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OK,
I wandered the group site for a long time and went to the HOW TO on the wiki site, Nothing helped me.

I have the steps for measuring an inductor down solid. And have probably done a hundred by now.

But I now want to measure a variable cap. to learn it's max and min values.

But not having any success on how to do that.

Anyone?

Joe WB9SBD

Hello
One walk around.
Get a known inductor.
Put unknown capacitor in parallell
See what frequency it resonate

C = 1/(L×fxf)

Regards


El jue., 16 de septiembre de 2021 7:38 a. m., Joe WB9SBD <nss@...>
别蝉肠谤颈产颈ó:

If you have successfully measured numerous inductors, you are also set to
measure the variable capacitors. They are both a source port (S11)
measurement. Connect the same as you did for the inductors. Instead of
plotting on the top of the Smith Chart, it will plot on the lower half of
the Smith Chart. Use the cursors to measure at a specific frequency.

Dave - W?LEV

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

I have gotten good measurements on a variable capacitor doing it exactly the same way I measure an inductor. I use only CH0 and measure reflection (S11). I connect the 2 conductors from CH0 to either end of the capacitor. I get a pretty stable result at any frequency from 100 kHz to several MHz. My capacitor (14 - 387 pF) hits self-resonance up around 21 MHz, and then shows inductance, when turned to the low-C side. I get the C value from the CH0 Smith chart.

I did some measurements on a variable capacitor in my junkbox. It was from an old MW receiver.

I measured the closed position on a calibrated DE-5000 LCR meter and it was 367 pF.
In the open position it measured 8.5 pF

I set my NanoVNA-H4 to a stimulus range of 1 to 10 MHz. I used a RG-58 cable with alligator clips on the end. This type of test jig is OK at lower HF frequencies but stray capacitance/inductance affects the results as you go much higher.

A SOL Calibration was done using the alligator clips and a 50 ohm carbon resistor (non inductive). When connecting the alligator clips to the variable capacitor make sure the ground alligator clip goes to the frame of the variable capacitor.

The NanoVNA gets the most accurate reading when |Z| is 50 ohms. In the fully closed position the capacitor measured 365 pF when the magnitude of Z was 50 ohms (bottom of Smith Chart). Only 2 pF difference compared to the DE-5000. Connecting the the NanoVNA to a PC and using the NanoVNA app the measured capacitance from 1 to 10 MHz was plotted. The measured capacitance ranged from 371 pF to 363 pF which is quite close.

Next the variable capacitor was set to the open position. It is not possible to measure at |Z| = 50 ohms because this occurs around 375 MHz.. However the NanoVNA-H4 still gives a reasonable estimate ranging from 8 to 8.3 pF over the 1 to 10 MHz. range which is close to the 8.5 pF measured on the LCR meter.

Experimental results are attached.

Roger

A cheap LCR meter is also a good option for measuring variable capacitors.
The value of most types of capacitors does not vary wildly with measurement
frequency. The value of inductors does, which is why the LCR meter is not
always a good option for measuring them.

On Thu, Sep 16, 2021 at 1:43 PM Roger Need via groups.io <sailtamarack=
[email protected]> wrote:

I did some measurements on a variable capacitor in my junkbox. It was
from an old MW receiver.

I measured the closed position on a calibrated DE-5000 LCR meter and it
was 367 pF.
In the open position it measured 8.5 pF

I set my NanoVNA-H4 to a stimulus range of 1 to 10 MHz. I used a RG-58
cable with alligator clips on the end. This type of test jig is OK at
lower HF frequencies but stray capacitance/inductance affects the results
as you go much higher.

A SOL Calibration was done using the alligator clips and a 50 ohm carbon
resistor (non inductive). When connecting the alligator clips to the
variable capacitor make sure the ground alligator clip goes to the frame of
the variable capacitor.

The NanoVNA gets the most accurate reading when |Z| is 50 ohms. In the
fully closed position the capacitor measured 365 pF when the magnitude of Z
was 50 ohms (bottom of Smith Chart). Only 2 pF difference compared to the
DE-5000. Connecting the the NanoVNA to a PC and using the NanoVNA app the
measured capacitance from 1 to 10 MHz was plotted. The measured
capacitance ranged from 371 pF to 363 pF which is quite close.

Next the variable capacitor was set to the open position. It is not
possible to measure at |Z| = 50 ohms because this occurs around 375 MHz..
However the NanoVNA-H4 still gives a reasonable estimate ranging from 8 to
8.3 pF over the 1 to 10 MHz. range which is close to the 8.5 pF measured on
the LCR meter.

Experimental results are attached.

Roger