Richard Fry wrote:
"art" wrote

I would also add that copper/braid itself does not turn into
a dielectric or contain a diode thus it also WILL also
pass a RF current at its centre but of course does NOT radiate.

_____________

art, you really need to buy and read Terman's RADIO ENGINEERS' HANDBOOK
or similar source, instead of relying on your intuition. Terman
provides the following equation for the r-f attenuation of
air-insulated, copper coaxial transmission line:

a = 0.00362 SQRT(f)*(1+ D/d) / D*log(D/d) dB per 1,000 feet

where f = frequency in MHz, D = inner diameter of outer conductor, d =
outer diameter of inner conductor.

Note that the attenuation is the same whether the inner conductor is
solid or tubular. This is the result of "skin effect," which for r-f
frequencies 1.8 MHz and higher confines the r-f current on the inner
conductor from its outer surface to a depth of less than 0.18 mm.


One should be aware that this formula applies only to "large" coaxial
transmission lines, where the skin depth is a small fraction of the
conductor thickness.

It's not like the current is confined in a uniform band of the skin
depth, and zero elsewhere. The skin depth is a convenient mathematical
fiction.. it's the depth at which the current density is 1/e, so you can
calculate things like voltage drop by assuming a uniform current density
in a layer that thick, instead of actually integrating it.

On a smallish round conductor, where the circumference isn't many, many
skin depths, there's a broken assumption in the skin depth formula of an
infinite flat plane. Actually solving for the true AC resistance (or
current distribution) involves elliptic integrals which only have
infinite series solutions.


Which is why there are nifty tables and empirical formulas for AC
resistance of round conductors (solid and tubular) that get you
arbitrarily close. See, e.g., NBS Circular 75 or Grover or Reference
Data for Radio Engineers.


Lest you think I am nit picking here.. take a piece of venerable RG-8
style coax, with the AWG13 inner conductor (0.072" diameter, 1.83 mm).
The skin depth at 1.8 MHz (per the above post) is 0.18mm, so the wire is
10 skin depths across, so it's probably a reasonable assumption.

However, let's take something a bit smaller, like RG-8X or RG-58 type
coaxes, which have a inner conductor on the order of 0.9mm. Now, you're
talking only 4-5 skin depths, and the assumption of an infinite plane
probably doesn't hold.


So.. Terman's equation probably holds for coax where the inner conductor
is 20 skin depths, and, as posted, it would make no difference whether
it's a tube (with wall thickness5 skin depth) and a solid conductor.

RF