guest, #421) [Link] ...the amount of radiation that gets through is proportional to the fraction of the surface area occupied by the holes Whoops, sorry, it's worse than that: the transmitted radiation is proportional to the hole's fraction of the surface area multiplied by (d/λ)4, where d is the hole diameter and λ is the wavelength. (H. Bethe, Phys Rev. 66, p. 163, 1944.) However, once the hole occupies a significant fraction of the solid angle around the antenna (as when you bring your radio close to a window), then the problem completely changes because you are in the near field and you can't treat the incident field as ~constant amplitude in the vicinity of the hole. This shouldn't be the case for an object wrapped in foil, however.
Stopping antennae with antennae Posted Nov 21, 2005 20:43 UTC (Mon) by kirkengaard (subscriber, #15022) [Link] So, essentially, we're building a closed waveguide around the transmitter to perfectly collect its incident radiation; any openings in the surface become possible transmission points. We're also blocking transmissions from outside, such that the casing perfectly receives the incident probe and does not transmit it to the interior.
How about a double-layer design, two conducting shields spaced by a dielectric? Up to the dielectric's ionization point, we get no conduction of incident energy between shields, and past that, we're left with a better Faraday cage.
Do we need to worry about resonance?
We obviously need to worry about how to reclose the enclosure securely; thinking at most basic of a plastic easter egg, you have two cups, one precisely smaller than the other, such that their open sides nest. The opening betwen these sides would have to be smaller than our wavelength. This would require designing a latch mechanism that effectively blocks useful frequencies while allowing useful mechanical operation. Even a bent slot is an effective transmission gap at the right lengths. What frequencies are we working with/against?
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Stopping antennae with antennae Posted Nov 26, 2005 1:20 UTC (Sat) by giraffedata (subscriber, #1954) [Link] Later responses to the same comment give some numbers. They show that there really isn't any tightness of seal concern.
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