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'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
https://github.com/espruino/EspruinoBoard/blob/master/Pixl.js/pdf/pixljs_sch.pdf
Maybe contact Taoglas direct as the Mouser link didn't seem to provide the necessary detail either.
EDIT:
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
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Robin
Cool thanks. I was thinking of trying to use this antenna:
https://www.mouser.com/ProductDetail/Taoglas/FXR05A?qs=sGAEpiMZZMuBTKBKvsBmlEY7deXXZu90agKoWShIJNEIRe76Xn%2Ffzw%3D%3D
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.