Ok, if u want to evaluate antenna efficiency, from antenna theory u know that
G=ηD, being G the gain, D the directivity and η the efficiency that keeps into account for losses. If u are using the definition of 'realized gain' in HFSS, u are including all dielectric, ohmic and mismatch losses. Otherwise, the normal gain definition allows to keep into account only for ohmic and dielectric losses. Then, the efficiency is η=G/D. If G=D, ur efficiency is 1 but this is just an ideal case. What I suggest is to create an output variable called 'efficiency' defined as the ratio between the gain (the one u choose) in mag and the directivity in mag along the direction of the global maxima (i.e. broadside).
I.