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If I have a stack of papers each with an RFID tag in the corner, is there a way for an RFID reader to only read the RFID tag on the top piece of paper, without the RFID tags below interfering?

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    \$\begingroup\$ probably not. can you use barcode instead? \$\endgroup\$ Mar 3, 2019 at 0:19
  • \$\begingroup\$ No, because I don't want it to require line of sight. If I put the RFID tag in a different orientation than the one below it, that can work, but the problem is it needs to work with a stack of 50 papers. \$\endgroup\$
    – donut
    Mar 3, 2019 at 0:24
  • \$\begingroup\$ yes there is a way .... put all the sheets into an RF shielded bag except the top sheet \$\endgroup\$
    – jsotola
    Mar 3, 2019 at 2:20
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    \$\begingroup\$ Explain your application a bit if you can. Why must the papers be in an aligned stack at time of scanning if you only wish to read the top tag? Is this for a human user or would it benefit from automation? \$\endgroup\$
    – K H
    Mar 4, 2019 at 2:30

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OK I said it was impossible. Now I'll tell you how to do it!

Paper HF RFID tags are a resonant circuit with a printed coil and a two plate capacitor Shown here with a paperclip of unknown size for scale!

to make these directional you need to increase the size of the lower plate of the capacitor such that when such a tag is above another tag of the same type the lower plate couples with the tag belows lower and upper plates de-tuning it. thus leaving only the uppermost tagke in a working state.

anti-stacking paper tag

the large green circle is the top capacitor plate, the large brown circle is the lower plate, the slots reduce the shorted turn effect of such a large plate inside the coil

the small circle is a join between the brown and green layers

Because the capacitance effect needs to work through a sheet of paper the capacitopr will probably need a fairly thick dielectric for the effect to be significant, or perhaps a foil sticker can be placed on the back of the paper.

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  • \$\begingroup\$ The problmem of how to get this deck of cards onto the blackjack table is left to the reader \$\endgroup\$ Mar 9, 2019 at 4:54
  • \$\begingroup\$ I doubt this would work. In addition to the capacitive coupling you also have magnetic coupling which would detune all the tags. \$\endgroup\$ Mar 9, 2019 at 6:05

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