John, what with the unwanted interference from Richard Clark, you'll
just have to accept the fact that the ferrite material has little or
no effect on transmission through the short length of transmission
line, be it coax or twin-line, wound on the ferrite core.

I can only repeat that, insofar as the normal transmission current is
concerned, the currents in the pair of wires flow in opposite
directions and therefore neutralise each other. No flux is induced in
the ferrite.

The unwanted longitudinal current flows equally and in the same
direction along both wires and therefore comes under the influence of
the choke.

It is all so beautifully simple!
----
Reg.