5 ms·
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
by 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 agoHmm, 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 agoIt 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 agoYou 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 agoProtip: stack X sheets of foil on top of each other, measure, divide by X.
- tgflynn 16y agoI 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.
- gvb 16y agoExperimental evidence, sample size of 1: a single layer of ordinary aluminum foil completely enclosing the card prevents a "normal distance" (4" square) reader from reading my card.
- tgflynn 16y agoAre you sure it's a 125 kHz device ? Maxwell has never failed me before.
- gvb 16y agoNo, I'm not sure, but it does physically match the cards and readers in the article. With no foil, the badge is read at 3-4". With a single slice of foil ~18"x12" held in front of the sensor, the badge is read at ~1" (a noticeable reduction in signal). With the foil folded in half and the badge in the fold so that it forms a single layer on both sides of the card, it was effective (could not read the badge). With the badge in an aluminumized mylar antistatic bag, there was no evidence of a signal reduction (read at full distance). I would theorize that Maxwell hasn't failed us, but the signal is weak enough that the foil attenuated the signal sufficiently to disrupt reading. The RFID reader works by modulating the sensor tuned loop by detuning on/off, which is going to be a pretty weak signal.
- tgflynn 16y agoThe read range you're seeing also seems compatible with a low frequency device. I'm quite perplexed. I don't think the foil could be attenuating the signal directly. It might be reflecting it/altering the field pattern so as to reduce the effective gain between the transmitter and receiver antennas.