NFC - ST25R95 dead spot

[StuN1]

Hi,

I have a new PCB design using the ST25R95 chip.

The application : Communicate with a moving tag with a reader (ST25R95). The distance is fixed (5mm) and the tags move under the reader. The problem : there is a dead spot at that distance when the tag is exactly in the center of the reader (at 5mm). I can read the tag coming from left to right, but there is no communication when the tag and the reader are aligned.

Is there some parameters I can change like this similar post ? It's not the same chip and this problem occurs with NFC tags on cardboard (no metal bags) https://community.st.com/t5/st25-nfc-rfid-tags-and-readers/nfc-reading-distance-has-a-dead-spot/m-p/675706#M10795

Thanks!

[Ulysses HERNIOSUS]

Hi StuN1,

I think it would be helpful if you could share some pictures showing your not-working position and the antennas in relation to each other.

In general there is always a dead spot when the two antennas are overlapping in a way that the inside and outside flux are equally strong and cancel out each other.

BR, Ulysses

[StuN1]

Hi Ulysses,
There's actually 3 dead spots :

- 2 when the middle of the tag is in front of a coil of the reader (which correspond to "the two antennas are overlapping in a way that the inside and outside flux are equally strong and cancel out each other").

- The last one is when the tag and reader's coils are in front of each other. The reader's antenna is sightly bigger.

The reader to tag height is around 5mm. This is simetrical left and right of the reader. 

Thanks

[Ulysses HERNIOSUS · Accepted solution]

Hi StuN1,

IMO the two dead spots cannot be avoided when having only these two antennas.

The other dead spot could be an effect of too strong coupling and de-tuning. Here I guess it would be worth to investigate based on VNA, field sniffs, antenna voltages on tag and reader, etc.

BR, Ulysses