Consider a transmission line with Z0=R+jXL and Y0=G+jXC. The propagation constant and the characteristic impedance are:
γ=Sqrt(Z0Y0)
ZC=Sqrt(Z0\Y0)
Considering the transmission line is symmetric, we regard the two-port as a T-network with Z0/2 on both sides. Then the ABCD parameters are:
A=D=1+(Z0Y0)/2
B=Z0(1+Z0Y0/4)
C=Y0
For a reciprocal two-port Z0 and γ are:
Z0=Sqrt(B/C)=Sqrt(Z0\Y0*(1+Z0Y0))
γ=ln(Sqrt(BC)+Sqrt(AD))=ln(Z0Y0(1+Z0Y0)+1+Z0Y0/2)
But here comes the problem. If ZC=Sqrt(Z0\Y0) then Z0Y0=0 and this means that γ=0. Something is wrong and i don't understand what.