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giridharan
Visitor II
May 21, 2026
Question

External Antenna Suggestions for ST25R3916B NFC Reader

  • May 21, 2026
  • 1 reply
  • 122 views

Hi STM teams , i have developed a custom NFC board using the STM32L476RCT6 and the ST25R3916B NFC reader, based on the reference design of STEVAL-D25R16B (https://www.st.com/resource/en/schematic_pack/steval-25r3916b-schematic.pdf). I am using an external sticker antenna (Manufacturer Part Number: V2089-013-A-01).

 

Currently, the board detects only the STEVAL-SMARTAG2 NFC tag card (https://www.st.com/resource/en/schematic_pack/steval-smartag2-schematic.pdf), while other NFC cards are not being detected.

 

However, when I connect the custom NFC board to the DK antenna, it successfully detects the STEVAL-SMARTAG2 card as well as all other NFC cards. so i think issue may be sticker antenna

 

Could anyone suggest a suitable sticker antenna or recommend an alternative external antenna solution?

 

I have attached pictures of the sticker antenna, the STEVAL-SMARTAG2 card, and the other NFC tags for reference.

Screenshot 2026-05-21 170931.pngScreenshot 2026-05-21 170940.pngScreenshot 2026-05-21 171005.png

1 reply

Travis Palmer
Travis Palmer
ST Employee
May 21, 2026

Hello Giridharan,

 

unfortunately the smith chart you have shared is not ideal for antenna matching. 

M1 should be set to 1MHz => correctly done

M2 is optional and should be set to 13.56MHz => correctly done

M3 should be set to the self resonance point => I cannot identify M3

The M3 should be set where the Impedance is almost 0:

image.png

Using M3 and M1 parameter you can follow the steps in AN5276 and input the parameters in the antenna matching tool. The AMT then provides you the correct matching values for your antenna. 

If the self resonance is very low, you have to play with the input values of AMT. It is possible that not all combinations of EMI cut-off, inductance value, target matching impedance... result in a valid mathematical solution.

If you change between the antennas (EVAL kit default antenna and other antenna) also the matching components need to be updated.

Please let us know, if you succeeded.

br Travis

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