How to calculate S-parameters of coupled lines?

I have a question: how to obtain S-parameters of coupled lines without simulation software? In case of stubs and transformers RLC-distributed elements could be used on a Smith chart. What about coupled lines? Is it only real part of impedance, or coupled lines may have complex impedances as well?

It's not easy.
There are 2 sets of s-parameters depending on "even mode" and "odd mode" excitation.
Even more, there is a textbook of Mr. Bhartia about coupled lines and structures."RF and Microwave Coupled-Line Circuits. P.Bhartia,I.Bahl,R.Mongia"