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Uncertainly of 3.5 mm cal kits.

Question asked by drkirkby on Aug 12, 2014
I've been looking at the uncertainty specifications of amplitude and phase in the manual for my VNA. There are different graphs and tables based on the cal kit used. The uncertainly is lower for the standard 85052B cal kit than the economy 85052D. I can understand this at frequencies where the sliding load is used, but at low frequencies, below 2 GHz the sliding load can't be used. So why should one kit give a lower uncertainty than the other? 

One possibility I thought of is that  older versions of the 85052B used a lowband load, which perhaps had a better spec than the broadband load of the economy 85052D. 

The data attached is taken from the 8720D manual.