Negative real input impedance in CST MWS antenna design

Hi everybody,
I have a problem with antenna design in CST MWS. I get the radiation efficiency larger than 1, something like 1.150 or even 4. !, also I get negative real input impedance at some frequencies. Obviously, I'm doing something wrong. This problem appears only when I use PEC wire or copper wire, and vacuum. When I add some lossy dielectric between, then the results are ok.
I really don't know what to do, please help....
thanks in advance

This could be due to mode convergence failure.
Some times solvers gives unrealistic data when trying to solve the equations.
Extend the circuit may (with a transmissiion line) solve the issue.

Thanks kspalla, I'll try to do that...