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Design & Match a Wire-Wound Antenna (Air Coil) for ST25TV02KC with ST25R3916B ?

MERSI
Associate III

‎2025-07-28 2:40 AM - last edited on ‎2025-07-28 3:38 AM by

Hi everyone,

I'm currently working on an NFC reader design using the ST25R3916B in single-ended configuration to read and write ST25TV02KC tags (ISO15693, NFC-V).

Instead of a PCB antenna, I'm using a round, wire-wound air coil antenna and now need to properly design the antenna and match it to the reader IC.

My questions are:

  1. How do I correctly design a wire-wound (air coil) antenna for 13.56 MHz in this setup? Should the antenna's resonance frequency be at ~13,56MHz or including the antenna network circuit (--> f_0 of antenna >13.56MHz)?

  2. Which parameters (e.g. inductance, Q, ...) are most important to measure or calculate for use in the ST matching tools?

  3. The STM32 NFC Inductance Tool seems to be designed for PCB trace antennas – can it be used for air coils as well, or should I rely on external tools? The resonance frequency of the antenna must be ~13.56MHz?

  4. How do I calculate or match the antenna network correctly for single-ended use with the ST25R3916B, once I have the coil parameters? Resonance frequency including antenna must be ~13,56MHz? Or must the antenna have by itself a resonance frequency of ~13.56MHz?

Any help or clarification on how to approach this would be greatly appreciated – especially from those who have worked with non-PCB antennas in combination with ST's design tools.

Thanks in advance!

Best regards

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1 REPLY 1
Travis Palmer
ST Employee

‎2025-07-28 8:31 AM

Hello Mersi,

Air coils or wire wound antennas can be treated the same as PCB antennas. 

There are two application notes available which are relevant for you:

  • AN5276 Antenna design for ST25R3916/16B... devices
  • AN5592 ST25R single ended antenna matching

AN5276 describes the general flow of antenna matching.

1.) Measure antenna parameter

2.) Calculate the matching components (using the AMT)

3.) Fine tuning of the matching

4.) Design verification

AN5592 compares the differential and single ended antenna topology and give further information about the matching component calculation.

Regarding your questions:

1.) The self resonance frequency of the antenna (without matching network attached) should be significantly higher than 13.56MHz. An ideal range is between 50MHz to 200MHz. Outside of this range, you might face limitations in the selection of component values (e.g. CP being very high if SFR > 200MHz) or the antenna is not even match-able with this topology if the SFR (self resonance frequency) is too low.

2.) Most important parameters to measure or simulate at:

  • Antenna inductance at 1MHz.
  • DC resistance of the antenna at 1MHz
  • Parallel resistance of the antenna at SFR
  • SFR

3.) The tool has been developed to calculate the inductance of PCB antennas. Nevertheless when using the correct Epsilon_r, an air coil inductance could be estimated. The tool itself is limited to rectangular antenna geometry. So the result might not be very accurate.

4.) I would recommend to follow the design process in AN5276. In the AMT (Antenna matching tool) you then can simply select "single ended antenna":

TravisPalmer_0-1753716417665.png
There is also an Online version of the tool available: https://eds.st.com/nfc-tuningcircuit/
TravisPalmer_1-1753716510132.png

Please let us know, if you have additional questions.

 

br Travis

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