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When I put a piece of aluminum foil underneath an RFID card, the RFID reader is unable to read the card when it's placed in front of it like this (or around it):

The reader IS able to read the card when it is at this distance WITHOUT the aluminum foil underneath.

The only way I could get the reader to read the card with aluminum foil underneath was when I put the card directly below it like this:

  • What is going on to make it work this way?
  • Could it be the antenna's configuration on the RFID reader? The RFID card? or both?
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  • \$\begingroup\$ Both aluminum and copper are effective RFID shields as long as they are in between an RFID tag and a RFID scanner or reader. Thin copper plated PCB will work as well. The metal also 'detunes' the RFID antenna if it is right next to it, so it also aids in blocking an RFID scanner. I have read where a fine brass mesh will work also, but I have not seen any official studies of various metals vs. blocking ability, and on what axis related to the RFID tags antenna. \$\endgroup\$ – Sparky256 Mar 4 at 2:06
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Basically RFID technologies powers the RFID chip by inducing an electromagnetic field as the principle of a transformer, where the reader is the primary and the RFID is the secondary coil. Also common wireless charging or inductive heater works this way.

When a conductive material, like an aluminium foil, is placed nearby, this electromagnetic field will be absorbed by the material and be converter in Eddy current which will convert into heat on the material, although the power of RFID is small, no noticeable heat will be seen.

Since the aluminium foil has lower impedance than that of the coil+electronic of the RFID, most of the magnetic field will be absorbed by the aluminium, leaving very little for the RFID.

This is why, RFID won't work, even if the foil is below, because most of the energy sent by the reader will be converted into Eddy current into foil and the RFID chip won't be able to have enough power to power up and transmit data.

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  • \$\begingroup\$ But why is the RFID card with foil underneath successfully read when placed below the RFID reader? \$\endgroup\$ – donut Mar 4 at 3:31
  • \$\begingroup\$ Because closer is the reader, stronger is the magnetic field, thus if it's close enough the card will get enough energy to power up despite the loss in the foil. \$\endgroup\$ – Damien Mar 4 at 6:55
  • \$\begingroup\$ Are you sure the EMF is absorbed by the aluminum? I've read this in many places: "However, you do need to remember that although Aluminum does block EMF radiation, it does not absorb it." (emfacademy.com/aluminum-foil-emf-radiation) \$\endgroup\$ – donut Mar 4 at 7:42
  • \$\begingroup\$ Aluminium foil will absorb electromagnetic field, it will reflect radio waves. It's all about different frequencies of the field. \$\endgroup\$ – Damien Mar 4 at 9:21
  • \$\begingroup\$ No eddy currents in 0.1mm foil at RFID frequencies. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 4 at 16:06
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Since Al foil is conductive and the RFID wavelength is much greater the geometry of the foil ( or the coil) it acts as a reflector and a ground plane

This is such that the receiving antenna is no longer matched and is detuned by capacitance and also significant attenuation of the localized E-Field in near proximity to the coil by the ground plane "shielding" effect.

Since the skin depth for 1 MHz in Al is 82 μm and foil is about 0.1mm so there are no eddy currents.

So the foil shunts the E field across the board, but there is at least some fringe signal when the Tx is brought close to the card.

In fact with Tap Technology on credit/debit cards, putting a layer of foil in your wallet is an excellent way to prevent strong card hackers that try to read your smart card's security info which requires very close proximity.

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  • \$\begingroup\$ Does that mean that with a higher powered signal from the reader, it could read the card in front as shown in the first photo? Or a higher powered return signal from the card? \$\endgroup\$ – donut Mar 4 at 20:58
  • \$\begingroup\$ Attenuation is bidirectional, so both. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 4 at 21:03

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