From beneath 'Information' heading at:
Cool thanks. I was thinking of trying to use this antenna:
In the PDF you linked, it shows two capacitors (C19 and C21), one for each wire. I can't find what capacitance is needed. Do you know if I even need C19 and C21 when using an antenna like what I linked above?
Thanks for all of your helpful responses, Robin.
'Do you know if I even need C19 and C21'
I'm not an antenna expert, but I believe this might be a tuned circuit requiring both the inductive (implied resistive) and capacitive parts to function, otherwise, why would the manufacturer place those in their documentation image if they weren't in fact needed?
They were probably left unlabeled, such that *any* antenna with the appropriate parts could be connected.
Just checked the NFC Pixl arrangement and the schematic shows TBD
Maybe contact Taoglas direct as the Mouser link didn't seem to provide the necessary detail either.
Just found this related pdf on a Google search: (what capacitor is needed for an NFC antenna)
Hunch correct p5: 'they form a parallel resonant LC tank' caps in picofarad range
To connect NFC you just need to add an NFC antenna to the pins D9/D10 labelled with 'NFC' here: http://www.espruino.com/MDBT42Q#pinout
Now getting NFC tuned perfectly is more or less magic - there are so many different things that affect it (including measuring probes!) that you basically need to put the aerial on, see where it resonates (using a grid dip meter or similar) and try different capacitance values. It's why I don't put capacitors on there.
The good news is that actually unless you're going for super high range, none of that actually matters. I've had a lot of success just wrapping a bit of wire around a 4cm diameter circle 8 times, and then connecting it to those 2 pins with no capacitors at all - in fact that's what I use for checking NFC on the MDBT42Q breakout.
I'd definitely give that a go first, before buying a specialist antenna.
If you really wanted to try capacitors then I'd suggest using 2x 100pF capacitors, but from my experience the actual value you need to get it perfect could be anywhere from 50-200. If you bought an antenna you'd hope that the antenna manufacturer would give you a rough idea on the capacitor value to use though.
Can anyone point me to info on how to best add an NFC antenna to the MDBT42Q Breakout board?