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Daniel , 08-11-2026, 12:07 PM

Hi Everyone,

I am developing a circuit board with an ESP32-S3 and an ST25R100 that is designed to read 5 antennas (30x30mm, 4 turns, 0.8mm width, 0.5mm spacing) sequentially via two multiplexers. I want to use it to read NFC tags placed directly on the antenna at a maximum distance of 2 or 3 cm.

I used the ST25R Antenna Matching Tool for the calculations. However, I have absolutely no experience with NFC yet, so I wanted to ask for your opinion on whether this could work.

Thanks!

, 08-11-2026, 12:13 PM

🤖 In the meantime, here’s a response from the [FEDEVEL AI HW Assistant]().
It helps to find relevant resources for your question, but it may make mistakes as it’s still in early development.

This is the response from the AI Assistant:
While the videos don't specifically cover NFC antenna design, several key principles from similar RF antenna implementations apply to your NFC application:

1. **Antenna Matching**
The ST25R Antenna Matching Tool is a good starting point (as mentioned in your approach), but real-world tuning will likely be required. For NFC at 13.56 MHz, proper LC matching is critical - the videos emphasize that even pre-tuned modules often require final tuning with a VNA [as shown in antenna measurement examples](https://www.youtube.com/watch?v=6nRpBZu2T0k&t=2792).

2. **Multiplexer Considerations**
Your dual multiplexer approach adds complexity. Ensure:
- Trace lengths between ST25R100 and antennas are matched
- Multiplexer's RF characteristics (insertion loss, isolation) are suitable for 13.56 MHz
- Parasitic capacitance from multiplexers doesn't detune your matching networks

3. **Antenna Geometry Validation**
Your 30x30mm 4-turn design appears reasonable for short-range NFC, but:
- Verify resonance using a VNA (critical for inductive coupling)
- Maintain consistent 0.5mm spacing - even small variations affect inductance
- Consider ground plane effects - NFC antennas typically require keep-out areas [similar to 2.4GHz antenna placement principles](https://www.youtube.com/watch?v=vX3l0iRxi8c&t=183)

4.

, 08-11-2026, 12:13 PM

**Practical Implementation Tips**
- Use 1oz copper for better Q factor
- Include test points for L-match components
- Consider shield can over NFC circuitry if using metal enclosures
- Validate with actual tags at varying orientations (NFC is sensitive to alignment)

The 2-3cm range should be achievable with proper implementation, but be prepared for multiple tuning iterations. As shown in [ESP32 antenna testing examples](https://www.youtube.com/watch?v=6nRpBZu2T0k&t=2643), real-world testing with your actual hardware is essential - simulation tools alone won't catch all environmental factors.

Would you like me to elaborate on any specific aspect of the implementation?

You can continue chatting with the AI Assistant here:
https://fedevel.com/hw-assistant

Robert Feranec , 08-12-2026, 07:12 AM

i didn't know multiplexing antennas is possible. interested to read more in this post and learn some new things 🙂 but cant help with answer 🙁

Daniel , 08-12-2026, 09:47 AM

Thanks anyway for your reply. It certainly seems possible. I just found this document on the subject: https://www.st.com/content/ccc/resource/technical/document/application_note/group3/58/ba/29/e5/47/8d/48/42/DM01078018/files/DM01078018.pdf/jcr:content/translations/en.DM01078018.pdf
I'm going to look into this a bit more before I send my first prototype into production.

QDrives , 08-12-2026, 03:48 PM

I doubt you can use a 'normal' analog multiplexer for RF.
There are special RF switches: https://www.digikey.nl/en/products/filter/rf-switches/865