Hi there,

I bought a NanoVNA in the hope to upgrade a RFID board I have. It has a RF module with horrible AT commands. I would like to integrate to the PCB a simpler chip from ST.
I have a custom antenna on this that is purpose made. I have no datasheet for it, it has been matched with the module.

I thought simply measuring the impedance of the bare antenna + output of the chip would allow me to design a matching circuit for the 2 that would work.

First thing, the antenna seems oddly tuned. It reasonnate at 12.85MHz when RFID should be 13.56Mhz. Is the matching circuit able to shift the resonance  freq ?

Would you know of any good tutorial to do impedance matching ?

Best Regards,
Vincent

On Tue, Aug 11, 2020 at 10:16 AM, <vincent@ecovelo.com> wrote:

>
> First thing, the antenna seems oddly tuned. It reasonnate at 12.85MHz when
> RFID should be 13.56Mhz. Is the matching circuit able to shift the
> resonance  freq ?
>
> Would you know of any good tutorial to do impedance matching ?
>
>

The matching circuit will not shift the antenna's resonant frequency. It only "matches the impedance of the generator/source and the load" (load - your antenna at a frequency) such the power transfer efficiency from the source to the load is the "best one".
There is a nice tool called SimSmith, you may import your s1p file (you get from nanovna measurement) into it and find the optimal matching. There are youtube tutorials how to do it.
Also you can use various available impedance matching calculators to match the impedance of your antenna at certain frequency to your source.
The optimal solution would be to tune your antenna's SWR such the SWR is the lowest at the required frequency.

Thanks for your reply!
I need to investigate...
The antenna definitely change the frequency when it is loaded with the RFID tag. Does this makes sense to you?

On Tue, Aug 11, 2020 at 12:31 PM, <vincent@ecovelo.com> wrote:
The antenna definitely change the frequency when it is loaded with the RFID tag. Does this makes sense to you?

Yes. As I wrote above the best approach would be "to tune" the antenna into "resonance" with the generator (source) frequency.

If your generator is 13.56MHz you should tune your antenna to 13.56MHz..

https://www.youtube.com/watch?v=TVDUuyEllTw

You may do it with your NanoVNA too - connect your antenna to S11 input (CH0) and do the same as the guy shows above..

Warning: do NOT send any signal from an outside source into your antenna when connected to NanoVNA!!
You can damage your NanoVNA!!

Tuning to 13.56 MHz may not be the optimum for maximum system performance. The tag responds in two sidebands, a few 100 KHz below and above the 13.56 MHz carrier frequency. When the antenna has a high Q factor, desirable to energize an RFID tag, the bandwidth is small. I have some experience with HF RFID and I saw that tuning the antenna just a bit out of band helps in maximizing the readout distance. Now one of the sidebands with the tag response is received stronger, at the cost of the other sideband response. Reader chips usually select the best signal.
Also mutual coupling between tag and reader antennas may shift the resonance frequency (of both). Pre-compensation of the reader antenna may help.

And for reference, matching circuits can be used to tune the resonance of the System consisting of the antenna and match; for instance short sub resonant antenna radiators used by hams. Automatic antenna tuners use this method, as well as most smart phones now, 5/8 wave mobile antennas, HT antennas, etc. Not always the best radiation efficiency tho.   Maybe I'm preaching to the choir, but felt the need to comment.