fr4-epoxy VS rt-duroid

hi every body
i want to analyse an antenna using hfss 12.

the problem is i want to use fr4-epoxy instead of rt-duriod but all the frequencies and other damn things chang.
would any one help me to chang my antenna to get the same result as i got using rt-duroid?

HI,
Check pls thes topic (frequency versus pcb material) too:
https://www.edaboard.com/viewtopic.p...393&highlight=
Especially Vfone`s proposed side: http://www.fairchildsemi.com/product...ip/fr4loss.pdf
K.