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There are various RFID tags that are designed to work in close proximity to metal but generally they hope for free space or at least limited loading in close proximity.

As a work around for RFID tag sniffing various metallised wallets and card sleeves are available.

I am curious if anyone has insight into the best metal to select for making such a case?

Copper, aluminium, maybe gold and untarnished silver are good conductors. Mu-metal is a good magnetic field absorber. The MetGlas type materials have some funny characteristics and are used in fluxgate magnetometers and as far as I know the strange shop anti-theft tags with the two bits of foil inside.

Would the low frequency tags prefer a magnetic shield and the VHF and UHF tags prefer a conductive shield? Could or should one use both types to cover all of one's bases?

EDIT:
Personal privacy and bank card and passport sniffing are going to become more relevant in future. If I want to market protective wallets one day I would like to make them from the right stuff. Persons movements can be tracked, funds withdrawn and some of their identifying details may be visible to determined parties who are up to no good. Having a flexible shield is preferable but even a trustworthy hard case would be of use in many cases.

The Wallets would be like your everyday bill fold, bank card holder, purse or wallet. Basically any place that you might store your banking cards or card style biometric identity cards. A larger version that can hold passports and airline tickets would also be an option for travelers.

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  • \$\begingroup\$ Could you explain what the purpose of the wallet is? \$\endgroup\$ – Andy aka Jun 10 '18 at 17:52
  • \$\begingroup\$ Like a bill fold or a card holder or a purse or wallet. Where you might store your banking cards or card style biometric identity cards. \$\endgroup\$ – KalleMP Jun 11 '18 at 17:24
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    \$\begingroup\$ I know what a wallet is but what is it intended to do for the RFID card? \$\endgroup\$ – Andy aka Jun 11 '18 at 17:36
  • \$\begingroup\$ @Andyaka Sorry, on re-reading my original question the purpose is not clearly mentioned. I assumed it would be obvious but I should know better than assume. The idea is to attenuate signals coming and going to the point that the card or passport is not readable when inside the protective wallet. \$\endgroup\$ – KalleMP Jun 12 '18 at 11:09
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The first place to look would be the frequency of the RFID system that the system would block. If there is one frequency this may make things easier, if the blocking needs to happen for several cards this might make design more difficult.

Here is a list of some of the systems and their frequencies:

enter image description here Source: http://www.rfid-101.com/rfid-frequencies.htm

Now you want to build a faraday cage around the card to block any frequency from entering that area, a faraday cage works by conducting an electric field through the cage leaving the field in the middle at zero. Another thing to not is magnetic fields cannot be blocked, only attenuated, so if the RFID system only uses an inductive field, its best to stick with a magnetic attenuator like mu-metal. The field also can be attenuated by a material with a parameter called beta.

\$\Large{\delta = \frac{1}{\beta} = \sqrt{\frac{2}{\omega \mu \sigma}}}\$

Mu and sigma are values that are material parameters and can be looked up for most materials. With a higher skin depth you have less attenuation, so you want a material with a low skin depth that won't conduct far into the material a the RFID frequency you want to block. Here is an example of some materials and their skin depth:

enter image description here
Source: https://commons.wikimedia.org/wiki/File:Skin_depth_by_Zureks.png

Here is an example: If we had a card with an RFID signal of 433Mhz that needed to be blocked (attenuated) with an 410 steel as a material, the signal would only travel ~0.02mm so that would be the minimum amount of material to block the signal.

A low frequency signal of 125kHz would need more material or ~0.05mm.

Its important to note that a faraday cage with an opening is no longer a faraday cage, this makes it harder to analyze and would take a few pages a lot of EM theory to describe, so try and approximate a cage as close as possible with your design. My gut feeling is this won't matter much if you keep the opening small or extend and make the sleeve longer than the card.

Testing should be pretty simple if you got a card and a reader, build a sleeve out of the material and see if it can read it.

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  • \$\begingroup\$ Just wondering, would the fact that the Faraday cage is not (or cannot be) grounded be a differentiating factor here? \$\endgroup\$ – mehmet.ali.anil Jun 12 '18 at 12:33
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    \$\begingroup\$ Faraday cages do not need to be grounded to be effective. They still redirect the field. \$\endgroup\$ – laptop2d Jun 12 '18 at 15:28
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All you need to do is have any conductive "metal-foil" on (paper or plastic) in a bill-sized piece or kitchen Aluminum alloy foil in the billfold section of the wallet so it encloses the credit cards with RFID. Cost $0.01 Trump Value $100

You can prove this by taking your credit card and try all the Tap Approve machines in your area with and without kitchen foil in wallet billfold, the size of 1 bill.

I could prove the attenuation vs error threshold, but I don't get paid for this.

enter image description here
(example of fake gold-plated metallized bill ) not metal color , that is not conductive <1 Ohm)

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  • \$\begingroup\$ This does not cover the low frequency inductively coupled design requirement. \$\endgroup\$ – KalleMP Jun 15 '18 at 18:24
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While my answer will not provide you with technical information (I believe that laptop2d covered it pretty extensively), I can however give you pointers about RFID usage in general and how important it is to understand the implications so that you can pinpoint your market and tailor your marketing strategy accordingly.

First and foremost, I believe that you will never want to carry with you any uhf or hf tags that is directly linked to private/important information. Currently, those kind of tags are used to track inventory, movement, prevent stealing and production line. There is a lot of focus on some special kind of shopping center where you simply pick up what you want and it will get charged on your tab when you leave the shop. The idea is nice, but it is PRONE to fraud. All in all, they are made to be readable at long range and the information that is actually in needs of protection is server side and not on the tag itself.

The only danger here would be a complete tag replication (from my understanding it is extremely difficult to do so), but then again (and this is somewhat personal): I wouldn't carry a uhf/hf tag on me that is linked to important information or allow someone to spend on my behalf. I know however that people will do it while being uninformed or gullible about it.

On the other hand, LF tags are used extensively already. One thing for sure, most of the stories so far were about people using quick tap or even a password protected usage on a hacked POS terminal. It is a lot easier to orchestrate than trying to read your card at short range in a subtle manner. Cloning the card after is apparently easy. Would selling a protective device simply be a sale made on fear or is it really a risk for LF? I'm not too sure about it to be honest.

At the end of the day, the technicality of the product is more than doable. The key challenge will be any kind of marketing around it and the branding of it.

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  • \$\begingroup\$ You make sound points. What may not be evident is that some of the reason for my question is exactly to be able to have a good marketing angle that the product is the best it can be. I am well aware that with good spin even aluminized Mylar can be sold as an RFID shield and is likely adequate for most applications. I am trying to pre-empt the hard questions of when it is not enough and have good answers for the advertising copy writer to pass on to the prospective customers. \$\endgroup\$ – KalleMP Jun 15 '18 at 18:20
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I shopped around and was not happy with copper-lined or foil lined wallets, so I made my own with fine copper mesh. First I trimmed it to fit the width of the wallet slot. (I dedicated an entire slot for this) I pushed it down to the bottom with 3 credit cards and trimmed away any copper mesh that was taller than the wallet slot.

I took the credit cards out and used black RTV (a black wallet) to bond the copper mesh to the wallet. I know it will pass even an up close test because it is a Faraday cage with a very narrow open top, and to get a powerful RFID scanner to go in at that angle and read the card is impossible.

Another item not mention is that the close proximity of the mesh to the cards antenna effectively shorts-out the antenna such that not enough energy can be sent to it to wake up the chip. Even if they were touching your back pocket with the wallet (which could be annoying and a clue they might have a RFID scanner), they could not read the RFID chip.

Not only can it not receive a wake-up call it cannot transmit because once again the copper mesh de-tunes the RFID chips tiny antenna so much it could not transmit, even if it was self-powered.

I was thinking of using a brass mesh as it is shiny and does not turn to black then green like copper, yet brass is an allow of copper and more corrosion resistant.

I also found out it is best to leave a bit of mesh over the top (1/5th inch or 3 mm)and bend it down and mash it flat and wipe away excess RTV, to any edges will not scratch the cards. Try McMaster-Carr or Home-Depot for these raw materials.

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  • \$\begingroup\$ Hmm, a good DIY Faraday cage solution that will probably work in most cases. In a production situation I would look to a knitted tube that is 90mm when flat made from gold plated copper wire. Conductive mesh alone may not be enough for the low frequency transponders though. \$\endgroup\$ – KalleMP Jun 12 '18 at 11:15

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