Network Analyzer - Reading Impedance from Smith Chart

[ArbitraryGenerator]

Hey guys,

I'm new to Network Analyzers and there operation but I can't seem to figure out what it going on here. Before I began my experiment I had to calibrate the port using a short, open, and load. The experiment was just to attached an unknown impedance, run a sweep from 10 MHz to 1 GHz. Then find out what the impedance is. 

So I started out just fine di everything and took a picture of my results seen in pciture one. A couple of hours later I did the same exact thing (everything the same and now it is showing that my results are in figure 2). I made sure everything was the same. You can see that even the mean value has changed. I'm going to contact someone about this I'm just wondering if I'm doing something wrong?

Thanks.

[ASEAN]

Hi,

Did you calibrate again the VNA during your 2nd measurements?

Any temperature variations or changes in the environment, such as electromagnetic interference (EMI)?

How about the standards and connectors you use?  Are they of high-quality components?

How about the torque, did you use the same force as the 1st when connecting the RF cables?

Are you using phase-stable RF cables? Phase-stable cables help ensure that the reference plane remains stable across shifts in cable position after calibration.

All these and other several fators can impact your VNA measurements.

And do check with Agilent support about this VNA, its specs and performance since HP8753A is already discontinued.

http://www.home.agilent.com/en/pd-8753A%3Aepsg%3Apro-pn-8753A/network-analyzer

May I know when was this VNA sent for the Factory (Annual) Calibration?

Regards,

James