13.56 MHz RFID tag in CST

hello everyone
I am trying to simulate in CST a RFID tag antenna which should resonate at 13.56 MHz(NFC antenna)
After designing it (at this stage I picked an antenna from a IEEE journal ) I have run the simulation (Time domain Solver) but the antenna impedance value does not match with the one in the IEEE journal. Think I am missing a point here and not sure where to look for my mistake.I have attached some print screens and the journal (pdf)

any help or suggestions would be much appreciated

kind regards

The paper is rather vague about the tag circuit. With minimal knowledge about RF engineering you should know that a small loop << wavelength is just an inductor and can't resonate without an additional capacitor.

The measured inductance of about 90 nH doesn't seem to fit the shown multi-turn loop geometry, however.

Thank you FvM for the quick reply.Yes,it does make sense, but in CST environment do I just insert an capacitor (equivalent as per chip spec)? Or there is other method of accounting for this?

Kind Regards

A capacitor is necessary.

hello again, i have been trying to get this implemented in CST but with not much luck.How do i incorporate the capacitor in order to get the tag to resonate at 13.56?
your help would be much appreciated

one simple way:
get your RFID inductor S params, and put a cap near the RFID inductor, and tune the cap value, then you get it.
This way, you can get a coarse cap value.
And you need make a PCB to tune one board. Simulation can help, but not the ultimate step.

Hi,

Do you mean physically place the capacitor in relation to the inductor? Navigating your way through the RFID jungle has block schematic pictures of HDX and FDX readers and transponders on pages 2 and 3 of the app note. There, the capacitor is connected across the antenna.

I thought the RFID is at 13.56M, right? Which is much different from UHF.
I remembered my colleagues, when designed a RFID for a mobile phone, just maintained the antenna area is above 1200mm^2, and at test, just measured the inductance is about uH, then it's OK for phone. Hope I am right in these data.
The NFC in phone is just a function, not like a reader, which has much more requirements.
About the circuit, you can search some phone NFC circuits as refer.

STM electronics has application note AN1806 with detailed description how to design and tune 13.56 antenna. And other tools - search for it. Other companies who manufacture reader and tag IC's likely have it too.

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