HFSS:a question on antenna resonance and S11 e Z11 results

Hi,
I'm using HFSS...
first of all a question.
If I've an antenna fed by a microstrip and I want to use a lumped port inside the model between the strip and ground what's the dimension of the port?
If I use a waveport at the edge of the model I know I've to take dimensions approx. 4h x 5w but if I use a lumped port inside can I take its dimension just h x w?so just large as the width of strip and the heigth as that of substrate?

Changing the subject, I've simulated an F-antenna which resonates near 2.4 GHz and I've put a lumped port between the feed point and ground.
Then I've measured S11 and Z11.
I expected that the frequency corresponding to the minimum of S11 (the resonance point) should be equivalent to the point where the imaginary part of Z11 goes to zero. Instead the frequency of S11 minimum is 2.485 GHz while the frequency where Im(Z11) goes to zero is approx. 2.50 GHz.
What's wrong? Is a problem of the simulator or I'm in error thinking about the equivalence of the two measures?
Moreover I've found this problem some other times with other models...
Thanks in adv to anyone will help me!
Bye!