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Just a general thought: We are not running a metrology lab.

Sure, accuracy is worth chasing. But there is a practical limit to what
amateurs can and should expect of affordable measurement instruments.
Monday, yesterday, I was working with another PhD EE and PhD in Plasma
Physics in designing a piece of equipment for a specific purpose. The
comment was made that the latest NANOVNA - the full 3-GHz version - comes
mighty close to challenging the equivalent HP (Agilent.....Keysight)
equipment. We both have an 8753C with S-Parameter Test Sets and HP
precision cal standards. And the cost ratio: maybe 1000:1 or greater.
The NANOs are the best piece of test equipment to add to any ham shack or
professional design effort since the advent of the DMM (Digital MultiMeter).

BTW: The "OPEN" cal standard for RF systems and measurements has been and
remains the most challenging of any "standard". A short or strictly
resistive standard is relatively easy to fashion at RF and microwave
frequencies. The "OPEN" will remain a challenge for metrology labs aimed
at higher RF and microwave frequencies - even for amateurs. Frankly, the
cal standards provided with most of the NANOs compare rather
(embarrassingly) well against the HP (Agilent.....Keysight) standards to
the frequency limits of the instruments.

Dave - W?LEV

On Tue, Nov 23, 2021 at 6:37 PM Roger Need via groups.io <sailtamarack=
[email protected]> wrote:

On Mon, Nov 22, 2021 at 07:06 PM, WB2UAQ wrote:

I agree with Roger that the reactance is very high making it tough to

measure

the min C accurately.

I think you misunderstood me. My comments about high reactance were an
answer to the question posed by Joe WB9SB. I said...

For a air variable R is very small and for a very small capacitance like

8 pF the reactance X is very high under 10 MHz. At >10 MHz. it is
1/(2*pi*8e-12*10e6) = 1,990 ohms and 19,900 ohms at 1 MHz. . So on the
Smith chart when you plot from 1 >to 10 MHz. it is just a fat dot at the
right hand side.

I did not say that the NanoVNA could not measure the minimum C of the air
variable cap. In fact the nanoVNA gives a good estimate as shown in the
screenshot/plot below. The 8 pF is close to the 8.4 pF what I measured
with my DE-5000 LCR meter.

Roger

--
*Dave - W?LEV*
*Just Let Darwin Work*