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My parents bought credit card case which protects against RFID sniffing (aluminium foil but only on one side of the case), i was skeptical because i thought it would work only on one side.

So i got my RFID reader and surprisingly (for me), i could not read the card while it was in the case (on one side or the other), can you help me understand why?

Thank you.

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  • \$\begingroup\$ Adding capacitance with foil next to an antenna will de-tune the antenna. Whether that de-tuning affects the signal strength enough to prevent communications depends on too many other variables to say for certain. \$\endgroup\$ Mar 21 '18 at 19:29
  • \$\begingroup\$ The foil reflects the waves back into the antenna and load. So it only leaks out the edges. It also detunes the coil as a capacitor to lower the resonant frequency thus further attenuating even more at normal f. Path loss is significant. \$\endgroup\$ Mar 21 '18 at 21:19
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Any flat metal such as aluminum or copper sheet up against the credit card will de-tune the antenna, as Brian Drummond mentioned in his comment. I made my own by cutting a piece of raw PCB, 15 mils thick with copper on both sides, and trimmed it to fit into my wallet.

I used a slot behind my credit card so the metal would not scratch the card. It works just fine, even with a card reader one foot away.

It works because of the capacitive loading of so much metal close to the antenna. This both de-tunes the antenna and blocks the signal. An RFID chip must receive a strong signal to wake up and answer back, so this metal 'barrier' prevents both reception and transmission of the signal.

Note that this does not include use of custom built RFID sniffers that are far more powerful in transmission of RF and extra sensitive to RF. However such a person would still have to be almost touching you to get close enough to overcome a metal shield.

Such closeness would only be common in shopping lines or transit lines. Beware of strangers who want to get very close to you for no particular reason. Sometimes they want to see you type in your PIN code.

I had thought of building a RFID sniffer detector, but it would be triggered by going through any store that has detectors by their doors.

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