Negative RLGC parameters for transmission line !!!

Hi

I did some simulation on coplanar transmission lines.
I calculate ABCD parameters from S parameters and the using follwing equations I caculate RLGC parameters of the line

Zc=sqrt (B/C)

Gamma = arccos(A)/l ; l is length of the line

R= real(Gamma*Zc)

G=imag(Gamma/Zc)

when my TL is too long I get the negative values for RLGC parameters!

even for short lines when I use slow wave structurs I get negetive value for G only.

Can any body help me

Many thanke in advance.