Designing a single side antenna for M24LR04E-R

[IsuruWickramasinghe]

I am attempting to create an NFC antenna for the M24LR04E-R chip, following the circuit diagram of the x-nucleo-nfc02a1 board. My challenge is that the x-nucleo-nfc02a1 board uses a double-sided antenna, but I want to design a single-sided antenna that I intend to print using silver ink on a flexible PET sheet.

I have already followed a YouTube video on STMicroelectronics' YouTube channel (https://www.youtube.com/watch?v=Dtq8fKiGMUU) and designed a 42x42 mm antenna with 11 turns, which results in a 5.01 uH inductance according to the eDesign suit (the calculations give a value of 5.009 uH). However, when I printed and tested it, it appeared not to be working correctly. It doesn't seem to operate at the desired frequency of 13.56 MHz. Can you point out what I might have missed or any mistakes I might have made?

[JL. Lebon · Accepted solution]

Hello, 

What do you mean exactly by "not working correctly" ?

The M24LR requires a 4.8uH antenna, so you should be good with 5uH.

The following application note explains basically what you have seen in the video, with more details. You can have a look at chapter 4, which explains how to check the resonance frequency of the antenna you have design and try to measure your antenna tuning:

https://www.st.com/resource/en/application_note/an2972-how-to-design-an-antenna-for-dynamic-nfc-tags-stmicroelectronics.pdf

Best regards.

[IsuruWickramasinghe]

Hello,

The problem I am facing is that, according to the eDesign suite, the antenna should have a 5 uH inductance and a resonance frequency of around 13.56 MHz. However, when I print the antenna and test it with a NanoVNA, it shows a resonance frequency of around 300 MHz (I do not know the exact inductance value or how to measure it). So, I cannot figure out why this is happening. If it is occurring due to the conductivity of the silver ink I use, how should I modify the design? What is the relationship between the impedance of the antenna and the conductivity of the conductor? (I assume that there is no effect of the substrate I use since this is a single-sided antenna.)

[JL. Lebon]

Hello, 

The conductivity of your ink is probably not as good as copper, and it may increase the resistance of your antenna. The resistance of the antenna as no effect on the resonance frequency (it is visible if you look at the equations in the application note). With a higher resistance you may have less power transmitted to the tag, but no change in frequency. The substrate has no effect in your case as you guessed since you are in single layer.

Concerning your measurement with the VNA, did you measure the antenna alone or the antenna with the M24LR connected to it? The measurement of the resonant frequency is to be done for the pair antenna+M24LR, not with the antenna alone. When you do the measurement, do you have any parasitic capacitance? (for example long wire between the antenna and the device? Or socket that may add some pF capacitance to the system?)

Best regards.