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Hi qrp.ddc and Rune
I think it is time to get this discussion straightened out..
Doing a simulation of an SMA female adaptor with e.g. the program FEMM and by basing the simulation on mechanical dimension/drawings from a well know manufacturer e.g. Rosenberger, the Fringe C of the female adaptor is 32.24fF equal to an one way offset of 1.612ps. If the designer of the NanoVNA is using the fringe C of 50fF as a delay=2x the Offset then it correspond to a Fringe C of 25fF = an offset of 1.25ps and not that bad
I have just measured these parameter using my HP85033C 3.5mm calibration kit and a good VNA and the SMA female had an one way offset of 1.03ps=20.6fF. Calibration is of course pending the state of the calibration kit which has served my for some years and provide superb calibration when doing T-Check and sweep of my APC7 airlines
Measuring the Open adaptor supplied with my NanoVNA (in the center of image) the one way offset was 1.45ps=29fF an increase of 0.42ps=8.4fF
Measuring an open adaptor from Fairview Microwave or Pasternak (lower part of Image) no increase registered compared to the female SMA left unterminated
Measuring 3 pc. Huber Suhner Open adaptors (top of the image), which has a disk fitted towards the calibration plane, had a one way offset as 2.16 +-0.1ps equal to 43.2fF average.
These Offset are pending the mechanical design as shown in attached image. The open adaptor supplied with the NanoVNA is in the center and as the cylinder mating the reference plane has same inner diameter as the inner diameter of the mating ring of the calibration plane. then it will interact with the field lines form the center conductor.
These field line does not range outside the end of the SMA female adaptor so that is why the Fairview Microwave /Pasternak Open adaptor/Dustcover has not influence. (I have tested to 8GHz and the impact is hard to detect)
An open adaptor from Amphenol RF having a disk (like the Huber Suhner) even caused a short when using my torque wrench.
So I plainly love to use nothing as Rune mention, but the big question is what the designer of the Firmware for NanoVNA has defined the referred 50fF (I have even seen 60fF mentioned) to be an offset = one way or a delay = 2x Offset.
It also depend what the NanoVNA user gets delivered or purchase later on. If it is one with a disk, then throw it away
Regarding the Short which is supposed to be 0ps delay/offset I measured the supplied short to be -0.65ps (yes negative) whereas my simular short from Farview Microwave measured -0.1ps. The reason being the insufficient thickness of the shorting disk and the turned center pin for the one delivered has a small chamfer.
All in all clarification of the (bloody) 50/60fF would be nice
That was my humble contribution
Kind regards
Kurt OZ7OU (calibration nerd)



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Fra: [email protected] <[email protected]> P? vegne af qrp.ddc@...
Sendt: 18. september 2019 12:11
Til: [email protected]
Emne: Re: [nanovna-users] Filter measurement

Rune Broberg, the problem with "nothing connected to CH0" case is that it has different geometry configuration than usual SMA connector. It leads to insufficient capacitance and different delay line, which leads to incorrect measurement. But it depends on what you needs. If you're work in 1-30 MHz range, you may keep "nothing connected to CH0" it will not affect result much. But if you want to use frequencies above 30 MHz, you're needs to use proper open terminator.