Hello,
We are producer of access control system. But we are having issues with our latest delivery of fobs. We are already in contact with our supplier, but I'm not 100% everything they are saying is correct. So this is a bit of a sanity check.
We ordered from our supplier combi fobs (desfire EV3 + EM). According to the supplier they have a read distance of 2cm. But we always use desfire with our AID and data written to it.
We noticed in the latest batch a lot of fobs have a terrible read distance (0cm).
Luckily we have a liteVNA and tested a couple of fobs with it (using this method: https://hackaday.com/2021/11/10/nfc-performance-its-all-in-the-antenna/ )
Some old fobs that work great have a signal looking like this:
The antennas are tuned at 13.8MHz and 13.6MHz, and have a signal of -38 and -20 dBm
(first one is desfire only, second one is desfire + EM).
These look like "normal" values. They work fine.
But when testing fobs from the latest delivery look more like this:
The first one is desfire only, second one is desfire + EM. They have a frequency of 14.03MHz and 14.5MHz with a signal of -4.2dBm and -1.6dBm respectively.
According to the supplier these values mean nothing. But I think the lower the signal, the more the tag "collects" the signal.
When I scan the fobs with NXP taginfo they report capacitance of 17pF. But according the supplier that is correct, but they manually add a 100pF capacitor to increase the read distance.
Am I correct to think that these fobs have badly tuned antennas?
And can they just add capacitor to 17pF variant to increase read distance?
