On Thu, 04 Feb 2010 09:20:10 +0000, Jeff
wrote:

One final point, you say "It's the separation of internal noise signals
from external wanted signals that matters, same as for anyone using
coax." I suspect that is about the last thing that most people think
about when choosing coax, they normally think about the impedance first,
then the transmission loss, size and cost. I doubt if the "the
separation of internal noise signals from external wanted signals"
crosses their minds at all!!

Hi Jeff,

You will undoubtedly have two camps there. As for those expecting
"the separation of internal noise signals from external wanted signals"
then they will be dissappointed or live happily with illusion; and the
others will, as you say, will select their transmission line (not
solely a coax) on the basis of those qualities they can expect it to
deliver.

The reason why I opened this up to include parallel line is that too
much superstitious quality has been attached to a shield. This has
been tangentially supported by measure of the cable transfer impedance
with the hope of using that to predict shielding efficiency. The
ordinary reader is left with the impression that by focusing on a
shield that the state of shielding is defined at the alter of the
coax. It is not.

"Cable transfer impedance" is measured in a highly defined manner with
an example of a very good graphic found at:
http://www.emcconsultinginc.com/docs/beldenTiAndSe.pdf

Replace the well grounded coax with a parallel line with its balanced
load and balanced source, and the transfer impedance for that system
will reveal shielding efficiencies easily equal to, or better than,
coaxial cables. That efficiency will vary by the degree to the
proximity of the parallel line to the ground plane, and its geometry.
This geometry is manageable with parallel lines, the coax has to live
with what it has.

Now, this counter argument is based upon the premise of the near
sighted quest for some goal that is achieved by the coaxial line alone
- in other words, a folly. However, what this counter argument does
is penetrate the balloon of complacency surrounding the investment of
superstitious qualities in successive layers of shielding. Without
care, those extra layers can inject MORE noise into the system than
that which exists in the environment. I have already written and
supplied reference to that unfortunate side effect to no obvious
comment about this paradox.

73's
Richard Clark, KB7QHC