On Sun, Mar 21, 2021 at 05:56 PM, jmr wrote:
I tried to repeat Roger's test of a 'Hi Q' 470pF cap with my nanoVNA and
managed to see an indicated Q that was very noisy (as expected) below 10MHz.
At 10MHz it was probably averaging a indicated Q of 2000 with the nanovna. By
20MHz the indicated Q dipped below 800 and the trace was still very noisy but
it was easier to spot the average.
Manfred's plots of the 2000pF 'Hi Q' capacitor (on the EFHW transformer
thread) obviously had a problem somewhere with the test setup or maybe his
2000pF capacitor was faulty? I'd expect the nanoVNA to perform better than
that across the HF bands if given a fighting chance with a decent 2000pF test
capacitor. Obviously, the nanoVNA can't measure the Q of the 2000pF cap down
at lower frequencies but it should perform reasonably well across the HF bands
for example.
In Roger's case, a decent 470pF cap is probably best tested across 5MHz to
20MHz. Below this the nanoVNA will increasingly limit the measurement and it
becomes meaningless to even try measuring the Q of the cap much below 5MHz
unless it is not a high Q cap.
It turns out the old cap in my parts bin that was marked HiQ was not a high Q cap. I found out afterwards doing a Google search that HiQ was a trademark of AeroVox corporation some years ago for their ceramic capacitors. This particular cap was a large (3/4") ceramic disc cap with a high voltage rating. Not much of a high frequency performer as it turns out but the test still showed that it could not give the same results as my DE-5000 LCR meter at 100 kHz.
I don't believe that testing a "real high Q" 470 pF cap from 5 to 20 MHz. on a NanoVNA will work very well. The reactance at 5 MHz. is 67 ohms and at 20 MHz. 17 ohms ohms. Even to measure a modest Q of 300 the capacitor ESR will have to be .223 ohms for 5 MHz. and .057 ohms for 20 MHz. At 5 MHz. the results will be marginal and at 20 MHz. not good at all. But if someone has results that show otherwise I would be interested in seeing them.
Roger
Roger Need