2 port s-param charectrization of RF trannsformers

Hi,
I trying to understand the characterization procedure for a 2-port characterization of transformer.

The two ports of the transformers are the P+ and S+ side of transformer. The P- and S- side are connected and grounded.


I have come across a formula for the mutual inductance shown in the attached image. Can someone point me to a reference where this formula is derived or explained.

Thanks