attenuation coefficient calculation

Hello, I'm testing CPW these days.
I heard there would be a way to calculate TL attenuation coefficient(alpha) by CST eigenmode solver with periodic boundary condition. The TL is CPW and I'd like to see the attenuation change according to the frequency sweeping. With other solvers I couldn't get an exact alpha due to the port impedance mismatch during frequency change. Thank you in advance.