Further information on double sided NFC Antennas for ST25DV

[PSaal.1]

Dear Community,

as a hardware developer I'm working on a project with ST25DV Tag.

Now I'm facing some problems with the NFC Antenna and would like to try out some double sided ones. I have some example boards and also a list of ST approved antennas but I want to fit the antenna size to my application. That's why I need some formula to calculate.

Unfortunately, I can't find any useful information on designing double sided antennas.

Do you have any experience with it or do you know good whitepapers or tools to calculate?

Of course I know the ST Antenna calculating tool but that's only for single sided antennas.

Thank you very much!

[JDirk.1]

Hi,

in the pdf attached you find some formulas and some best practices. Your question is good. Reading this i realized that i also should think about. I designed my first antenna these days using the stm tool. I simple did the antenna double side, so the same layout on both pcb sides. Once i receive the pcb i will check if it works - but i am pretty sure it will. I also compared my layout with the ANT7 reference board - its exactly the same and they also did it both sides ....

anyway ... maybe the pdf attached helps you

good luck!

[PSaal.1]

Hi JDirk.1,

thank you for your answer.

AN2972 has all the needed formulas for different shapes but only for single sided Antennas.

Of course one could simply take the same antenna to both sides and connect them. There are some unknown factors (e.g. PCB thickness, coupling, capacitance) that have impact on the inductance of the antenna. To determine a inductance of a double sided antenna I would like to pack these factors to a formula.

Maybe ST is working on the Antenna Design tool for double sided antennas?

I have also copied a ST antenna and it works fine.

[JDirk.1]

Hi,

finally my pcb arrived and i tested my antenna: Unfortunetly it is not working. Attached you find the layout which i calculated with the st antenna tool. Was my fault that i applied it on both sides of the pcb?

The ANT7 Antenna looks very similar and it is also double side, so i am wondering why my Antenna doesnt work ...

In case its because of the two side, what do i need to consider?

many thanks

[PSaal.1]

Hi,

the antenna tool is only working for single sided antennas. If you apply it on both sides you get a very different inductance than calculated.

Actually that is exactly my question, how to calculate double sided antennas.

I also copied some ST antennas but they have also a very different inductance than the original one.

[JDirk.1]

Hi,

i did some new antennas with 4,845 uH and some more with +-5% to test. It's working fine but only single side.

I am also missing to calculate the inductance in case it's double side, something to think about (or maybe st will give us a hint?)

In additon i had a look on the gerber files for ANT7 antenna. I did a 1:1 copy (at least what i thought) but still its not working.

PCB material is 0,74mm, cupper 35um.

size 13x14mm on both sides. width 0.2mm spacing 0.2mm. turns 9.

Probably a very small difference will lead to a not working antenna.

In case you find out how double side ones are calculated, please let me know. For single side its simple 4,8uH and that's it ...

[LHell.1]

I am also looking for this, and I can't find a single reliable resource with formulas for two-layer antennas. However, I found a PDF from ST (see below), which lists the parameters for a few different single and double layer antennas. I am interested in using the ANT14x14_Double layer antenna as my board is 1.6 mm thick (unlike the ANT7 reference board, which is 0.8mm). Unfortunately, the inductance for this design is not listed, it is simply stated as "ANT8" which I have no idea what means. I can't find a reference design or anything called "ANT8".

Using ST's single-layer calculator, it seems that for the double-layer antennas with 0.8mm substrate thickness, the listed inductance is approximately (but not exactly in all cases) four times the calculated inductance in the single-layer case. I.e. for the ANT15x15_Double layer antenna, putting the parameters into ST's single-layer calculator gives us 1.12 µH, and the PDF lists 4.48 uH, which is exactly 1.12*4. I don't know what the multiplication factor would be in the 1.6 mm case.

Also, looking at the Gerbers, the ANT7 reference design does not seem to use any of the antennas listed in the PDF, at least not exactly.

I am tempted to use the parameters for the ANT14x14_Double layer and making layer traces overlap as in the ANT7 Gerbers. However, it would be very helpful if someone from ST or someone else with more solid knowledge could pitch in and validate this.

A question for @JDirk.1​ : When you measure on the ANT7 gerbers, what do you get for the length and width parameters? One of them seems to be pretty clearly 14.00mm, but the other one is either approx. 12.67 mm or 13.07 mm, depending on whether you include the last "half-segment" in the measurement or not. See my image.

 

[JDirk.1]

I used the gerber file and for me it was 14,2mm x 13,0mm. 9 windings. ANT7 Antenna. The width of the wire is 0.2mm und the space in between also 0.2mm. I produced this Antenna on 0.8mm PCB. The Antenna is not working ... So in my diameter there is an error. I have the ANT7 evaluation board available as well, its working very well. its given with 14x14mm but i was not able to measure it in the gerber files.

[PSaal.1]

Have you checked whether a tuning cap is necessary?

Is your inductance below or above the needed 4.8µH for ST25DV...

If it is below you can tune with a cap.

[LHell.1 · Accepted solution]

FYI I was just told by ST support that increasing the PCB width from 0.8mm to 1.6 will decrease inductance by about 0.4 uH, and that this could be compensated for by adding a 10th inner winding on both layers. I will also add a footprint for a parallel tuning capacitor, just in case.