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ST25R3911B, range of air gap between transmitter and receiver antenna

Associate II

Hello, commuity,

We want to estabilsh a data link between two devices over NFC. This is, because we want to power a kind of sensor device over the air via a gap of abt 1 mm.

The sensor consists of a ST25DV04KC and a STM32L031 with a low drop regulator.

On the other side or transmitter is a ST25R3911B driven by a STM32L031 (the controller will change later). Antenna matching network was designed by using the ST's Antenna Matching Tool. Anntenas itself are PCB coils of 4 windings of about 55mm diameter

Testing the power transmission we found, that this works fine in the range of more than 5mm. But the closer the antennas are, the more the field of the transmitting antenna drops, until there is no supply voltage available on the sensor board.

Are there limitations in the use of power transfer in this small ranges or did we miss any other restriction of this?

Thanks in advance




Hello Travis,

many thanks for Your reply.

The question with the insufficient energy at the power receiver is nearly solved. This must be caused to the sub-optimal SW configuration of the initiator. If I change to other screen in the GUI, the voltage at the power receiver is much higher.


Could we please get also some reply to my questions, which I posted on 04.12.2023? (In which I write about different Smith-diagram of the evalboard antenna compared to the appnote.)

Many thanks in advance!


Hello Peter,

I hope, I do not miss any of your questions - otherwise please repeat them.

Yes, when being un-powered the RFO's are in high-Z and do not disturb the smith chart measurement.

Yes, if the device is un-powered also the AAT pins are in high-Z and you see the matching as without AAT-caps populated. That's, why we recommend to power the chip and put the driver in high-Z mode.

Your approach of measuring the smith chart and comparing powered and un-powered state of the chip are completely correct. But your smith chart look somehow distorted. If you still have an un-modified eval board ( I assume you are talking about the ST25R3911B-DISCO) can you please measure it un-powered. There is an AAT application note (AN4914) and the un-powered smith chart should look like figure 2 (or very similar). 

I measured the antenna parameters (AN4974 - Figure 7) in the past. It should look very similar to figure 7.

Regarding your antenna measurement (first figure in your post): It looks distorted to me. Maybe there is a problem with the calibration of your VNA? Did you also compensate the cable?(Autoport extension)? Can you measure O-S-L as a reference? You could also measure a de-soldered inductor (like EMC inductor) and maybe a 10Ohm resistor. Also your antenna might have a resonance below 1MHz. Typically the curve starts in the left part ( = 0R spot) of the real axe. Yours is starting from the right side ( = infinite Ohm spot). Maybe your antenna is not a closed loop. You can also check the resistance with a normal multi meter.

Regarding your ST25R3911B-DISCO antenna measurement (second figure in your post):

It looks better, but your beginning of the curve (1MHz marker) is very high in the inductive region. Again there is a strange distortion.

Please let me know if your problem is solved.


br Travis


Hi Travis,

many thanks for Your reply.

I checked it again, the plots I posted were measured at an official EMC facility, with $$$ calibrated equipment. We did a 3-point calibration just before the measurement, at the end of the cable, which was then connected to the evalboard using two short pins. So the cable was also involved in the calibration, too.

We did the measurement at the EMC facility in order to verify our (new) VNA before posting the graphs. And we got pretty similar results. I'm considering to make a short video about the measurement and post here.

We measured also other antennas (not only the one of the ST25R3911B evalboard) e. g. by Abracon, Pulse and 2x of NXP evalboards to have golden units for repeated verifications of our VNA.

So this issue with the different results is still strange for me.

Many thanks for Your support!

Best regards,



P.S.: The development of our boards is still in work and we still plan to use the 3911B. As stated in an earlier post we changed the circuitry at the energy receiver side and got good results. The main reason for the weak received energy (and low voltage) was the wrong configuration of the 3911B, which seems to be solved. Now my SW-dev. colleague is working on the FW to take the 3911B evalboard out from the test setup. (Which is now used "as MCU" sending the SPI signals to our RF board.)