(Moderator action: Link above is modification of original link below which appears broken) efaidnbmnnnibpcajpcglclefindmkaj/http://www.ece.uprm.edu/~rafaelr/inel6068/HFSS/HFSS_Antenna_v2015_v1/lectures_trainee/ANSYS_HFSS_Antenna_L05_0_Dynamic_Link.pdf)
Usually you change the LOAD and SOURCE impedances to 500 ohms, then the graphical outputs plot it that way. Each program has a different way to do it though, you have to read the instructions