The close together is to minimize any inductance?
The clips are on the stud for the stators and the frame of the rotors, they are like 1/4" apart from each other. the ceramic in this photo is 3/16" thick.
Joe
WB9SBD
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On 11/22/2021 10:25 AM, Andrew Kurtz via groups.io wrote:
I have had good success measuring C of variable capacitors doing what you describe, except I don¡¯t understand ¡°connecting the leads on each section as close together as possible.¡± Just to fill in a few things you didn¡¯t mention:
- I get the C value from the Smith chart trace.
- I use S11 (reflection) involving only CH0.
- I use a range of about 0.5 to 5 MHz, and believe the most accurate C is pretty low in that range, although C is quite stable and almost constant.
- Any range I use for any measurement is also the range I used for calibration.
One unlikely possibility is that you have approached self-resonance at the lower C so you are seeing a parasitic inductance that gets combined with C. For these sorts of measurements I always have one of the other traces tracking reactance X so I can see how close I am to X > 0¡
Andy
On Nov 22, 2021, at 10:12 AM, Joe WB9SBD<nss@...> 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.
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.
Not sure why. Or if I am doing something wrong when measuring these caps.
Anyone?
Joe WB9SBD
