[HFSS]How to see the impedance of a CPW

Hi guys. I am simulating a CPW with an extra SiO2 layer.
The 2 wave ports are set to touch the two opposite surfaces of the trace.
I first looked my s11 parameter, it's below -20dB, which seems all right.
However, when I checked the z11 plot and found the real part is not 50ohms while the imaginary part is quite large.
This is really confusing because if the impedance mismatches, the s11 should be very large.
Thanks for your help.

I would surmise you're not exciting the correct mode. Can you post your HFSS simulation file or field plots on the ports?

Thanks everyone, I have figured it out.
Because this is a 2 ports network I should use smith chart plot of s11 to see if the real part close to 1 and the imaginary part close to 0.