A coupled transmission line is described by two identical transmission line ABCD-matrices, one for the even mode (or common mode) and one for the odd mode (or differential mode). Because the coupled lines are a symmetrical 3-line system, the matrices are completely independent of each other. Therefore, its Y-parameters write as follows.
The S-parameters (according to the port numbering in fig. 9.12) are as followed .
with the denominator