Help in the design of a printed 13.56Mhz antenna for the ST25DV04K

NDvas.3 · ‎2022-12-21

Hello,

We are now entering the stage of PCB editing and I wanted to ask for your help in the needed calculations and outcome of a printed antenna to be used with the ST25DV04K IC.

I have read the app note titled "an2866-how-to-design-a-1356-mhz-customized-antenna-for-st25-nfc" but I find it hard to understand and so I turn to your professional help since I do not want to make any mistakes with this.

Attached is an image of our PCB. As you can see it is a round PCB with a diameter of 34mm and a thickness of 0.8mm. I can see from the ST25DV datasheet that the Ctun internal capacitor value of the ST25DV is 28.5pF but from this point I am lost.

Can you please assist me?

Thank you,

Nir.

NDvas.3 · ‎2023-01-12

Thank you JL, this is great!

NDvas.3 · ‎2023-01-13

Sorry one very important question came to my mind:

Assuming the PCB is 4 layers and the NFC antenna is printed on its top side, can the other layers contain copper like a normal PCB layout would be and the bottom side contain all the need electronic components or should the antenna area be kept total 'clean' in all layers? Does the PCB thickness come into play?

Thank you,

Nir

JL. Lebon · ‎2023-01-13

Hello Nir,

The antenna must be free of any ground plane. This is explained in chapter 3.5.2 of the application note.

A ground plane would "block" the magnetic field, so you need to clear all the area under the antenna on all layers, but also around the antenna on all layers so that the field can loop around the antenna traces.

If only some traces are present on the bottom side, this is not ideal, but if there is enough space for the magnetic field to flow through it usually works.

As for the PCB thickness, it is only important if you plan to do your antenna on several layers. It has no impact on a single layer antenna. In your case, I recommend a one layer antenna (anyway all the calculation made before are for a single layer antenna).

Best regards.

NDvas.3 · ‎2023-01-16

In this case, I have much less PCB space for the antenna from what I initially thought. Please see the attached. The only area completely free from components is on the left side of the red line. (there are more components on the bottom side of the PCB which are not visible here)

What is the best course of action here? Is it a small rectangular antenna or is there some way

to calculate how to design a sort of a semi circle antenna?

Thank you,

Nir

JL. Lebon · ‎2023-01-18

Hello,

An antenna on just the left side will be very small. From what I remember, you have only 7mm between the external edge circle and the internal one. With so few space, your reading range may not be good. You can try to do a rectangular antenna so that at least one of the dimensions is higher, but even like that, it is would be a very small antenna with probably poor performances.

A semi-circular antenna would not change that fact significantly and would require a professional simulation tool to calculate.

I think your best option is to find a solution to have your antenna around the center circular hole.

Best regards.

NDvas.3 · ‎2023-01-19

In this case, I think I will turn to a ready made antenna and not try to print it on the PCB. Given the mechanical size we have, do you have any recommendation for a P/N of an antenna I can source from Digikey or others?

BTW, I saw this on Sparkfun: https://www.sparkfun.com/products/18991 where they used the W3102 NFC antenna. Do you think that would that be a good solution in our case?

Nir.

JL. Lebon · ‎2023-01-19

Hello,

Using a discrete coil like the one in the Sparkfun is a possibility. You will have to peek a non-shielded one with the correct inductance (~4..8uH). Biggest diameter is better. With such antenna nevertheless, reading range is small, so it needs to be put close to the reader (in case you have a box, place it close to the box border)

Otherwise, there is a lot of ready to be used antenna on Digikey. I don't have a specific P/N to recommend, it all depends on your physical constraints. Just search for "RFID antenna". They are all 13.56MHz tuned, so should all fit. Using such antenna will probably give you better reading range than a small coil.

Best regards.

NDvas.3 · ‎2023-01-23

I understand. Hope this will be my final question:

Would an NFC antenna work well when placed above a Lithium Polymer Battery? Is there something in these batteries (except for the protection circuit) that might be an issue?

Nir.

JL. Lebon · ‎2023-01-25

Not sure as I'm not an expert in battery, but I would think it is ok as long as there is not any metal enclosing the battery.

Regards.