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How do I detect a Faraday bag coming into or leaving my store?

The intention of detecting Faraday bags is to be aware of customers who have the ability to walk out of the store with an RFID-tagged product that hasn't been paid for.

ocrdu
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Michael
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  • Have all bags pass through a magnetic field... But you might loose a few customers... with pacemakers... – Solar Mike Jan 10 '23 at 17:15
  • @solarmike what do you mean by "loose"? – user263983 Jan 10 '23 at 17:29
  • @user263983 Like a catapult, or archery...LOOSE! lol. I think he misspelled "lose". – Aaron Jan 10 '23 at 17:31
  • Aren't most anti theft tags magnetic rather than RF? – user1850479 Jan 10 '23 at 17:32
  • @user1850479 Some are RF resonant, some are actually powered by the scanner and communicate, some are RFID. There's been many designs over the years. In the early years, Best Buy had one that you could actually see the coil (inductor) on in their DVD cases. – Aaron Jan 10 '23 at 17:38
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    You could put a box prominently labeled "Faraday Bag Detector" with flashing LEDs by the exit and observe any suspicious behavior on the part of customers. – Spehro Pefhany Jan 10 '23 at 17:40
  • Arent the 58 kHz magnetostrictive shopping tags too low in frequency to be blocked by any such bag in the first place? – winny Jan 10 '23 at 18:07
  • I think this is a good theory question, even if the implementation of it is not necessarily something that may be achieved by a simple Q&A. – JYelton Jan 10 '23 at 18:16
  • OP asked specifically about RFID, which uses higher frequencies than old-school Sensormatic tags. – hacktastical Jan 10 '23 at 18:27
  • @SpehroPefhany Not by the exit ... but at the entrance! – Antonio51 Jan 10 '23 at 19:08
  • With a clearly readable warning at the entrance, use a "big" EMP at the frequency of the RFID tag which would "burn" everything inside ... <<< Make "open" ALL bags before the exit, please >>> :-) – Antonio51 Jan 10 '23 at 19:13
  • Lots of people carry shielded wallets for their credit cards, etc. -- all perfectly legitimate. Do you plan to stop all of them? – Dave Tweed Jan 10 '23 at 20:28
  • @Dave Tweed yes, that’s an important point. I made note of that - you as a shop owner only have the right to detain anyone if certain ‘probable cause’ is met - eyewitness, surveillance, etc - as defined by local law. Detecting a Faraday bag isn’t by itself probable cause; and there’s plenty of legit reasons to want to use one. – hacktastical Jan 10 '23 at 20:39
  • Let me re-phrase: Is RFID a must or is 58 kHz magnetostrictive tags an option for you? – winny Jan 11 '23 at 09:23

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First, the problem/challenge. The Faraday bag suppresses RF energy in both directions: RF to the tag, which energizes it, and RF return from the tag.

A classic conductive Faraday cage works by distributing RF energy across its surface, cancelling any effect on the interior. These may be grounded (like a shield room) to shunt energy away, or ungrounded, relying only on distribution of charge and skin effect.

More about Faraday cages here: https://nationalmaglab.org/about/around-the-lab/what-the/faraday-cage

In practice, a shield bag won't be a Faraday cage, but instead will be made of a lossy, absorptive material which converts RF to heat.

Example: https://slnt.com/products/faraday-bag

Anyway, I suppose you could sniff out such a lossy bag by measuring the overall return signal with a beam aimed at bag-height. If you see a dip in this signal compared to a person walking through without such a bag, then you might have something to act upon, assuming you have other evidence (such as surveillance footage showing them placing the item in the bag) that would legally entitle you to stop them.

hacktastical
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    Faraday cage doesn't need to be grounded in order to shield what is inside of it. – Aaron Jan 10 '23 at 17:35
  • What @Aaron said -- a grounded Faraday cage is nice, but one that's free-floating in space should still isolate the inside. That's not saying that what's out there isn't made of lossy material. – TimWescott Jan 10 '23 at 17:37