Hi Dave,
Yes I agree that the 20:1 VSWR impedance ratio for my 1K hi Z test is higher than for the 5 Ohm low R which is half that at 10:1 VSWR. I selected the values of my test loads based on the range of impedance I would like to have the ability to measure with a reasonable degree of accuracy. I never made up any loads for less than 5 Ohms as with typical antenna matching tasks one would not often run across the need to measure a load less than 5 Ohms. I guess there would be exceptions for example a short mobile whip where the radiation resistance is less than 5 Ohms. Of course the input Z of solid state RF power amplifiers might be another example where there would be need to measure very low Zs. I agree that if one were to try and measure a very low resistance load it would, as you point out, be also difficult. Series inductance and I suppose skin effect R could become significant factors.
I do think the need to measure high impedance loads is relatively more common for antenna building which I think is the most common application for the nanoVNA. For example a coaxial stub might be needed to match an end fed antenna. The use of 450 Ohm balanced line for feeding non resonant multiband wire antennas with VSWR exceeding 3:1 is not that uncommon. Most antenna feedpoints present a series impedance bucket curve that approaches 50 Ohms at resonance and off resonance the Z typically increases dramatically due to reactance. Thus 50 Ohms is more or less the minimum encountered in that case. My impression is that there is more of a real world need to have the ability to measure very hi Z loads than for measuring very low Zs. There are probably methods to measure hi Z loads using impedance step down techniques but much more convenient to be able to make the measurement directly.
My original intent was to raise awareness of the hi Z measurement difficulty at higher frequencies. I think if in the future parallel equivalent impedance data is provided by software/firmware that will be very helpful for those making hi Z measurements. In my case it would have given me a much better perspective. I would have become immediately aware that the real R value measurement was close and the problem was just due to parasitic shunt capacitance. But by just looking at the serial Z data I gained the false impression that the real R measurement accuracy had fallen off around 100 MHz when in fact with my nanoVNA it remains quite good up to 450 MHz - what a wonderful little instrument!
Best Regards,
Tom
VA7TA
