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I have an RFID-enabled ID card for work that is used for building access. While I can use a lanyard, cord-reel, etc. to carry the card and still have convenient access to it, I'd like to simply put it between my phone and its case. (I have legitimate reasons for this that are tangential to the question.)

My concern is that I already have a small metal plate in there that I use to hold the phone to a magnetic mount when I'm in the car. The plate itself is of no concern to me - it's just a small piece of ferrous metal and does not impact the functioning of the RFID chip.

My question is: will the regular very close proximity (1-2mm) of the strong magnets in the car mount cause any issues with the functioning of the RFID tag in the card? It would be exposed for about an hour 2x per day, plus other random trips.

My guess is that it would not be impacted any more than the solid state circuitry in the phone itself, but I'd like to get confirmation.

Additional notes: You now know as much about the particular RFID chip in my card as I do. You also know as much about the magnets in the mount as I do. Unfortunately, I can't provide any more specifics than this because I don't know them.

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The magnetic field from a magnet is static. In order to induce a current and/or voltage you need a changing electro/magnetic field.

Typically the field that RFID uses for operation has a frequency of at least 120 kHz but typically 13.56 MHz. That's much faster than you can practically generate by physically moving the RFID card around a magnet.

So near a permanent magnet you would be inducing a field but with a much lower frequency. The RFID tags are much less sensitive for such low frequencies as the antenna will not pick up that low frequency very well. So not much energy can reach the RFID chip and it should be nearly impossible to damage it that way.

So as far as I can see there can be no issue.

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  • \$\begingroup\$ I'm not sure, if the antenna itself is already selective to specific frequencies. This might be the case for some tags having dipole-antennas. But some tags use wire loops (e.g.: 5.imimg.com/data5/NL/SM/MY-45708806/rfid-glue-tag-500x500.jpg). To my opinion theses wire loops are not frequency-selective and a quickly changing magnetic field (fast relative movement of magnet / antenna) might induce a high voltage. However, I can't tell either if this will potentially be enough energy to destroy the tag. However, antenna inputs of ICs are typically very sensitive to ESD... \$\endgroup\$ – SDwarfs Feb 7 at 1:37
  • \$\begingroup\$ @SDwarfs To my opinion theses wire loops are not frequency-selective Well, they are, loops have inductance and that determines their frequency response. A voltage will be induced but that isn't an issue, RFID tags are powered by that induced voltage. \$\endgroup\$ – Bimpelrekkie Feb 7 at 8:43
  • \$\begingroup\$ However, antenna inputs of ICs are typically very sensitive to ESD For RF inputs (f > 1 GHz) I could agree but even then it depends. I worked on an IC which had an on-chip inductor to ground at the antenna input making it very ESD robust. RFID is at a very low frequency so adding proper ESD protection will not harm the chip's performance. For RFID this isn't so much an issue as it is for radio receivers like for GPS, WiFi etc. \$\endgroup\$ – Bimpelrekkie Feb 7 at 8:44
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No, you're right, RFID does not work by "magnetism" in that sense. The antenna picks up energy from the electromagnetic field. There's nothing in a standard magnet that can emit such fields.

Depending on the specifics of your tag it may not work while mounted on the magnet, but I don't think that's what you're asking. There is a related question about that with good answers.

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  • \$\begingroup\$ I did read that question and meant to mention it. You are correct - I'm not looking for the card to operate in the presence of the magnets - it's more of a storage near situation. \$\endgroup\$ – FreeMan Feb 6 at 15:50
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A static magnetic field caused by a normal magnet should not cause any harm to a RFID-tag. Its all about speed of the movement of the RFID-Tag relative to the magnetic field.

The antenna (a coil) of the RFID chip and the magnet form a generator. If you quickly turn or move magnet and antenna relative to each other a voltage is induced in the antenna, which is connected to the RFID chips power supply input and antenna input.

Usually, a HF or UHF signal is received by this antenna and supplies any passive RFID tags with energy while transmitting the request data. The RFID tag has not much control over the received signal strength (nearby or far away RFID reader) and therefore it must cope with a broader range. So a higher signal strength induces a higher voltage and the RFID chip must have some over-voltage protection. But, this protection most likely does not work for infinitely high voltages/energy amounts...

So it's all about how fast you move the RFID tag relative to the magnet, how strong the magnet is, how far away/near by the magnet if relative to the RFID-antenna and how well the over-voltage protection of the RFID-chip is.

Without any real data one can only guess. But, I assume usual movement speeds and magnets should not induce energy that is too high for an RFID chip. Also the over-voltage protection usually works by converting electrical energy into thermal energy. Hence, it becomes hot... and hotter... and finally too hot. If the induced energy (due to continuous motion) would not be interrupted for longer time, to let it cool down again. But, the usual motion would be putting the phone / RFID-chip in place or take it away once in a while.

So my guess is: There should be no issues. But, it's not safe to say without any real data at hand.

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