I have an old 85056A that I bought 15 years ago from a company that has never used it since the first day they bought it new from HP. With it I also bought their 8510B 40 GHz that I converted into an 8510C. 20 years ago I had bought an 8510C 26.5 GHz with its 85052B still coming from an other company that was looking to modernize with a PNA.
Up to 26.5 GHz it's possible to do a fairly simple calibration with fixed loads and open and short circuits. But if you want to be precise, it's better to do a full calibration with the same elements plus sliding loads. Then I stocked up on the elements needed for TRL calibrations, but I've never had the courage to use them yet, as they're relatively more complicated.
I also had an 85053A individually calibrated for over $3000 less than 10 years ago at Keysight Roseville (the only place where they know how to do it according to Keysight), so I was able to check several times the difference between with and without sliding loads.
On the other hand, I was able to verify that for 40 GHz, it's really mandatory to use sliding loads because fixed loads aren't good enough to really know what we're doing. That's why I have also one (unfortunately) 2.4mm air line.
I think we should bear in mind that up to 50 GHz this is even truer, and that without sliding loads, without air lines, and without an individually calibrated verification kit (which is almost never the case) to check the result of what we're doing, we can only be in the fog.
For some years now, as I've run out of space, I've stored the two 8510Cs at my country house, and all I have at home is a small (but fantastic) 8720ES.
The 8510C is a slow, impractical instrument, with no computing power, no possibility of storing tons of configurations in internal memory, and no ability to interpolate if the frequency limits are changed. However 8510C is still a very accurate instrument that I was able to compare to several from R&S and HPAK... It's only PNA-X that was better in term of stability along time.
For better verification of all calibration parameters, option 010 is practically mandatory. Fortunately, I have it on all three of my VNAs.
F1EKU.
