Using A Standard Coil For NFC Tag Implant Reading

A few months ago Hackaday covered the xNT crowdfunding campaign which aimed at making an NTAG216 based NFC implant for different purposes. I actually backed it, found that standard NFC readers don’t perform well and therefore decided to try using a standard coil as an antenna for better reading performances.

Most NFC readers typically only have a small sweet spot where implant reading is possible. This is due to what we call coupling factor which depends on the reading distance and reader & NFC tag antenna geometries. Having a smaller antenna diameter increases the coupling factor and makes implant positioning easier.

In my detailed write-up you’ll find a good introduction to impedance matching, a process where a few passive components are added in series/parallel with an antenna to bring its complex impedance close to a RF signal transmitter’s. This usually requires expensive tools but allows optimal power transmission at a given frequency.

You may find our xNT coverage here.

18 thoughts on “Using A Standard Coil For NFC Tag Implant Reading

    1. Antenna design is one of the weirdest things out there, on the one hand scaling antennas is pretty easy, or angling things for better reception in a specific direction. It gets downright spooky when you try to design new antennas. So many weird effects on what makes it work.

  1. What sort of read range did you get? And will you be releasing / selling your final design for the rest of us implantees to use?

    I’ve been testing a few cheap NFC readers recently and have typically managed about 3-5mm from the skin if you’re right by the PCB trace (rather than in the centre of the coil). Some better than others. The Samsung Ezon locks seem to read fairly well (probably due to the small tags they use) so I was considering looking at their antenna. I was planning to write this up, but haven’t got round to it. If you get a good reliable read range I suspect your work may make this a bit redundant.

    1. Of course, everything I make is open source!
      Actually it’s not really the reading range that annoys me but the way to position my hand. I therefore made this PCB as small as possible So I could easily move it around.
      In my tests I just had to put the implant on top of the antenna and I was sure to be able to read.

      Please share your experience so we can learn from it!

  2. Yay, I can’t wait to help Big Brother to reach 1984 by sponsoring him with my private money and by chipping myself without any real reason. Smart/Rich enough to afford/access a 100k$ network analyzer but apparently to stupid to see the consequences.

    1. You do realize that, by studying and understanding the type of technology that might lead that way in advance, that gives everyone a distinct advantage over it.
      Knowledge of physical limitations, structure of the potential technology, and what it is actually capable of in a real world setting, is a bit of a step ahead I’d burying your head in the sand.

  3. Anyone know how good android NFC readers are at reading RFID implants? I was thinking of getting one to unlock doors and whatnot, but it would be cool if it could unlock my phone too.

    1. My Nexus 5 reads reasonably well but I have to place it against the implant and know the sweet spot on the back of the phone. My wife’s HTC One Mini 2 is much the same. If I’m showing it to a friend and I don’t know where the NFC antenna is located on their particular phone then it sometimes looks like it doesn’t work as I move it around randomly trying to find the right location.

        1. There ought to be a testing mode where you could put your phone into NFC mode and then emit noise at the right frequency while waving the emitter over the phone case until you generated an error message in the NFC app or something… wouldn’t take long to narrow down the location and then you could just put a dot of nail polish or something on the back of the phone to indicate the detected spot.

          Alternately, why doesn’t someone design phone cases which are marked for the location of the NFC coil? Wouldn’t be too hard to get this info for each of the popular phones and then produce the cases with a divot or logo or symbol of some sort moulded into the case material. That might be a little too esoteric for manufacturers, but it ought to be able to be done on a small scale…

    2. Smaller antenna is better the best readers i worked with are acr122t usb (sort of like a pendrive) and seedstudios modules with small antenna on a cord is great like NFC Shield V2.0 but there are more modules on that antenna. DFrobots nfc rreader is not that good with reading implants.

      Im still wondering abiout nfc locks inserts (standard size insert that can replace regular key mechanisms)

  4. I have wondered about using a high Q tuned loop as an intermediate element between a reader and an RFID device, acting as something akin to a director element, not unlike those sometimes used by AM radios in weak signal areas or crystal sets, but I haven’t had a chance to try it.

  5. Ferrite rods also work :-)

    I did some work with RFID a while back and NFC is remarkably similar, the lack of far field signal is indeed fixable using a diamagnetic material like pyrographite glued to the ferrite rod to act as a field buffer, and a heater to tune the setup further.

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