I have a 125 kHz RFID antenna consisting of a coil and series tuning capacitor. The manufacturer supplied plots of the SWR and Z of the antenna showing a nice 50 Ohm 1:1 SWR at resonance. I have tried to replicate these plots using my newly purchased NanoVNA and I get radically different plots showing a load closer to 25 Ohms and a 2:1 SWR. Measuring the L and C separately gives about the same readings I get with my HP4801 vector impedance meter at 125kHz, but it is near the upper frequency limit of the HP4801 so results are shaky. I calibrated the NanoVNA over the same range as shown in the manufacturer's plots. My Smith plots are centered on the horizontal axis like I would expect them to be. The manufacturer's plots are shifted up to the inductive half of the Smith chart. Both show a nice resonant dip at 125 kHz.
Am I doing something wrong? Is the antenna manufacturer lying?
SherpaDoug WA1UWP
Douglas Butler
