Specifying Solution Settings > Setting Adaptive Analysis Parameters for HFSS       

Use Radiation Boundary on Ports

If the design includes waveports, the Use Radiation Boundary on Ports option is enabled under the Advanced options tab of the Solution Setup dialog box.

• If you select this setting, edges which are assigned to ABC and touch a port have an radiation boundary condition applied during the port solution.

• If you do not select the setting, a perfect conducting boundary condition is used during the port calculations.

In most cases this setting has a limited effect on the overall fields or post processed quantities.