How to calculate the electrical length of a transmission line by HFSS simulation

e.g. a piece of waveguide with certain length. How to accurately calculate its electrical length by HFSS simulaiton?

Thanks.

Apply two waveports, one to to the end of the waveguide, then simulate.

The phase of S21 will tell you how long the transmission line is, within one wavelength. To find out how many integer number of wavelengths there are, simply plot the electric field and count maxima.

Yes, i figured it out afterwards. Play the physical length until seeing the integer number of 360 degree.

Thanks anyway.