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Hi I’ll get straight to the point... theoretically if I built an antenna loop that went around a door frame including under the carpet so I could walk through the loop, then connected the antenna to a NFC payment (POS) terminal would it be able to read and take a payment from my contactless bank card, I have a bet with a colleague who says it is not possible?

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It’s not possible. I believe it was designed to only work <4cm gap and never work >= 10 cm gap, limited by the card’s antenna radius and path loss for a much longer wavelength (20m band)

RFID tags in smart cards operate at 13.56 MHz with a magnetic loop antenna and subcarrier at about 3.3% of this carrier with an AM data rate that a bit more than 6% of this in ISO 15693. Another version uses ISO 14443 at twice the subcarrier Freq and with QPSK at about 4 times the data rate. Effectively this limits the Q of each antenna from 9 to 16.

The real problem is the transformer coupling factor is too weak at the distances of say a 1m exit with a pair of 1m square antenna loops. The squared ratio of the card antenna radius to the say 1/2m gap to the base loop from the card loop has a ratio of attenuation of magnetic coupling est. more than 20 dB below base station threshold.

Graphical example of parallel loops with wide separation showing divergence of H field.

Other info

  • if you happen to do any research in “wireless power transfer” or WPT there are tens of thousands of PhD and research papers on this topic. (I got daily google auto-search updates at one time).

  • have yet to see any transfer meaning power much beyond the diameter of the sending coil which must be match to the size of the receiving coil for coupling.

  • However with ferro-magnetic repeaters, it is possible to transfer inefficiently power beyond these magnetic loop dimension limits from my experience of others doing this.

  • Electric fields from a dipole or Yagi are different than Loop magnetic antenna patterns and are defined by Friis Loss with a quadratic loss factor.

  • Further the card orientation is likely orthogonal to the side coils in the front or back pocket. Then there is the antenna defining factor of adjacent cards.

To add more power in the coil is possible (>5W) with a Q of 16 but then is it safe for the eyes to stay in this range long ? (security guards near exit) (no).

Even if the card got lots of power the Zener limiting voltage on the coil would lower the Q and detune the sensitivity.

Maybe if you have liquid nitrogen cooled LNA’s and multiple coils and Tempest EMI sniffing design experience , you’d be hard pressed to do this and you might get red eyes trying to do so for weeks on end from the caterogenic effects of high Q RF power near field in testing.

Although it was possible at one time to pickup EMI from old 25kV CRT’s and retrace the pixel modulation and reconstruct a QVGA screen. Now EVERY VGA cable has a Ferrite CM choke to limit radiation on the VGA cable.

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  • \$\begingroup\$ P.S. if you bet a beer, you owe me one too (lol) \$\endgroup\$ Commented Apr 9, 2021 at 0:47
  • \$\begingroup\$ -1 to the loser without comments \$\endgroup\$ Commented Apr 9, 2021 at 3:16
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    \$\begingroup\$ Wasn't me, Tony. This is way outside my area of expertise. \$\endgroup\$ Commented Apr 9, 2021 at 11:32

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