HELP: HFSS simulation problem

Hi,
I want to simulate an antenna with the absorbing material in HFSS, but I have no idea of setting absorbing material in HFSS. Can anyone give me some suggestion? Besides, I need the absorbing material to be 15dB attenuation. Please??many thanks.

And someone said I could use PML to setup, is it?

absorbing material is really hard to simulate
some papers say that u can do some simplification