ST25R3916 Antenna Design Guidance?

[willem buitendyk]

I've created my own differential AAT board (3 turn 80x36mm antenna) that is working, but with a range of only 4cm compared to the NFC06A1 range of 8cm using a similar 15693 tag . With my own board powered off, I used my miniVNA to measure the matching network and antenna and see a minimum VSWR 2.365 and matching frequency of 16.52 MHz. When I measure the Nucleo NFC06A1 it yields a VSWR of 3.727 and a matching frequency of 13.3192. When I use QUCs to simulate my board with the same matching components its different (more capacitive on the smith chart). I was hoping the QUCS simulation would be more inline with what I'm seeing with the miniVNA. Which is more trustworthy, QUCS or the miniVNA for tweaking the matching network? I've attached some charts to show the differences. Any suggestions of how to proceed would be much appreciated. Thanks

[Ulysses HERNIOSUS]

Hi Willem,

in general the measurement should tell the reality. Simulation results may not always give accurate results as also suggested in 6.3 of AN5276.

I will ask one of our experts - he may have a more profound answer.

BR, Ulysses

[willem buitendyk]

Thanks Ulysses. I discovered that Figure 27 'Fine Tuning of the matching circuit' has been the biggest help. I also hooked up my board to the discovery board so I can now use the discovery gui. I have ordered a large sample size of caps, resistors and inductors so I can dial it in. I also ordered some desoldering tweezers because I kept desoldering the varicap by mistake and they are almost impossible to hand solder back on the board.

[Travis Palmer · Accepted solution]

Hello Willem,

It is good to hear, that the fine tuning chapter of the application note helped.

One remark from my side: The NFC06A1 has variable capacitors which are controlled by the ST25R3916.

To precisely measure the smith chart there is a special sequence mentioned in the application note. An important point is to turn off the driver, which could damage the VNA.

If you have a fixed matching (no variable capacitors), you should be fine measuring the matching unpowered.

The variable capacitors are for pin devices. even with a solder tweezer it will be hard to solder. I would recommend a hot air solder iron and heating plate below the PCB.

Please let us know if you still have open questions or need help optimizing the matching.

Did you achieve the desired read range? It would be good, if you could share your antenna dimension and the achieved read range.

BR Travis