It should also be noted that, when using manual calibration kits, there is a risk of human error invalidating the calibration results, since manual calibrations can take a long time. In calibration, patience is a virtue: it is worth accepting that the process cannot be rushed, and adopting a methodical approach.
The avoidance of errors in the first attempt at a calibration will save the time that would be wasted on a repeat performance of the process.
At fault: man or machine?
With VNAs, as with almost any field in which technology is applied, the most common source of error is the human user rather than the machine.
Fortunately, the careful observation of the practices described above, combined with the use of high-quality and undamaged components such as cables and connectors, will eliminate all of the common sources of user-generated error, and will lead in almost all cases to many years of trouble-free and accurate measurement with a VNA.
The author, Adam Purkiss holds a BSc and is a Specialist VNA Support Engineer at Anritsu (UK) — www.anritsu.com