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S-11 Trace Drifting Upward After Calibration

Question asked by abouv31 on Mar 11, 2019
Latest reply on Mar 26, 2019 by Dr_joel

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?