I'm trying to become reacquainted with microwave circuits and how to analyze them, especially with a VNA. Just to get some practice I'm using Keysite ADS simulator.
To get the feel of it I used the circuit below as a test, and analyzed it. It's a simple band-pass, the resonates at 5GHz.
My issue that I'm having is interpreting the Smith chart results. By my understanding at 5GHz, the input to the filer, S11 should be 0, with maybe a phase change, and also the reflection coefficient should be 0. In the Smith chart below, at resonance, 5GHz, it shows S11 as being ( 1 + i0 ). This doesn't seem to make sense to me.
Looking at the other graphs of S11 and S21, they seem to makes sense, where at 5GHz S21 goes to 0 dB and S11 goes to very low, meaning %100 of the signal from port 1 goes to port 2 as you would expect. The Smith chart doesn't seem to show the same thing. I would expect it to show 0 at resonance.
So what is the meaning of the S11 plot on the smith chart? Or, is the Smith chart not appropriate for plotting S parameters? I figured that with S parameters being complex they would normally be plotted on the smith chart, but what I'm seeing doesn't seem to make sense to me. Or is the issue just with the way that ADS plots S parameters? Would I expect the same plot on an actual VNA on the Smith chart display.
BTW, I'm also including magnitude and phase charts of S11 to show that those reading are behaving as expected.
S11 and S22 below, with marker at 5Ghz resonance.