I cannot refer to HFSS 10 neigher, but in newer versions there is an option in the port properties to renormalize the port impedance.
Anyways, if you tell HFSS to assume a certain impedance at the antenna port, which does not equal the real Z_0 of your design, your simulation might work, but certainly gives you wrong results regarding S11, gain etc.
As far as I know, the magnitude of Z_0 should be matched to the feeding network. Therfore, you should modify your antenna design in order to match your mag(Z_0)=|20-j160|~=161 Ohm.
If you feed your antenna through a microstrip line, you might use a microstrip line imedance calculator (to be found a google) to get an idea about the dimensions necessary to match this port impedance.