On 11/22/21 7:26 AM, Joe WB9SBD wrote:
Hi Jim,
Thanks for the answer.
Yes someone else mentioned about the added "C" from that short piece of coax.
Is there a way to Null out cancel out, calibrate out, whatever so it is just measuring what is between the tiny alligator clips?
You need a short, open, load at the clipleads.? So that's fairly easy - open is just the clipleads in the configuration and position you'll use them. Short is, well, shorted. Load is the only tricky one, if you have a 50 ohm resistor, it's easy. If not, you can try and cliplead to your sma load.
Fixturing on this kind of thing is tough.? You might want to have the capacitor close to, but not connected to, the clip leads (because the C from the lead to the frame of the capacitor).
when measuring I kept everything away from the plates area. and tested on a wooden table.
Joe WB9SBD
On 11/22/2021 9:20 AM, Jim Lux wrote:
On 11/22/21 7:12 AM, Joe WB9SBD wrote:
?I have a NanoVNA-H4
Using it I am trying to measure the capacitance of a large variable Cap.
I have read about as much as I can find, and watched dozens of You-Tube videos.
My Test lead is one of those 6" long pieces of coax that comes with the unit.
It ends in a very short like 1" long leads with micro alligator clips on the ends.
Now i do the whole calibrate thing and all that,
and simply connect the leads on each section of the cap as close together as possible.
when you calibrate, was it "at the clipleads" or did you calibrate "at the connector on the VNA".
If the latter, then you've got the capacitance of the coax and the "fixture" (i.e. the clipleads).
That will make a bigger difference when measuring small capacitances than big ones.
I have two caps, Manufacturer specs say
250 to 40 pf
and
170 to 20 pf
Using the VNA when I measure them the max "C" values are dead on, 249 and 169 pf
But the min's are double what they are supposed to be, the 20 measures at 40
and the 40 measures as 80.
Fixturing error? - although being dead on at the high values and too high at the low values is a bit weird.? Where are the test leads and surroundings relative to the plates?
When you're at minimum C, the plates are "unmeshed" - maybe they're closer to something in that configuration.
Is your capacitor sitting on a table or suspended in air well away from everything?
Not sure why. Or if I am doing something wrong when measuring these caps.
Anyone?
Joe WB9SBD
