It's... definitely a filter. I can say that much.
Not sure what's up with the 0 dB regions at start and end of the sweep, it's clearly a bug (The waveform behavior doesn't match, it's definitely not ...
@nprofile1q... You have a lot of test equipment, you get it calibrated right? And people who see your stuff sometimes go out and buy the same kind of stuff right? I'd call up your R&S and Keysight reps and say "Hey, I'm wondering about a 20GHz VNA, could you bring one by and show me what you got? If so let me know when so it doesn't conflict with <other vendor> coming over, thanks!" 1/2
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