ST25R3911B with antenna size 30x40cm

[michaelhagmann9]

Hi

I am new to NFC solutions and I have to design a product that can read NFC tags inside an area of about 30x40cm. The tags won't be far away, I think less than 10cm. But they are all placed inside this area, with the wire-antenna around them.

Is it possible to fulfill this requirement with the ST25R3911B NFC reader?

What would be the best solution for the antenna (wire diameter and umber of turns)?

Thank you very much for your help.

Best regards

Michael

[Travis Palmer · Accepted solution]

​Hello Michael,

This would be the perfect use-case for the ST25R3911B in combination with ST25TV . Its high output power and high sensitivity are perfect for this application.

I would suggest the following procedure:

1.) Doing a one turn wire wound antenna with the dimension of 30x40 cm.

2.) Measure the antenna parameters and design a matching network using application note AN4974

The following inputs are recommended:

Target mathching impedance: 15Ohm

Target antenna Q: 30

4.) Solder the calculated and simulated matching network on a ST25R3911B-DISCO board, disconnect AAT and the onboard PCB antenna

5.) connect your wire wound antenna to the ST25R3911B-DISCO

6.) crosscheck and finetune the antenna matching circuit to achieve maximum output power.

7.) adjust the capacitive voltage divider to 2.7Vpp to achieve best sensitivity.

8.) Check tag reading the your volume (30x40x10 cm)

BR Travis

[michaelhagmann9]

Dear Travis

Thank you very much for your answer and the recommendation for the procedure how to get the best performance. It helps me a lot to be sure that the ST25R3911B is the right choice for this application.

Do you have any recommendation what wire diameter to start with?

Thank you for your support.

Best regards

Michael

[Travis Palmer]

​Hello Michael,

There are no special requirements on the wire. Only the diameter should not be too small, otherwiese the antenna Q-factor will be too small. i expect 0.5 to 1mm should be sufficient. Otherwiese you can also use two wires in prallel. e.g. 2x0.5mm diameter.

BR Travis

[michaelhagmann9]

Dear Travis

Thank you very much for your support.

I followed your guidelines and the antenna works perfectly. The reading distance is actually a bit too big (>25cm with ISO15693, >15cm with ISO14443). Thankfully too big instead of too small.

Thank you very much.

Best regards

Michael

[Travis Palmer]

​Dear Michael,

Thanks for your feedback!

If you want to reduce the read range and power consumption, you can simply increase the matching impedance.

It is likely that there is a minimum H-field in the center of the antenna (z=0 distance). You have to take care, that there is sufficient field strength left to power the tag.

BR Travis

[michaelhagmann9]

Dear Travis

Thank you for your hint.

Is it possible, that with the "ST25R3911B Discovery" Tool, it is not possible to detect more than 10 ISO14443A tags? Ten tags can be read without problems. If I add one more tag, it gets detected once, but disappears after this one detection. If I add more ISO15693 tags beside the ISO14443A, all of them get detected without problem (up to 20 tags).

Do you have an idea what could be the problem?

Best regards

Michael

[Ulysses HERNIOSUS]

Hi Michael,

for ISO14443A the loop to detect multiple tags is implemented in the GUI: iso14443ASelect(REQA), iso14443AHalt(), iso14443ASelect(REQA), iso14443AHalt(),....

So there is no limit in software. This is different for iso15693 where the loop with fixed number of tags is in FW.

I have no hypothesis on the reasons for your observed behavior. I rarely managed more than 5 ISO14443A tags with default Disco antenna. Typically all the tags detune the system heavily and some tags may not get powered by the field anymore.

If you want to analyze this behavior in more detail you can use the ISO14443A tab to execute this loop manually and observe every single iso14443ASelect().

Regards, Ulysses

[michaelhagmann9]

Dear Ulysses

Thank you very much for your answer.

If I step through manually in the ISO14443A tab, I am able to detect all 20 tags (all I have).

Seems like the polling tab limits the amount of tags to 10.

Best regards

Michael

[Ulysses HERNIOSUS]

Hi Michael,

indeed, I checked there is a safety counter in the Polling tab of 10. The reason for it is to cope with tags which ignore the HLTA command and repeatedly answer leading to an endless loop.

Regards, Ulysses.