Problem with VNA calibration

The VNA I was trying to calibrate is with N-type output port and I added an N-type to SMA adaptor so the cal kit can be applied. What I am trying to do is a SOL full one-port calibration over the band of 2.3-2.6GHz.

After I completed the cablibration, I put the O and S cal kit back to the port that had been calibrated and found the S11 of open and short are not at 0 and 180 degrees on the Smith Chart. However, they are at -90 and -270 degrees accordingly, which means rotating clockwise.

Another thing is when I did the same calibration over the band of 230-260MHz, everything looks ok. The S11 of open and short sits where it should be.

What is the possible reason for this problem?
Can it be because the calibation kit is not good enough? (The kit I am using is not from the manufacturer of the VNA)
Or there is something wrong with the VNA, which needs to be inspected?

Thanks.

Even a bad calibration set should display s11 of +/- 1 when checking the calibration. I guess, you managed to add a port extension to the displayed measurement.

FvM said:

Even a bad calibration set should display s11 of +/- 1 when checking the calibration. I guess, you managed to add a port extension to the displayed measurement.

Click to expand...

Thanks for your reply.

I did not add anything when I checked the calibration. I just put the O and S kit back and found what mentioned above. If the VNA itself has not been calibrated for a long time, will this happen?

Did you leave the VNA in heating ?? If it's not been calibrated, you should leave it at least 2 hours to be heat up..

The VNA has a definition of the calibration kit elements, because the short and open are not perfect. High quality cal kits come with documents that describe these small imperfections, and this can be stored to the VNA.

Check if there is some offset length is programmed in the cal kit definition stored in your VNA.

Most likely is in VNA stored cal.kit electrical length offset wrong, if actual correction parameters not is stored in VNA, as Volker pointed out.
In some VNA's is not the electrical delay set to zero as a part of SOL calibration.
If coaxial cable is replaced or if total length changes with a few mm is it much more visible at 2.4 GHz then at 240 MHz.

For a frequency range of 2.3 to 2.6 GHz, the effect of a port extension or calibration offset delay can be easily seen in the short and open measurements, by the way. It changes the expectable point at +/-1 into an arc segment along the |1| circle.