Different Port Zo between using 1 mode and 5 modes in HFSS

Hi all,

Just a few questions in HFSS. As I ran Port Only simulation and I have found a suitable wave port dimension (only 1st mode propagates at the frequency of interest) for a microstrip line simulation. Then, by fixing to this optimum port dimension, I ran Port Only simulation again. Now, I set the number of modes in the wave ports to be 1 and 5, and I compared the results from these two settings.

I have found that, for 1 mode, the Zo of the port is equal to 54 Ohm, while for 5 modes, Zo became 50 Ohm ...

How does this difference come from ? And, on which case that I should rely on the result ? I mean, is it necessary to change the number of mode to 1 after obtaining the port dimension that results in no propagation of higher order modes ?

Thanks a lot for every reply,

DYL