nRF52 NFC antenna and its equivalent inductance

[Lauri Jämsä]

Hi,

We decided to add NFC functionality to our nRF52 open source sensor beacon design, RuuviTag: http://ruuvi.com

I would like to ask your opinion about the antenna design, because Nordic hasn't published (correct me if wrong) any white papers or design guidelines about the NFC antennas.

Our NFC design files can be found from here: github.com/.../nfc-antenna

First I used STM's eDesignSuite to match the equivalent inductance to 2uH:

After that bended the antenna to fit our design:

Would it work? It sure needs tuning capacitors in place, but AFAIK, we need to make some measurements to find the correct values...

[Anders Strand]

Hello.

As you mention we do not have any documents on this yet.

As long as you are able to tune the antenna to resonate at NFC frequency, i do not see any reason why this should not work. We provided this antenna with early nRF52 DK's, you might get some useful info from that datasheet. You might also find useful info on the NFC section of the nRF52 OPS.

-Anders

[Lauri Jämsä]

Did you find the values (130pF) of tuning capacitors (C42 and C43) using that particular antenna?

[Anders Strand]

Yes. There are some app notes out for other nfc devices, which explain the tuning process. For example this one from ST.