Sign of S Parameters obtained by an FDTD code

Hello,

I calculate S parameters using an FDTD code. I have monomode excitation and I calculate S-Parameters for more than 1 modes. I see that the |S11| below cutoff frequency of each mode is positive and very large. I know that this is due to the fact that each mode, below cutoff, is purely reactive. Are these results correct? How can I "correct" S11 in order to take negative |S11| below cutoff. Everything measured in dBs.

My technique is based on this http://ieeexplore.ieee.org/stamp/sta...number=1710860

Thank you in advance.