Calculating Antenna Zin in HFSS

[anz13556]

Hi,

I am simulating a high dielectric planar slot antenna using a lumped port, and I plot the Z11 and get this:This shows that the resistive part is very high. My dielectric constant is 85.2.

Is this the way to get the input impedance? I am trying to find where the resonance is. 
Another method I'm using is to make the port a voltage source and find the current on the port using . Is this a good way to go about it? Please walk me through the best way to find Zin so that I can be certain that I am looking at the right thing.