5 ms·

This is why Richard Stallman recommends that you wrap your RFID card in aluminum foil. http://slashdot.org/articles/05/11/19/149242.shtml?tid=133&tid=117 http:

This is why Richard Stallman recommends that you wrap your RFID card in aluminum foil. http://slashdot.org/articles/05/11/19/149242.shtml?tid=133&tid=117 http://slashdot.org/articles/05/11/19/149242.shtml?tid=133&#...

ben1040 16y ago
Not just RMS -- the RFID trusted traveler pass I used to use (a NEXUS card to be specific) came with a foil storage envelope that DHS/customs recommended you use whenever the card wasn't in use.
gvb 16y ago
Hmmm, how effective is aluminum foil in blocking the RFID? Despite having "RF" in the name, the signal is primarily magnetic induction. One google hit says "helps a lot, but doesn't prevent": http://www.omniscienceisbliss.org/rfid.html http://www.omniscienceisbliss.org/rfid.html Note that the picture of the reader and the unshielded read distance (18") indications it is the long distance version. I'll have to experiment at work tomorrow. ;-)
- tgflynn 16y ago
  Electromagnetic signals are attenuated exponentially in metals with a frequency dependent decay constant called the skin depth. The formula is given in this wikipedia page : http://en.wikipedia.org/wiki/Skin_depth http://en.wikipedia.org/wiki/Skin_depth If you know what frequency RFID uses and the thickness of the foil you can easily calculate how much attenuation you'd get.
  - gvb 16y ago
    125KHz => 0.024mm skin depth Looks like typical aluminum foil is thicker than that. The box in our kitchen doesn't say how thick it is, but the Wikipedia page indicates it is likely thicker than 0.025mm since it needs to be thicker than that to be impermeable to oxygen and water. Ref: http://en.wikipedia.org/wiki/Aluminium_foil#Properties http://en.wikipedia.org/wiki/Aluminium_foil#Properties
    - tgflynn 16y ago
      Hmm, one of us is off by an order of magnitude, I get 0.24 mm. For Al foil thickness I'm seeing conflicting numbers from 0.2 to 1.1 mm. If my cheap digital micrometer hadn't died I'd measure some. If my skin depth is right then the attentuation probably won't be complete, if yours is then it might nearly be. Of course that's only the case if the receiver and/or transmitter is completely wrapped. Although at low frequencies like that you could probably tolerate quite a large gap.
      - Deadsunrise 16y ago
        It can't be 0.24mm, that would be too thick. On the wikipedia page there's a picture that says: A roll of aluminium foil, with micrometer showing a thickness of 0.013 mm (0.00051 in) http://en.wikipedia.org/wiki/Aluminium_foil http://en.wikipedia.org/wiki/Aluminium_foil
      - gvb 16y ago
        You are right, 0.24mm. I did the m->mm conversion in my head (d'oh). My cheap digital micrometer only has two digits of precision in mm mode. Measuring my cheap (not "heavy duty") aluminum foil is at the limits of its resolution, 0.01mm.
        fr0sty 16y ago
        Protip: stack X sheets of foil on top of each other, measure, divide by X.
        tgflynn 16y ago
        I folded a sheet of heavy duty foil into 16 sheets and tried to measure the thickness and it was still << 1mm, so I think your estimate of about 0.01mm is reasonable. In that case I would predict that a single sheet would have little affect on an RFID device operating at 125kHz. Note however that RFID devices appear to span a very large frequency range : http://www.rfid-handbook.de/rfid/frequencies.html http://www.rfid-handbook.de/rfid/frequencies.html and the foil would certainly block the higher frequency devices.

ben1040 16y ago

Not just RMS -- the RFID trusted traveler pass I used to use (a NEXUS card to be specific) came with a foil storage envelope that DHS/customs recommended you use whenever the card wasn't in use.

gvb 16y ago

Hmmm, how effective is aluminum foil in blocking the RFID? Despite having "RF" in the name, the signal is primarily magnetic induction. One google hit says "helps a lot, but doesn't prevent": http://www.omniscienceisbliss.org/rfid.html http://www.omniscienceisbliss.org/rfid.html Note that the picture of the reader and the unshielded read distance (18") indications it is the long distance version. I'll have to experiment at work tomorrow. ;-)

tgflynn 16y ago
Electromagnetic signals are attenuated exponentially in metals with a frequency dependent decay constant called the skin depth. The formula is given in this wikipedia page : http://en.wikipedia.org/wiki/Skin_depth http://en.wikipedia.org/wiki/Skin_depth If you know what frequency RFID uses and the thickness of the foil you can easily calculate how much attenuation you'd get.
- gvb 16y ago
  125KHz => 0.024mm skin depth Looks like typical aluminum foil is thicker than that. The box in our kitchen doesn't say how thick it is, but the Wikipedia page indicates it is likely thicker than 0.025mm since it needs to be thicker than that to be impermeable to oxygen and water. Ref: http://en.wikipedia.org/wiki/Aluminium_foil#Properties http://en.wikipedia.org/wiki/Aluminium_foil#Properties
  - tgflynn 16y ago
    Hmm, one of us is off by an order of magnitude, I get 0.24 mm. For Al foil thickness I'm seeing conflicting numbers from 0.2 to 1.1 mm. If my cheap digital micrometer hadn't died I'd measure some. If my skin depth is right then the attentuation probably won't be complete, if yours is then it might nearly be. Of course that's only the case if the receiver and/or transmitter is completely wrapped. Although at low frequencies like that you could probably tolerate quite a large gap.
    - Deadsunrise 16y ago
      It can't be 0.24mm, that would be too thick. On the wikipedia page there's a picture that says: A roll of aluminium foil, with micrometer showing a thickness of 0.013 mm (0.00051 in) http://en.wikipedia.org/wiki/Aluminium_foil http://en.wikipedia.org/wiki/Aluminium_foil
    - gvb 16y ago
      You are right, 0.24mm. I did the m->mm conversion in my head (d'oh). My cheap digital micrometer only has two digits of precision in mm mode. Measuring my cheap (not "heavy duty") aluminum foil is at the limits of its resolution, 0.01mm.
      - fr0sty 16y ago
        Protip: stack X sheets of foil on top of each other, measure, divide by X.
      - tgflynn 16y ago
        I folded a sheet of heavy duty foil into 16 sheets and tried to measure the thickness and it was still << 1mm, so I think your estimate of about 0.01mm is reasonable. In that case I would predict that a single sheet would have little affect on an RFID device operating at 125kHz. Note however that RFID devices appear to span a very large frequency range : http://www.rfid-handbook.de/rfid/frequencies.html http://www.rfid-handbook.de/rfid/frequencies.html and the foil would certainly block the higher frequency devices.