That's why it draws that arc that heads towards the centre of the chart at higher frequencies. At high frequencies the inductor will attenuate the signal and also cause phase lag at port 2 and the phase lag gets worse with increasing frequency and so does the signal loss. In other words how well the transmission signal gets through the inductor to reach port 2 and what the phase shift is at port 2. You have to abandon the concept that the s21 chart is showing impedance because you are plotting the transmission coefficient of s21. 'so why does the Smith chart on S21 start out at 3 o’clock, where R is approaching infinity?'
