Hi
I built up an ATU-100. In general it seems to work. To learn more about it, I put a 50 ohm termination on the RF out port and put my NanoVNA on the RF Input and put the atu in test mode. I also simply put the components into SinSmith. I sequenced the inductance step wise up the range of values, setting SimSmith, and watching the NanoVna output. The inductance values agreed reasonably well with SimSmith until I got to 6.70 uh. The value on the nanovna was close at 6.33 uh at around 5.80 uh. I seemed when the inductance was 0 uh that I had about 40 nh of stray inductance. In any case, when i stepped to 6.7 uh the inductance value suddenly falls by 3 uh down to 3.70 uh on the nanoVNA and then as I step higher it continues to go lower. Eventually at about 8 uh reading roughly 0 uh of inductance. It seems to me this is not right and would cause the tuning algorithm to not work properly. I did check that the correct relays get powered on. The problem seems to occur when the 4.4Uh and the 2.2 uh inductors come on together and the others off. Clearly the inductors are approximately correct because when I track the values they are fine all the way up the scale to this point. So the question:
Am I doing something wrong measuring this way? SimSmith reports the reactance value should keep rising, although the SWR is very high at this point and perhaps that makes the measurement invalid? Is it working as expected or is there something wrong? What could it be?
The capacitance side seems to work correctly and tracks SimSmith well so I think this side is fine?
Does anyone have a suggestion about what is going on?
John
