Herb,
Thanks for your comments. You make good points to remember.
I am not presently in a position to have a "gold" standard available. That would be nice. And, I am not in need of absolute National Bureau of Standards calibration either. However, I am interested in knowing if the data is believable. Is the device doing better than generating random numbers. Well, I think the T-test I did suggests it is much better than that.
When I just look at the impedance of the T-Test circuit I used, the nanoVNA shows the impedance to be within an ohm of 25 ohms up to about 170 MHz. To me, this seems quite acceptable even though there may be a 4% error in the measurement. But, as you point out, that is referenced to the load I used to calibrate the VNA.
That load does show 50.0 ohms on a very good BK Precision ohmmeter at DC so there is hope it is pretty accurate, especially at low frequencies. I must say that this ohmmeter is the most precise device I own. I do have some 50-ohm loads that don't measure nearly as well at DC and show it in comparison to the one I use. For example - When I measure a load (two 10-dB pads hooked together) that is 54.3 ohms at DC and the VNA shows the parallel equivalent resistance to be between 54 and 54.5 ohms at all frequencies below 300 MHz, I assume that indicates the VNA is pretty good and my calibration load is pretty good over all of those frequencies. This seems to be better than about 0.6% accuracy to me. I would call this excellent for a hobbyist device. This seems to be a real indication of the accuracy of the measurements near 50 ohms?
I don't really need to know the impedance my amateur radio transmitter is working into to a better accuracy than 4%. In fact, I think that precision is far better than the SWR measurement in my amateur radio transceiver. I am sure you know the VSWR measurement is supposed to be 2.0 in this T-Check measurement. The worst VSWR measurement is above 1.94 at at or below 190 MHz and at or above 1.9 all the way to 300 MHz. So, this suggests fairly good accuracy for amateur purposes, especially for an inexpensive device like this.
I am also impressed that the measurements with the nanoVNA are fairly stable and repeatable. If, today, I check a calibration made yesterday against the same loads or cables used to generate the calibration, I get a very similar answer, often far less than a degree in phase variation and 0.1 dB in an amplitude for an open. Using the same through calibration I get very repeatable results for S21 amplitude and phase as well. I assume that this is mostly testing the VNA device and not the loads or the cable. I am sorry to report that is not true for other hobbyist VNA devices I have tried.
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Bryan, WA5VAH
