I recently purchased a NanoVNA for playing with radio electronics, mainly HF radio for now, and I've already measured some filters and the result looks good. Now I'm trying to understand the behavior of toroid impedance transformers and baluns - it seems every amateur radio operator can wind them, but I never get it, so I decided to do some experiments.
I etched a single-layer FR4 test fixture with some holes and two SMA connectors, so I can solder different components and see how it measures. My methodology is basically: (1) solder a transformer on it, (2) solder a resistor dummy load, (3) do a one-port S11 measurement to find the return loss (or SWR), and keep tweaking it and repeats the steps. If the results look reasonable, I assume the transformer is doing impedance transformation correctly, then I can wind another identical transformer, connect them back-to-back, remove the resistor, and do a two-port S21 measurement to find out the insertion loss.
It worked fine at HF. But I started to get strange results when I move into VHF (output transformer needed for an upconverter, and it's only 30-60 MHz). As a sanity check, I removed everything from the test fixture, leaving only the SMA connector, and a 3200R metal film resistor and did a single-port measurement. NanoVNA reports the impedance 3200R-411X at 50 kHz as expected, but at 50 MHz, astonishingly, it becomes 542R-1250X (Z=1362R). I think it's why I was getting strange S11 measurement for my transformer - even a resistor doesn't behave like a resistor for me. See the Smith Chart attached.
I've never used a VNA before and I don't know understand the RF black magic here (I thought 100 MHz would be black magic, but apparently even 30 MHz is black magic). What could be the reason of these measurements? Is it a real result, or it's just a bogus artifact of the VNA (or, should a do a SOLT calibration on my test fixture)? Is it the parasitic elements in my test fixture? Parasitic elements of the resistor (but I heard ordinary metal film resistor works up to 400 MHz without seeing significant change of impedance)? Transmission-line effect? I suppose there can be some transmission-line effects, but I didn't expect to see an effect this great.
Update: Case closed. It doesn't work because I shouldn't attempt to do it. A VNA is not designed to work under significant impedance mismatch. If you measure S11 for a mismatched load, the results are bogus.