Large NFC Reader Antenna Design

[OpusOne]

Hi, I plan on designing a NFC reader based on the ST25R3916B, with an "extended" range.

So, the antenna will be a 150 mm x 150 mm PCB antenna, 2 turns, 1 mm traces and 1 mm gap.

According to ST's NFC Inductance tool, this should yield an inductance of ~ 2.4 µH. I have modeled it with a better approximation of its real shape, using FastHenry3, and get ~ 2.1 µH @ 13.56 MHz. Not too far away. I suppose FastHenry does a more accurate analysis, but does not model dielectric, so maybe that's what explains the discrepancy?

My question is more about the antenna's shape. I will be driving it in differential mode, and am not 100% sure if the coil I have designed (see below) will work well as a differential antenna. Can you let me know or give me pointers if it needs some adjustments as to how the turns should be layed out?

Thanks!

[Travis Palmer · Accepted solution]

Hello OpusOne,

Below are two posts where a similar wire-wound antenna was used:

https://community.st.com/t5/st25-nfc-rfid-tags-and-readers/how-to-use-st25r3911b-for-long-reading-distance-40cm/td-p/236114

https://community.st.com/t5/st25-nfc-rfid-tags-and-readers/design-questions-around-st25r3911b/td-p/370090

As long as the Self resonance frequency stays within a tune able range ( >30MHz, ideally >50MHz) you should be fine.

You should follow the normal design process as described in AN5276 Antenna design for ST25R3916/16B... devices.

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

Please share your experience with this kind of antenna design and please let us know, if you have further questions.

br Travis

[OpusOne]

Hi Travis, thanks.

Yes, I have used ST's tuning tool: https://eds.st.com/nfc-tuningcircuit/ to get prelminary values based on inductance estimations. I will use a VNA to actually measure it once it's prototyped to check actual impedance.

One thing I was wondering in particular is about "differential antennas". I have worked with the X-NUCLEO-NFC08A1 to get started, and noticed that its antenna looks a bit "unusual" with the way its 4 turns are layed out. So I was wondering if I had to do something similar here, but in my case, with only 2 turns, I don't know if it makes any difference.

I'll check the two other threads out to see what they did.

[Travis Palmer]

Hi OpusOne,

The antenna you designed should be perfectly fine.

It mainly depends on the self resonance of the antenna, if it can be tuned to 13.56MHz.

BR Travis

[OpusOne]

Note that I'll be using ISO15693 (ST25DV-based) and the goal is 10cm to 15cm max operating range (which, from your own tests, looks very doable), but... with very small tag antennas, so I think the 150mm x 150mm isn't  a luxury in my case.

Not sure I can simulate the self-resonance in any meaningful way, so I guess I'll have to prototype the reader antenna to figure it out.