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

[NDvas.3]

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]

Hey Support - I would be glad to have your help on this. We are holding off the PCB design, waiting for this.

Thank you very much again,

Nir

[JL. Lebon]

Hello Nir,

It is good that you already read the app note about design antenna.

What you want is to get the correct inductance of the antenna so that with the tuning capacitance of 28.5pF of the device, you get the maximum resonance at 13.56MHz

The app note gives you the formula to calculate the required inductance for your antenna:

ftuning=1/(2*pi*sqr(Lantenna*Ctuning)) => ftuning=13.56E6, Ctuning=28.5E-12

If you make the calculation, you will see that the antenna needs to have an inductance of ~4.8uH

(please note that a resonant frequency doesn't need to be exactly at 13.56MHz. It is usually not bad to tune the antenna slightly above 13.56Mhz (~13.7) to compensate high coupling when reader is close to the tag)

Now, you need to design an antenna with this inductance. For this you can refer to chapter 3 of the app note. In your case, you can make a spiral antenna or an octagonal antenna.

If you take spiral antenna, the formula helps you to find the correct parameters.

For example, in your case, you can make a spiral antenna with external diameter or 33mm and internal diameter of 27mm with 9 turns (which give trace thickness of 0.35mm and spacing of 0.35mm). According to the formula, this will give you a 4.7uH inductance, which is good.

If you need more accurate calculation of your resulting inductance, then you will need some simulation software, but those are usually complex tools with steep learning curve.

Once your PCB is done, it is important to check the resonant frequency of your antenna by using the measurement methods described in the app note.

Best regards.

[NDvas.3]

Hey JL,

Thank you very much for this detailed answer! Like many things in life, this project got a little more complicated than first anticipated... Please find attached the updated and hopefully final DXF of the desired PCB. Please note a few things:

1. The PCB area got smaller
2. On its top side, where the antenna is suppose to be printed, you can see a 3 leg device. This is a magnetic sensor which will sense the presence/absence of a small magnet (round, 3 * 1 mm) located at about 6mm directly above it.
3. All other components will be placed on the bottom side.

My question is if a circular antenna is still the best way to go here or will the magnetic sensor and magnet badly effect its performance and so we should design an antenna that does not do a full circle but rather "folds back" when near the magnet area. If that is the case, how will I be able to calculate its inductance?

Thank you again for your continued support,

Nir

[JL. Lebon]

Hello,

Sorry, can't open DXF files. Can you export to a PDF or a picture file?

About the magnet: the NFC is based on magnetic field modulation and magnetic field coupling between the reader and the tag. It is probably not a good idea to have a magnet too close to the antenna since the magnetic field of the magnet will probably have an effect on the magnetic field emitted by the reader.

Best regards.

[NDvas.3]

Sorry please find attached the DXF of the PCB

[NDvas.3]

Yes the magnet issue is bothering. Please see a top view of the PCB. What about placing the antenna

only on the right side of the black line I have drawn here so there is an air gap between the magnet fields and the antenna's fields?

Nir

[JL. Lebon]

Ok I understand.

Thinking about it, the magnet is probably not a problem for the NFC.

The NFC is based on modulating the magnetic field, so the static field of the magnet will not be an issue.

The only impact of the magnet may be that the relative inductance of the antenna may be increased a little bit. So I think it is not a problem to do a circular antenna that goes under the magnet sensor.

On the other hand, you may check that your magnet sensor is not perturbed by the magnetic field generated by the NFC reader?

Best regards.

[NDvas.3]

Thank you for the added info - that is relaxing to know that the magnet should not affect the NFC reading. There is no problem that the magnet sensor will not be perturbed by the magnetic field generated by the NFC reader since we will activate the sensor only when there is no NFC reader in range.

Two final questions:

1. Would you be so kind and help me re-calcualte the new circular antenna now with the new dimensions of the PCB I have sent above?
2. Will it be a good idea to add a small place holder for a tuning capacitor between the AC0/AC1 pins ?

Nir.

[JL. Lebon]

Hello Nir,

Please find attached an excel sheet that will help you calculate your antenna. It is based on the application note AN2972 formulas. You can easily expend it with more shapes formulas that are in the app note. Just enter rin/din, rout/dout and N and it will calculate the inductance and the tuning frequency for you.

As for a place holder for a tuning capacitor, it is indeed a very good idea if you have room for it. It will let you retune your antenna in case you are not satisfied.

Be aware nevertheless that you can only decrease the tuning frequency when adding a tuning capacitor, so it it helpful only if your antenna is tuned too high in frequency.

If you can, I recommend you to do a PCB matrix with several different antennas, each with a slight step difference. Then you can measure the real tuning of each antenna and choose the best one for your final PCB. This will save you time and money.

Best regards.

[NDvas.3]

Thank you JL, this is great!