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Kirkby calibration kit alternatives?
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Andrey_irk:
G0HZU,
If you are concerned about the RL accuracy when measuring low RL values then you should look at the residual directivity value. These are in order of 40-50dB for the most of commercial kits. Keysight, for instance, provide these values for every VNA calibrated with every their kit or ECal (as long as I remmember).
51.1 Ohms gives you these 40dB of RL  at low frequencies and the high frequency values will depend on the parasitics (which are usually much more difficult to make small). And it won't help you if you get a 49.999Ohm resistor as the HF performance won't be affected.
If you measure say 20dB RL DUT with with a VNA calibrated to have 40dB of directivity - it will give you an error of about 0.8-0.9dB, which is pretty good.
At work I have an SMP cal kit, which has a Load standard with (according to specs) 50+-0.25Ohms, but only 25dB RL beyond 4 GHz. And I'm not aware of any commercial kits with better specs for this type of connectors. So, with our 20dB RL device this kit gives us an error of 4-7dB (!).

P.S. I'm not a guru at this, so hope Dr. Kirkby will correct me if I'm wrong.
P.P.S. Is it possible to invite Joel Dunsmore here?
G0HZU:
I think you are missing the point I'm trying to make. DrKirkby said the resistance at LF is irrelevant.
Suppose I wanted to test some 100R resistors across temperature to see how accurate and consistent the resistance was over maybe 0-200MHz.
If I take DrKirkby's advice then it won't matter which Kirkby cal kit I use to measure the complex impedance as long as they all have the same 39dB return loss. So for each test at each temperature I could select a Kirkby cal kit at random (from a box of several) to use and not care if the real part of the calibration load was 48.9R or 51.1R between the kits.

But I'd see errors of > +/-2% for the real part of the impedance even for this simple test at a fixed temperature. I could correct the resistance in the user file for each cal kit  to minimise the error but DrKirkby implies all this is irrelevant.
Bud:
Can't you enter the reference impedance value in the VNA settings and call it a day. Not all VNAs may have that setting changeable though.
G0HZU:
I guess so but I don't know if a VNA would interpret this as a 51.1R resistor with a 50R Zo delay or as a 51.1R resistor with a 51.1R delay.

So it might cause subtle problems when being used up at higher frequencies. I had a rummage at work to try and find an SMA 50R termination as poor as 51.1R at LF but couldn't find one. The other option is to use a homebrew SMA load up to a couple of GHz that is selected to give close to 50R and then use the wideband 51.1R load up at higher frequencies if critical measurements are being done in either case.
Bud:
VNAs should not assume things, unless it is a Chinese one where anything is possible. I'd think the math certainly has delay variable in it but any delays should be specified by the user explicitly.
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