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ST25R3911B Antenna Response Distance and Impedance Matching Debugging

J-luu
Associate II

‎2025-09-02 1:48 AM - last edited on ‎2025-09-02 5:40 AM by

This is a custom-made PCB antenna board utilising the ST25R3911B chip, but I am unsure how to proceed with impedance matching debugging and parameter configuration.

Could you offer some guidance? Below are the antenna dimensions, the calculated impedance matching values, and the original impedance matching diagram. Communication is currently possible at very close range.

To increase communication distance and stability, what modifications should be made?

Jluu_0-1756802810927.png

Jluu_1-1756802820331.png

Jluu_2-1756802854425.png

 

*This post has been translated from Chinese to comply with the ST Community guidelines.

 

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4 REPLIES 4
Ulysses HERNIOSUS
ST Employee

‎2025-09-02 6:52 AM

Hi,

please refer to AN4974: Antenna matching for ST25R3911B/ST25R391x devices. 

BR, Ulysses

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Travis Palmer
ST Employee

‎2025-09-12 1:47 AM

Hi J-luu,

 

120nH antenna inductance seems very little for a 4 turn antenna with approx.. 30 to 40mm diameter.

Could you please share a smith chart of the antenna and antenna + matching network?

What is the read range you are able to achieve and what is your desired read range?

BR Travis

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J-luu
Associate II
In response to Travis Palmer

‎2025-09-18 11:42 PM - last edited on ‎2025-10-23 1:53 AM by Community manager

Jluu_0-1758263827265.png

Jluu_1-1758264033201.png

Jluu_2-1758264055347.png

The impedance matching network currently in use is shown in Figure 1, with STMicroelectronics' official analysis depicted in Figure 2. Compared to Figure 3, the present reading range is limited to 5 cm. How might we optimise the antenna to achieve a reading range of approximately 10 cm? Alternatively, could you provide a correctly designed circular antenna? Owing to the lack of equipment for measuring antenna parameters, we can only input calculated values for impedance matching.

 

*This post has been translated from Chinese to comply with the ST Community guidelines.

 

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Travis Palmer
ST Employee
In response to J-luu

‎2025-10-28 4:57 AM

Hello J-luu,

 

It is not very easily possible to tune a NFC reader without VNA.

There is a nanoVNA H4, which is quite affordable. 

Even if you use a pre-tuned 50Ohm antenna, it is recommended to check the tuning. Parasitics introduced with the layout or PCB layerstack can have very bad influence on the matching. 

One other solution is to rent a service at a university or consult an external RF design house.

BR Travis

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