We made a custom NFC reader / writer with the PN5321, copying the refrence design from AN10449_1.
We use Mifare Classic 4K cards in our application.
One of our prototypes is giving intermittent problems with card communication. We found that communicatoin with the card stops working if the card is too close to the antenna.
If the card is 6-15mm above the antenna (coil) communication is bad.
If the card is ~30mm away from the antenna communicaiton is reliable.
Any suggestions on what components to alter to reduce the field strength?