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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Dressing RG6
Date: Tue, 14 May 2024 19:51:38 -0700
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On 5/14/2024 3:08 PM, Jeroen Belleman wrote:
> I've been putting coax inside copper tubes or braids to measure
> and/or reduce the transfer impedance (leakage). I did that to
> measure small signals in a particle accelerator, which typically
> has kicker magnets and RF cavities with kA currents and kV
> voltages nearby.

But you likely weren't ALSO relying on the tubing for it's
mechanical characteristics?

I see no real electrical issues -- a copper pipe is just
a third (for RG6U) or fifth (for RG6'q') shield layer.
But, its (desired!) inflexibility adds other complications
to its deployment.

- it inherently eliminates any "service loop" so the
   endpoints are fixed, spatially, wrt each other.
   So, positioning those points then becomes important
   along with its constraints on their disconnection!
   (imagine having two or more such connections between
   "end devices", potentially orthogonal to each other!)
- it transfers any mechanical stresses encountered
   along its length to the supporting endpoints which
   likely weren't designed with that sort of "mechanical
   load" in mind (you're supposed to have a CABLE connected
   there, not some "cantilevered mass").  A passerby
   bumping it anywhere along its length poses a risk.
- it makes on-site manufacture (by "semi-trained
   installers) more difficult -- e.g., fabricating the
   cable *inside* the tubing (few folks are proficient in
   handling EMT and *that* has much less constraints;
   how much does the inner cable "shrink" with each bend?)

I assume your deployment was "one-off", done by someone
skilled in the practice (or, at least able to evaluate
the quality of their FINISHED work) and likely isolated
from other "traffic"?  You likely wouldn't expect some
"jamoke" to be able to install it correctly?  (dealing with
the consumer market is a whole different set of challenges)

It's also been suggested that "tubing" may not be rigid
enough for my needs -- any impacts might deform it and
leave lasting stresses on the connections.  I will buy some
type K (the thickest wall tubing commonly available, here)
and see how much abuse it can take.

I've been directed to explore plastic solutions as
an alternative.  And, technologies that would allow
some of the critical dimensions to be avoided in
their (on-site) manufacture.  Sadly, ENT isn't (?)
available in such small diameters...  Maybe PVC??

Or, sell "assemblies" which means having to settle on
a small number of configurations and expecting the
market to adapt to them.  :<

> A colleague developed a special low transfer impedance coax
> cable for this sort of application. It had two screens with
> intermediate magnetic shielding. It was unpleasant to work
> with, because part of the magnetic shielding was a steel
> spiral foil tape that was razor sharp. But it worked really
> well.