"Ed_G" wrote in
. 192.196:

That's because I added that to the mix. Bruce's comment was a

Ok, well here is a model to shape your thinking and moving the goal
posts.

At frequencies where skin effect is fully developed, and that is a
reasonable assumption for most practical coaxial cables at HF, the
current on the inside surfaace of the outer conductor is equal to but
opposite in direction to the current on the outside surface of the inner
conductor. This is TEM mode propagation.

At the end of the isolated outer conductor, this current must flow
somewhere, and it flows around the end onto the outside surface of the
outer conductor (effectively changing direction as it does so). So, at
that point, the current flowing on the outside of the outer conductor is
exactly equal to the current flowing on the outside of the inner
conductor.

Leaving aside the effects of changing Zo by substitution of coax for
plain conductors:

If you use two coax lines in parallel with the shields isolated, it makes
very little difference, the current that would have flowed on the two
plain conductors now flows on the outer of the coax lines. The common
mode current is the sum of the currents in both coax shields, as it would
be for plain conductors.

If you join the shields together at each end, the sheilds together now
carry the common mode current. A different equivalent circuit, but almost
the same outcome.

Most of these 'shielded solutions' arise from a lack of understanding of
how the coaxial transmission line works in TEM mode.

For example, I saw an ham advise someone that station ground connections
were subject to noise pickup and the best improvement he could make was
to shield the ground lead. In his case, his shack was on the first floor
of the building, and his 7m vertical ground lead to the earth stakes etc
was a source of noise, so he used 7m of RG213 with the shield and inner
bonded to the earth stake and the shield left isolated at the top end.

Firstly, this is not a 'shield' at radio frequencies, but what he did
achieve was to insert a s/c stub in series with his station ground
conductor. The impedance of that series stub at 7.1MHz is 3056.20-
j1509.30 ohms... not a good outcome.

It might have 'fixed' his RF feeback problem, but it didn't improve the
station earth at all, it degraded it severely.

Owen