aaron_do
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Hi,
I assume this is a two-port simulation with a t-line as the DUT. Your t-line is not 50-ohm. On the smith chart, your plot will trace a circle around some centre-point close to the real axis. This point is the characteristic impedance of your line. Because it is a circle and at DC S11 must be zero (perfect through), the S11 will periodically intersect zero with a period depending on the electrical length of the line. At this point, the imaginary impedance is zero, so you could possibly call it a resonant point, but the point where S11 has its smallest magnitude in dB would also be a resonant point.
If you were to change your port impedances to equal this characteristic impedance, then your S11 would be zero across all frequencies (-infinite dB, assuming lossless).
regards, Aaron
Edit:
Sorry i think i answered too quickly. The center of the circle is not the characterisitic impedance. The characteristic impedance should be the geometric mean of the minimum and maximum resistance......
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