hello,
When you simulate a physically half wave dipole in free space, you should find a real Z for a frequency somewhat below the half wave frequency (due to capacitive effects). You should expect between 50 and 70 Ohms.
When you put that dipole close to a ground plane (for example 0.05*lambda above it), you will see that the S11 curve travels close to the left edge of the Smith Chart. The ground plane cancels part of the radiation, hence given a certain feed current, the dipole over ground plane radiates less. This results in less real part in the impedance, hence larger S11.
When simulating full wave dipoles, you will notice high resonance impedance just below the full wave frequency. When you place it close above a ground plane, you will find even higher impedance (in the many kOhm range). So the curve travels close to the edge of right side of the Smith chart (around the full wave frequency).
Generally spoken, antennas close and parallel to a ground plane result in strong increase in S11 (or strong increase in return loss) when directly fed from 50 Ohms. You will also notice that the equivalent Q factor of the resonance increases when you reduce the distance to the ground plane, so this makes narrow band antennas.
Try to simulate a thick strip half wave dipole also and note the difference in d(Im(Z)/dfreq with respect to the thin dipole.
In all cases, |S11| < 1. Antennas in air over ground planes simulate well (and fast) when using MoM method. You may find strange results for the radiation pattern when you use infinite dielectric sheets (for example to simulate a patch antenna inclusive the substrate).