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Kirkby calibration kit alternatives?
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G0HZU:
FWIW at work today I tried measuring a number of 50R loads from our Agilent N and 3.5mm cal kits. I used a decent Agilent 4 wire bench DMM (calibrated at Agilent in April this year) and the 'worst' load I found in the group test measured 49.97 ohm at DC. The rest were slightly better than this but all very close to 50 ohms as expected.
I also tried calibrating an E5071 VNA with one kit and measuring the resistance of the other Agilent cal kit loads at low frequencies. They all measured within a few tens of milliohms of 50.00 ohm at a few MHz and some were within just a few milliohms of the reference load used for the VNA calibration. Some of our Agilent cal kits must be >15years old so this is a good result I think.
hendorog:

--- Quote from: G0HZU on July 23, 2018, 11:05:34 pm ---Here's an old plot of my Suhner 18GHz SMA load showing the LF performance. This load was hand selected from a large batch of identical Suhner 18GHz loads to be the closest to 50R and I just measured it again this evening (after many years) and it is 49.99R at DC on a Keithley 2000 4W meter.

This plot was taken at work (a few years ago) using an 85033E cal kit and the VNA was a 6GHz 8753ES. It shows very close agreement with 50R all the way up to 200MHz. So this implies the 8753ES/85033E can measure a real 50R quite accurately.

On my VNA here at home (using my N4431B-60006 Ecal to calibrate the VNA)  the same Suhner 18GHz load measures 49.94R at LF. So pretty close to 50R again!
 I also dug out another Suhner 18GHz load and it measured 49.87R on the VNA (with the same Ecal calibration) across about 2-10MHz. On the 4W Keithley this second Suhner load measured 49.89R at DC. So good agreement again I think.

--- End quote ---

This is what I did too when I got my 8753 up and running so I'm glad to hear it makes some sense.

In addition I entered the measured resistance into the VNA cal standard table, instead of just 'Load'.
IIRC that helped the VNA measure the other loads more precisely.

Interestingly the SDR Kits guys use this DC measurement to characterise their loads in their low cost cal kits - you get a little label in the box with the result hand written on it.

Of course this doesn't necessarily mean that it will still be 50 ohms at 6GHz, which is I think what Dr Kirkby was saying.

G0HZU:
I think you also have to correct for the time delay in the SMA load I'm not sure how muddy things get if you were to try and do some precision work with a full 2 port VNA with an uncorrected 51.1R SMA load as the calibration reference. I wouldn't want to use it like that even if it would be OK for many casual tasks. I'm not sure I want to waste the time working out how much it will degrade certain types of measurement. I'd rather source a decent load and keep it simple :)
Mechatrommer:

--- Quote from: hendorog on July 24, 2018, 08:01:07 pm ---Of course this doesn't necessarily mean that it will still be 50 ohms at 6GHz, which is I think what Dr Kirkby was saying.

--- End quote ---
what i suspect he's saying is, albeit the inaccuracy of the load at DC and to some extend at RF, the error should be close to insignificant (for normal Joe's purposes).. but to know exactly, one has to make a comparison test requiring both the inaccurate load and super accurate $$$ load in hand, calibrate VNA with both and make few practical measurements to see how the measurements will be far off. i cant do the test since i cant justifiably afford $2,000 and beyond calibration kit let alone $13,000. so i have to anticipate more ripply measurement than it should. my 2cnts.
Bud:
You do not have to do that elaborate test, and it will not be conclusive anyways because the best commercial calibration loads still have errors. All you need is to know your calibration load's Retun Loss within the frequency range of interest, then you can reference existing charts / tables that show +/- error of measurements for a given Return Loss figure. You can pull these charts from the Internet.
Now the problem is to know your cal load RL, that you need to measure against a decent calibration kit on a VNA or from the manufacturer data.
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