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ST25R39R11B

J-luu
Associate II

‎2025-09-18 11:53 PM - last edited on ‎2025-10-23 1:51 AM by Community manager

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Currently, we are developing a card reader featuring a circular NFC antenna. However, due to the lack of equipment for measuring antenna parameters, we are reliant solely on calculated theoretical values. The circuit board and antenna are depicted in Figure 1, impedance matching is shown in Figure 2, and simulation diagrams are presented in Figures 3 and 4. Is there any means to optimise the antenna design or draw the antenna? Alternatively, are any circular antennas readily available? The circular antenna requires a minimum diameter of 42mm and a maximum diameter of 50mm.

 

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

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1 REPLY 1
Travis Palmer
ST Employee

‎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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