The Smith chart doesn't seem to show the same thing. 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. In the Smith chart below, at resonance, 5GHz, it shows S11 as being ( 1 + i0 ). 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. My issue that I'm having is interpreting the Smith chart results. It's a simple band-pass, the resonates at 5GHz. To get the feel of it I used the circuit below as a test, and analyzed it. Just to get some practice I'm using Keysite ADS simulator. I'm trying to become reacquainted with microwave circuits and how to analyze them, especially with a VNA.