On 7/17/22 6:17 AM, Larry Martin wrote:
I work with RFID/NFC at 13.56 MHz, and have been using my NanoVNA with a nonresonant loop to check the resonant frequency of HF tags. I'm wondering if the radio experts here can tell me if other measurements are reasonable. Here is the setup (172KB jpeg):
The top picture shows the loop with nothing on it, a near perfect short with a little inductance. The bottom view shows the loop with a typical NFC inlay on top. Resonant frequency is a little high. That's the only objective measure I can really define with this setup.
I would love to be able to point to the depth of the LOGMAG traces as some kind of inlay strength indicator. Informally, I like to compare incoming tags with the one in the link to see if it has as-good LOGMAG deflection (this tag type is the best so far), but I can't convince anyone else of anything based on that observation. I'm wondering if someone with more radio knowledge can think of a way to make a sensitivity measurement out of that, so I could tell my customers whether they are getting good tags from their vendors.
What you're sort of measuring here is a combination of the coupling to the tag, and the mismatch of the load on the tag. Think of the equivalent circuit as being a transformer (your loop and the loop on the tag) coupled to some load. So the "mismatch" you see in the S11 is a combination of the coupling ratio and the actual load.
The actual load may or may not be 50 ohms in the design circuit. Since the receiver and transmitter are typically connected right to the tag's antenna, traditional "matched transmission line" might not be relevant.
You might be able to set up a test rig to do incoming inspection on theoretically identical tags (same make and model) - whatever the mismatch and frequency is, it should be the same.
But it's not necessarily a good way to compare different kinds of tags. Maybe one mfr designs for 150 ohms and another designs for 50 ohms and yet another designs for 10 ohms.
Couple notes:
* this works with _some_ EMI field probes.
* this also works at UHF (902-928 MHz), with _a few_ field probes.
* I know there is test equipment for this, $25k and up.
