I am working on a project using a VNA (old one but enought for my measurements : HP8720B) . But I had observed strange things on the screen on my VNA
I had done a One port calibration, after placed OPEN ,Load, Short kit and computing cal coef my calibration had been done between 130MH and 20GHz ( full bandwith on my VNA !) . After all calibration , I always placed the cal kit ( OPEN,SHORT, LOAD) just to verify if the cal is good. But recently, after a calibration when I placed 3 elements on the VNA and placed VNA in mode Smith Chart : Load is correctly centred, but OPEN or SHORT, I see the same phenomena : a large circle turning on smith chart !!!!?? Normally I must see a small line at left (Imaginary part ~0 )Xaxis ) for the OPEN and a small line at right (Imaginary part ~0 ) for the SHORT
Could you help to understand this?
For your information : My VNA is just coming from Agilent for a full chekup and calibration period. I had discussed on phone with the Agilent technician , the guy was not sure but he said to me that is normal!?
NB: sorry for my english