I designed an NFC reader board that reads and writeS to tags. Some of the parts with NFC tags have metal frames inside them (think of cup holders with NFC tags, but the object in the cup holder can be metal.) In order to keep the induced eddy current that creates interfering magnetic field which block the incident (from Reader) EM Field; i taped a ferrite sheet on the cup holder and stuck my tag right on it (see below.) It turns out the reader can't read the tag unless there is some gap between the ferrite sheet and the tag (space.)

  • I'd like to understand why this space between the ferrite sheet and the TAG is required.

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  • \$\begingroup\$ The whole point of ferrite is to absorb EM fields, and convert them to (usually) heat. What do you think it's doing? It certainly doesn't block them. \$\endgroup\$ Oct 19 '15 at 23:01
  • \$\begingroup\$ That still doesn't explain why it works better when there is space between the tag antenna coil and the ferrite sheet. I may have stated that wrong. It keeps the magnetic field from inducing Eddy current on the metal that in turns generates a magnetic field which cancel out the reader's. \$\endgroup\$ Oct 19 '15 at 23:09
  • \$\begingroup\$ When the tag is directly against the ferrite, the ferrite is absorbing most of the field, which is why it doesn't work in that configuration. Are you saying it works better when spaced away from the ferrite then without the ferrite present at all? \$\endgroup\$ Oct 19 '15 at 23:13
  • \$\begingroup\$ Yes, it works better when there is space between the ferrite and the tag \$\endgroup\$ Oct 19 '15 at 23:17
  • \$\begingroup\$ In fact it only works when i add that space between tag and ferrite sheet \$\endgroup\$ Oct 19 '15 at 23:54

I have to disagree with the route some of the comments are taking. I have used and continue to use ferrite sheet to enhance magnetic coupling between coils. I also use ferrite sheet to reduce or reshape the magnetic field so that eddy currents are not induced in structural metal parts.

Having said that, it depends on operating frequency and type of ferrite sheet that is used but, at the fairly common frequency of 13.56 MHz (used for a lot of RFID applications). I use Ferroxcube 5T1 materials - they have a magnetic permeability of 10 up to 100 MHz and they work just fine.

However, as with any tuned circuit system, if you place the ferromagnetic material close to a coil you'll shift the resonant frequency and suddenly find that it needs a re-tune. That is what I believe is the problem the OP has found.

Wurth also make this type of ferrite sheet but it looks even better at 13.56 MHz - material 364 has a permeabilty of 120 and seemingly very low losses too. Looks nice for RFID applications and, if you read the data sheet they tell you how much it can restore the tuned frequency when metals are close by. The clear inference of this is that it can also de-tune!

Here's what Wurth say about it: -

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Note the table across the middle of the sheet and how inductance rises from 8.4 uH to 14.2 uH with more ferrite sheet thickness. A shift of 69% in inductance is a shift in resonant frequency of 30% and might take 13.56 MHz down to 10.4 MHz.

  • \$\begingroup\$ Thanks @Andy aka for sharing the app note as well. I'm starting to understand a bit better how ferrite sheet are supposed to operate. The key words are redirection/concentration and Absortion. Depending on the material ideal permeability and it's losses over frequency, the ferrite sheet may redirect or absorb at the frequency of interest (in my case 13.6MHz). My ferrite sheet is 0.3m thickness which may also very likely be shifting my center frequency to the left of 13.56Mhz by increasing the overall inductance. \$\endgroup\$ Oct 20 '15 at 17:17
  • \$\begingroup\$ There will be some absorbtion losses in the 354 material but far less in the 364 material that I would favour at 13.6 MHz. Absorbtion in 354 is still relatively small at 13.6 MHz - the graph on page 2 shows you the red line (normal permeability) and the much lower loss line in black. I'd put my money on the shifting frequency because the red line in the graph is the dominant line numerically. \$\endgroup\$
    – Andy aka
    Oct 20 '15 at 17:18

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