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Thanks Roger and Owen,

Remember that the S11 formula from
and NanoVNA Saver (and my own
Excel spreadsheets) already takes away the 50ohm (Zload) of port 2.
The S11 2-port series through measurement (also described by k6jca,
NaonoVNA Saver and earlier link), is always close to the S11 1-port
measurement.

My calibration plane is at the end of the middle/earth tips of the PCB SMA
connector (also my calibration kit is at these tips:
). I solder the
DUT directly to these pins aka calibration plane (so no real extension of
the plane). This all to minimize variability of reference plane
capacitance/etc.

In my case Zload of port2 is very close to on average 50.8-0.1i ohm (300kHz
to 30MHz), which is based on a calibration load of 49.7 ohm (using DVM,
accuracy 1.5%). As my Calibration-load is a little smaller than 50~0iohm,
the resistances measured by the NanoVNA will be a little too high. So my
Zload comes closer to 50ohm. Anyway I compensate S21 for that Zload.
In my case Zsource is significantly different (on average 43.6+3.2iohm)
from 50ohm. So I compensate S21 for that (using Owen's method:
), S11 is not affected by this Zsource
as Zsource is calibrated 'out' by Calibration-load.

I put the DUT on a styrofoam underground (to minimize the effects of my
wooden table top's dielectricum). I also put a clip-on ferrite on the port1
or port2 wire, or shortened the port2 cable (from 16cm to 2cm), or put it
as far away from other equipment (the NanoVNA is though always some 16cm
nearby: IMG_1643.jpg). All these changes don't really have an effect on
2port series through measurements of S11 or S21. Only slightly at the end
of my measurement range (30MHz).
The DUT (in 2 port series through) has *certainly* hand effect (aka adding
stray capacitance), so I need to stay way;-). This hand effect is most
prominent on the S21 (not much on the S11) 2-port series through.

If I change the offset delay, this has almost no effect on S21 2port series
through (as expected), but on the S11 2port series through (it shifts the R
and X peaks to higher frequencies). If I put the offset delay to around
-56ps equivalent to 7.5mm, the R&X peaks of S11 map the R&X peaks of S21.
The below screengrab is with a offset delay of -56ps:
[image: afbeelding.png]
I still find it strange that while the S21 changes so much the S11 stays
quite constant. If I understand k6jca (


), he (as some other experts) likes S21 the best (also more stable than
S11...) for high impedances (like for chokes at say 10kohm). This makes it
even more confusing as others now tell me S11 is better;-)
It would be nice to use Benson's Y method; but that needs a S11, S21, S12,
S22 setup (and with that a 12-term calibration). This is not available with
NanoVNA, so that does not help as advice. Perhaps in a few years prices
will go down for such equipment;-)

I will digest k6jca pages in more detail (I recognise some of what he is
telling, looking at my own experiences).

Thanks for all the feedback provided. If I gain more experience, I will
communicate over the group.

All the best,

Victor

Op za 11 jun. 2022 om 20:08 schreef Roger Need via groups.io <sailtamarack=
[email protected]>: