On Sun, 14 Feb 2010 11:14:29 -0800, Jeff Liebermann
wrote:

A 450 ohm coaxial cable would be rather large.
Zo = 450 = 138 log(b/a) (where b=OD and a=ID)
For an inner conductor diameter of 1 mm, the outer shield diameter
would need to be 1800 mm or about 71 inches.

This solution for the formula also reveals the diminishing increase in
Zo for two-line or one line against earth as the separation grows
astronomically.

Let's put a 1mm wire 1000m in the air,
Zo = (138/e^.5) · log(4h/d)
(138/1.667) · log(4000/0.001)
82.8 · log(4000000)
82.8 · 6.6
546 Ohms

73's
Richard Clark, KB7QHC