How to choose 13.56MHz antenna for ST25DV64KC

[Danny0007]

Hi,

I designed a board with NFC Chip ST25DV64KC, which will be connected with a extra antenna, during the selecting of antenna products, one parameter confuses me very much: the self-Resonance Frequency, is it important? For example this product.

Or all antennas with specifical frequency 13.56MHz will work with ST25DV64KC? 

Thanks in advanc.

Danny

[JL. Lebon · Accepted solution]

Hello, 

What is important is that the resonant frequency of the antenna + the tag is at 13.56MHz (or close).
The self-resonant frequency of the antenna alone is not a very useful information as the tag is adding a capacitor to the couple "antenna+tag" which modifies the resonant frequency.
You have an antenna with a serial inductance L connected to a tag with a serial capacitance C and they should have a value so that 1/(2*PI*SQRT(L*C))=13.56MHz.
The ST25DV has a tuning capacitance of 28.5pF, so you need to find an antenna with a serial inductance of ~4.8uH so that the "antenna+tag" self resonance is 13.56MHz.
The antenna you are proposing has a serial inductance of 7.3uH, which would give a resonance for antenna+tag at ~11MHz.
Look for an antenna that has a serial inductance of 4.8uH.
I recommend you to read this application note to better understand this topic: https://www.st.com/resource/en/application_note/an2972-how-to-design-an-antenna-for-dynamic-nfc-tags-stmicroelectronics.pdf

Best regards.

[Danny0007]

Hi @JL. Lebon 

Thanks for the reply. that solves my question basically.

but one more question: since serial inductance is a so important parameter, why mouser doesn´t put it in the selection filter?

[JL. Lebon]

Glade I could help you!
For mouser, I'm afraid I can't answer for what they do and don't do   But I think this is because antenna parameters proposed by Mouser are more for "traditional" antennas (bluetooth, wifi...) that are not working the same way (no magnetic coupling as in NFC) and where the inductance of the antenna is not an important parameter. NFC antennas work like a transformer where the ferrite coil is replaced by the air. So important parameters are the same as for a transformer, not the same as for traditional antennas.

Best regards.