[Hartley118]

I guess that much depends on frequency. When I was involved in the design and manufacture of coax cable for the military and for mobile telecoms, it was always asserted that the signal (normally 100- 3000 MHz) actually travelled in the dielectric, the conductors merely acting as boundaries. I guess that model would fit with the maximum possible characteristic impedance being the 377 ohm free space value.

On the other hand, in the audio industry, we regarded the ‘standard’ 600 ohm impedance figure as a quaint and largely irrelevant hangover from the ancient days when the only audio was telephony and their long wires were spaced 9 inches apart in free air. In fact, the most useful impedance model of screened audio cable is a capacitor - easy to deal with.

I suppose that classical ‘textbook’ transmission line theory lies somewhere between these two extremes.

Martin