[Radio Wrangler]

A piece of 600 Ohm line is still 600 Ohm line, whatever its length.

It looks to its driver like a 600 Ohm resistive load right from the instant it is driven. no need for the delay for a signal to get to the far end and back.

It looks to its termination like a 600 Ohm resistive thevenin/norton source. If accurately terminated, there will be no reflection here.

If you can make a 600 Ohm line good for tens of kilometres, you can make one good for thousands. Only the losses and the overall delay scale up.

The calculation of cross-section dimensions is where things go a bit squiffy when you start pushing to extremes because if you make coax with an almost infinite diameter outer, the C per unit lrngth doesn't approach zero. A standard student exercise is to calculate the capacitance of a lone sphere.

Capacitance to what? I hear. It puzzled me too. Capacitance to everything! Whatever its initial potential with respect to anything unchanging, its potential increases by 1 volt per coulomb divided by its capacitance, if you pump charge onto it.

So infinite spacing can't give you Z tending to infinity. Infinitesimal diameter seems a better bet.

David