HFSS hafl wave dipole's imaginary impedance does not reach to 0

I have tried to simulate a half wave dipole with HFSS using PEC. The dipole is 38.75mm long with a small gap (0.4mm) between the two arms to allow a lumped port to be put there. So this should be a half wave at around 1.8GHz.

But the imaginary impedance does not even reach 0 (smaller than 0). How can I determine the input resistance of the anteenna? (I mean usually we determine the input resistance of the antenna by reading the real part when the imaginary part equals to zero)