The loss of real coax often doesn't fit simplified models for at least
the following reasons:

1. Plated or tinned center conductor, where plating or tinning is
thinner than several skin depths at the lowest frequency of analysis.
2. Roughness of a stranded center conductor.
3. Roughness of a braided shield and the necessity for the current to
migrate from wire bundle to wire bundle.
4. Shield thickness which is less than several skin depths at the lowest
frequency of analysis.
5. Tinned shield.
6. Multiple shields.

Numbers 1, 4, and 5 can be calculated, but require modified Bessel
functions and often some mathematical trickery to prevent truncation or
overflow errors even with extended precision calculation. The remainder
are often empirically determined, are complex functions of frequency,
and vary from one cable type or manufacturer to another. All can be
significant with typical cables at HF.

Roy Lewallen, W7EL