When I try to calibrate to 50 Ω the Smith chart says 51 Ω and when I do an open calibration the impedance is off the Smith chart. Is there a problem with my PicoVNA or the calibration file?

The calibration standards are not perfect and do not need to be.

To begin with, they have finite length – so reflections from the short and open must take finite time to travel from the reference plane to the short/open and back to the reference plane. In our case the standards are physically long and the effective length is about 17 mm (SMA standards), or a bit shorter for PC3.5 standards . If in the Enhancements Menu you add Reference Plane shift of about 16 to 17 mm, the Smith chart will display short and open more closely at the short/open points. The explanation is that 17 mm is about 85 ps travel time given a wave velocity of about 5 ps / mm. There and back is therefore 170 ps. At 6 GHz that is about 360 degrees on the Smith chart, so our short and open display as a near full-unity circle on a full-span frequency sweep. After reference plane shift is added, the standards still do not display as a perfect short, open and load. This is because they are still not perfect zero, infinite or matched impedance – they all have finite imperfections – and this is most clearly revealed at higher frequencies.

Calibration works, therefore, through accurate knowledge of the standards' characteristics when tested on a reference VNA that is directly traceable to national standards. Then their length and their impedance imperfections are accurately captured. That accurate knowledge is captured in the .kit file that is supplied with the calibration kit. That file is loaded to the VNA to tell the calibration process exactly what the short, open and load should measure. You can inspect a .kit file with a text editor and will find it to be a concatenation of the s-parameters for each of the four SOLT elements. Full details are given in the NMT Kit User Guide Appendix 2.

The data that we supply with our kits is better than that supplied with most available calibration standards. Typically, full data, unique to serial number, is supplied by others only with premium kits. Competitors tend to use instead only polynomial models, or sometimes only average models on lower-cost kits. We instead supply full and unique data with all our Premium and Standard kits and this allows more accurate calibration.

In particular, note that when polynomial models are used, these do not model the load. This places great need upon the load standard to be very good (and therefore expensive) and for the VNA to assume that it is perfect (having nothing to define otherwise) and to display it as a perfect dot on the Smith chart. This is of course is impossible at any price and is in fact an erroneous display.

So perfect dots on a Smith chart should almost never be expected or trusted. A a perfect dot implies significantly less than 1% loss or reflection and for the short, open and through significantly less than 1° phase delay at the maximum frequency of the sweep. To do that, you would need something that sits wholly inside the test port connector right at its reference plane and to have no physical size of its own!

Above are typical plots of our Premium Female PC3.5 Calibration Standards out to 8.5 GHz.