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Victor, I notice that in these examples you are using the nanoVNA with the
saver software, connected to a computer via USB cable.

When I do a similar setup, I note that the USB cable to the computer can
significantly affect the measurement at HF frequencies. I found that I
had to use a high quality cable and put ferrite beads on it to decouple
that RF path. You may wish to experiment with that to see if it gives you
better stability in your results.

On Sat, Jun 11, 2022, 4:22 PM Victor Reijs <pe1atn.victor.reijs@...>
wrote:

I forgot to add that the NanoVNA measured Impedance (based on the S21 (or
S11) 2port series through):
[image: afbeelding.png]
matches quite nicely the theoretical simulation of a choke:

[image: afbeelding.png]

<the theoretical simulation got one parameter (stray capacitance of the
choke, not of the testing gear!) which was derived by matching the peak

in

the simulation with the peak in the measured S21 curve>

I think I am more or less understanding what/how is measured with the
NanoVNA. Will try to make a web page out of that.

All the best,

Victor



Op za 11 jun. 2022 om 08:32 schreef Victor Reijs via groups.io
<pe1atn.victor.reijs@...>:

Hello Roger,

Using Series though, the NanoVNA measures both a S11 and S21 (for that
configuration, which I [of course] calibrated). The link I gave (
), calculates the Zdut

using

these Sx1 from the 2port series through measurement. In the case of the
resistive DUTs the S11 measured with 2 port (red blue dotted) is almost
identical with the S11 measured with 1 port (orange dotted). The same I

see

with the toroid DUT.
But I am not able to map the Zdut calculated with the S21 (green

dashed;

measured in the series through 2 port).

So the Zdut (Zx) using with 2port series through and S11 based is

different

from Zdut (Zx) using (the same) 2 port series through and S21 based. So

all

stray capacitances, etc are the same (which I see part of the DUT).

All the best,

Victor

Op za 11 jun. 2022 om 07:26 schreef Roger Need via groups.io
<sailtamarack=
[email protected]>:

Victor,

I am not sure how you are measuring the DUT using S11 reflection and

S21

series methods.

You have to measure them with the correct reference plane calibration

and

this usually requires two different test jigs. What are you using

for

S11

and S21 test jigs? Attached is one I use for S11 up to 30 MHz. I

tack

solder the DUT as shown. My example uses an inductor but you can

solder

the leads to your choke. The second one I built into a box and it

works

well too. I posted my S21 series jig in the last topic

Roger

Hello Roger,

Using Series though, the NanoVNA measures both a S11 and S21 (for that
configuration, which I [of course] calibrated). The link I gave (
), calculates the Zdut using
these Sx1 from the 2port series through measurement. In the case of the
resistive DUTs the S11 measured with 2 port (red blue dotted) is almost
identical with the S11 measured with 1 port (orange dotted). The same I see
with the toroid DUT.
But I am not able to map the Zdut calculated with the S21 (green dashed;
measured in the series through 2 port).

So the Zdut (Zx) using with 2port series through and S11 based is different
from Zdut (Zx) using (the same) 2 port series through and S21 based. So all
stray capacitances, etc are the same (which I see part of the DUT).

All the best,

Victor

Op za 11 jun. 2022 om 07:26 schreef Roger Need via groups.io
<sailtamarack=
[email protected]>:

Victor,

I am not sure how you are measuring the DUT using S11 reflection and S21
series methods.

You have to measure them with the correct reference plane calibration and
this usually requires two different test jigs. What are you using for

S11

and S21 test jigs? Attached is one I use for S11 up to 30 MHz. I tack
solder the DUT as shown. My example uses an inductor but you can solder
the leads to your choke. The second one I built into a box and it works
well too. I posted my S21 series jig in the last topic

Roger

:

Thanks for the comments to spur me on...I earlier felt that I had better
clean up the spreadsheet just to avoid more confusion, but never took the
time. Glad that it's turned out to be of some use. Wasna sure when I posted
it if anyone would be interested.
--
~R~
72/73 de Rich NE1EE
The Dusty Key
On the banks of the Piscataqua

Others have found similar issues using the S21 series method using the
NanoVNA and even with higher performance HP VNA equipment. Here are some
interesting links. The K6JCA experiments find differences similar to what
you have measured.






In my experiments I found that parasitic capacitance had considerable
affect on the S21 measurements and I tried several different test jigs to
see what happened. The issue of where to establish the reference plane
took some consideration. S21 Gain and S21 phase determine the complex
impedance of the DUT and even with some type of source (Port1) and
termination (Port2) correction the phase was often incorrect. Phase
measurement was the problem and in my tests was due to an offset delay
across the jig. Another issue I found was that for some reason high DUT
impedance resulted in common mode current on the surface of the coax cables
connecting the NanoVNA to the test jig. I tried snap-on ferrites and the
results changed.

You mentioned that you could do a S11 reflection and S21 series
measurement at the same time. You will get different results when the DUT
is in series between Port 1 and Port2 compared to when it is across Port 1
alone. Best case is that it will be 50 ohms higher. TIn reality the
complex impedance measured when in series is the DUT plus the input
impedance of the cable attached to Port 2 (which will deviate from 50 ohms
due to Port 2 impedance not being 50 ohms) and any parasitic inductance or
capacitance between the DUT and the Port 2 cable.

I wish you good luck in your quest and hope you find a method that works
for you and that you find a solution that eluded me.

Victor,

I am not sure how you are measuring the DUT using S11 reflection and S21
series methods.

You have to measure them with the correct reference plane calibration and
this usually requires two different test jigs. What are you using for S11
and S21 test jigs? Attached is one I use for S11 up to 30 MHz. I tack
solder the DUT as shown. My example uses an inductor but you can solder
the leads to your choke. The second one I built into a box and it works
well too. I posted my S21 series jig in the last topic

Roger