This question has been partially answered in another document, but as a quick summary, of why the coefficients of a short open load thru (SOLT) calibration kit are so important to make even reasonably accurate S-parameter measurements. Some people think they can get away without knowing the coefficients if they only want magnitude information (attenuation in dB or VSWR), but this is not so.

- Even if the VNA was perfect, which they are not, the phase shift down the connecting cables means the VNA can not measure a device under test even remotely accurately. The electrical length of the connecting cables changes the phase. A true impedance of 20 + j 30 Ω may well be shown as 800 - j 40 Ω.
- The VNA derives a set of correction coefficients, from measurement of calibration standards. For this a short, open, load and thru are needed, which is why the calibration kits are often called SOLT (short, open, load and thru). Rohde & Schwarz however use the word
*match*rather than load. So what most manufacturer call SOLT calibration, Rohde & Schwarz call it TOSM (thru, open, short, match). The method used for calculating these corrections is not something the user of a VNA normally needs to consider. The method is non-trivual, but is given in a number of references, such as the book Handbook of Microwave Component Measurements: with Advanced VNA Techniques by Dr. Joel P. Dunsmore, who works at Keysight on the development of VNAs. (Joel is a regular contributor to the Keysight VNA forums, using the username Dr_Joel). - If the coefficients of the short, open, load or thru standards are not properly known, the error correction coefficients will not be computed properly.
- Unless the correction coefficients are computed correctly, the results are meaningless.

For all devices, both the real and imaginary parts of impedance are measured. All error correction is performed as vectors. This is why, if using a VNA, a good calibration kit is needed, even if only scalar measurements are made.