In general, you will not find good methods to get lumped element approximations to antennae. Lumped element approximations are good for objects less than about a tenth of wavelength, and most antennae are at least 0.5 wavelengths.
For HFSS, you can try the "low bandwidth" option of the "Equivalent Circuit Export" in the "Matrix Data" panel (right click on a solved Setup to get this option). It may work.
If that doesn't work, this thread has some useful info: