Hello, my lab is using a PNA N5225A with electronic calibration kits N4433A and N4692-60001. I am trying to make S-11 measurements in the frequency range 400-1300 MHz for unmatched, electrically small antennas which have will have a very high reflection coefficient at the input (close to, but less than 0 dB). In order to see the S-11 value, I scale the trace output to 0.1 dB/division. After putting my cables in place, I use one of the two above-mentioned electronic calibration kits. Next, I disconnect the e-cal, and connect one of the antennas. Initially, the readings are as I would expect: the S-11 value is very close to 0 dB at the lower end of the frequency range and decreases as frequency increases. However, without touching anything, the entire trace begins to slowly float upwards, to the point where the reading at 400 MHz reaches roughly +0.1 dB. This is of course not physical as the antenna is a passive device. I believe the N5225A is accurate enough to perform these measurements. I have tried using different ports and different cables but have not had any success with this issue. This issue is also present even without connecting the antenna, and simply leaving the end of the cable open after disconnecting the e-cal. Can you help?
Thanks,
Alex
So I tried an experiment:
setup the channel on my N5242B with start/stop frequency of 400-1300 MHz, IFBW of 1 KHz and 801 points. I then used my ECal connected to port 1 test port (i.e no cables) and did a 1-port S-Parameter cal. After the cal was done, I removed the ECal and connected a mechanical open standard directly to port 1. took a single sweep and then used the Math->Normalize function to basically do a S11 -> memory and then a S11/Memory operation. put on 2 markers to search for the max and min with tracking turned on and this is what I had at this point (note that my scale is also set to 0.1 dB/Division):
This next picture was captured 5 minutes later:
I see no perceptible drift in the measurement over time. I will leave this measurement on overnight and post another picture when I come in to the office tomorrow. I would suggest you do the same experiment with your setup. If you still see drift without any cables in the setup (especially in a 30-60 second time period), then there may be something wrong with the hardware in your PNA (for example a loose internal connector). However, if you do this experiment and get the same result as I am getting, then the drift is coming from the cable.