RFID Antenna Matching Circuit And Values? - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: October 23, 2024, 11:43:19 pm
content: Hi,I don't have any experience in RF design but I need to design the matching circuit for this RFID antenna (PDF attached) to work with the ST25R100 (PDF also attached).I saw the reference design but the values are different. And after trying to play with this tuning tool I still don't know what values to put into some of the fields:https://eds.st.com/nfc-tuningcircuit/From the datasheet, I assume the values to input in the software are as follows: antenna self resonance = 13.56MHz, DC Resistance 2,000mR and inductance 2,000nH. Correct?BUT... These are not clear to me in the tool input fields:- should I use series or parallel resistors? What are the pros/cons of each and the criteria for choosing?- what is the Parallel Resistance (whether I click on series or parallel resistors it still shows PARALLEL Resistance)? How do I choose that value (it is not in the datasheet)?- I can put the most extreme and different values in the Antenna input fields (top area) but it does not seem to solve, it always gives error of either Q being too high or Cp being less than 0?I would very much appreciate if you could please help me find the right values?Thank you

author: Tobias89
author_rank: Contributor
timestamp: October 24, 2024, 07:05:04 am
content: It's very hard to give the exact values, I wouldn't spend too much time on it. This depends on the actual antenna geometry and (even more important) any metal in the vicinity of the antenna.My best guess would be to put some approximate value components, you'll need to measure this and tune via VNA anyway if you want to optimize the performance.Quote from: ricko_uk on October 23, 2024, 11:43:19 pm- Antenna self resonance is 13.56MHz as this suits the tags.- DC resistance seems a bit high, as I think this denotes the resistance of the antenna itself, not including added resistors. Since you will probably implement antenna via copper traces, I think the resistance will be more in the range of cca 300 mOhms. You will add resistors that will introduce resistance (to limit the Q-factor).- Approximate inductance of 2uH is used so you can use reasonable-values capacitors to reach the resonance frequency of 13.56 MHz. To be less strict, it can be anywhere between appr. 800 nH - 3 uH (but this will alter the matching capacitors values).Quote from: ricko_uk on October 23, 2024, 11:43:19 pmTBH, doesn't make a difference, antenna will not know. Whether you use series or parallel resistors, it will change the values, but the effect is the same - lowering the Q-factor of the LC circuit.Quote from: ricko_uk on October 23, 2024, 11:43:19 pmIf you go with the parallel, put 10k for start. You will probably want to tune this once you get the prototypes, but this seems as a reasonable starting point.Regarding the calculator, here are my values. Not sure if it will report error as it runs the calculation and becomes non-responsive (but these values seem like a good starting point).

author: PGPG
author_rank: Frequent Contributor
timestamp: October 24, 2024, 09:56:38 am
content: My experience is (for other Ic) that using design procedures given I got the working RFID reader that was good for standard tag identyfication number reading.But when we came to reading mifare Desfire tags the only way I found to optimize read distance was to do many experiences. For each of serial capacitor values from 5pF to 60pF with about 2pF step I was changing the paralel capacitor searching for the highest read distance. It took me 3 days for one antenna and my PCB got be black after hundreds of de-soldering and soldering capacitors.Than I found that when antenna is located in destination place near the microcontroller PCB I should start my work from beginning

author: ricko_uk
author_rank: Super Contributor
timestamp: October 25, 2024, 05:50:58 pm
content: Thank you @Tobias89 and @PGPG

author: flyseason
author_rank: Newbie
timestamp: March 17, 2025, 09:19:49 am
content: 1. In terms of the antenna datasheet, 13.56MHz is not antenna self resonance, instead that 13.56MHz is the working frequency. And this is not indicated from datasheet, and so as to Parallel Resistance. You can refer to another NFC antenna vendor's datasheet (https://www.leankon.com/product/nfc-antenna-coil-30mm-by-20mm/), they provide much more parameters except DC Resistance. DC R=2000m Ohm, Inductance 2,000nH, you are correct.2. Normally use series resistors, this resistor is to adjust the Quality Factor (Q factor) of antenna, higher Q, smaller working bandwidth and vice verse.3.This is a NFC antenna measured parameter from VNA. The datasheet I mentioned above provides this parameter.4. Because the values you input are not reasonable, then the result you get is unreasonable as well. I have tried to put the parameter from above antenna, and the result are normal as attached, by the way I assumed the DC value is 500 m ohm. If you want to get the accurate value, you can reach out to them to provide.Hope this will help.

author: ricko_uk
author_rank: Super Contributor
timestamp: March 18, 2025, 07:49:42 am
content: Thank you @flyseason

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 18, 2025, 12:29:11 pm
content: Yeah the antenna datasheet from TE doesn't give sufficient information to design the matching network. But even if they did provide more detailed characteristics, you should measure its actual impedance in your setup (as it depends on its proximity to conductors) using a VNA or LCR meter.

author: ricko_uk
author_rank: Super Contributor
timestamp: March 18, 2025, 12:51:35 pm
content: Thank you @mtwiegam I correct in thinking that if all we need is to read a card at only very close distances within 10-15mm, with those antennas then the values can be off by quite large margins and it would still pick it up?Thank you