Generally speaking, and you may be aware of this, the impedance of an Antennae is best measured with network analyzer, displaying the smith chart. (IMHO)
Other methods often suffice, such as using a signal generator, directional bridge and and Spectrum analyzer. (vswr or return loss - here you look for resonance/tuning)
The antennae will only have a pure Resistance at resonance (best case), and even this may be quite narrow band, Other wise the resistance will actually be a complex impedance R +/- J.
Due the nature of antennae and RF, and the measurement thereof, a virtual analysis may not be yield real results unless your models are quite accurate.
