The ball-park impedance, Zo, of any isolated reasonable length of wire such
as part of a radio antenna, is 600 ohms.


It is first necessary to force yourselves to accept the uncomfortable idea
that any length of wire is a single-wire transmission line.


Zo = 60*(Log(4*Length/dia) - 1) is near enough for most purposes. Jot it in
your notebooks. Sorry I'm unable to provide a reference but you can quote ME
if you like. I found it jotted in MY tattered notebook. ;o)


Even Terman knew that. But perhaps not having sufficient confidence in the
matter he may never have said so explicitly. In which case, unless somebody
can work it out from the information he cribbed from Grover et al, hardly
anybody knows it. ;o)


In the case of a 40 meter dipole of length 20 meters, using 14 awg wire, the
wire Zo = 588 ohms.


To calculate matched-loss in dB/100 feet of single-wire lines would be more
complicated. It is akin to calculating what happens in the case of a
Beverage. It is non-linear versus length.


Cecil's 600 ohms was correct but his description could be confusing. He
need not have mentioned travelling-waves or any other sort of waves because
that depends on whether or not a line or anenna wire is terminated. A
line's termination has no effect on its Zo. Frequency does not enter the
argument. Like any other sort of line it's just a matter of Sqrt(L/C).


A single-wire non-resonant transmission line is used to feed the original
1920's (?) Windom. The line's input impedance is around 600 ohms. It is
correctly terminated by tapping into the resonant antenna at the appropriate
off-center point. The line is of course low loss but it does radiate a bit.
But then, who loses sleep about a bit of radiation from a feedline to a
multi-directional antenna - it's not wasted.

I'm on Chinese vinyards' "Greatwall" white tonight. They have certainly
woken up. ;o)
----
Reg, G4FGQ