Jim Jarvis wrote:
>So, answer me this, oh gurus...if I have a coax with a
>tight braid..98%, which is then covered by aluminum foil,
>how close to perfect is it?
>
>My experience is, 9913 flex, when used to replace all other coax
>in the station, reduced crosstalk, stray spurs pickup, and
>interaction with house phones. Is there a shield system that's
>superior to 9913F? How does LMR400 stack up? rg213?
>
>I've sampled only a limited number of types, and am truly curious.
>
>N2EA
>jimjarvis@ieee.org
>
>
As a bit of summary: Tom started this discussion off by saying:
"That's why our transmitters can be shielded perfectly with
copper shield or aluminum shields as long as the shield is
several skin depths thick."
Which for ham purposes is true.
Then nit picker Roger added:
"If that perfect shield has a finite thickness, which
it has to have in the real world, there will be an
external field. Actually, to really, really nit-pick,
unless the cable has an infinite diameter!"
Which is also true. Since the skin depth for the RF is proportional to
the square root of 1/F, you can never get to zero. Which means some
amount will penetrate the shield. You can get to something
insignificantly small, which is the case for most of our coax cables,
and I think that was Tom's original point.
Another factor, that is more significant than skin depth, is the amount
of shielding the cable has. Cables that have 85% braid coverage may
have limited use, but for most ham applications 85% shielding is OK.
(85% coverage is poor, most are better than this.)
Consider transmitting: You are going to get some signal penetrating the
shield, but compare this to the amount of signal that is being radiated
by your antenna, or being radiated by your coax because of a not
perfectly unbalanced-to-balanced interface and you can conclude that the
85% is good enough.
Consider receiving: You are going to pick up nearby stuff through your
coax cable, compare this to the amount of extraneous signal picked up by
your antenna. The only place you could have a problem is if your coax
is laying against a noisy line for a long distance.
Of course going to 100% shielding (solid foil shield) can reduce pickup.
A place where you want to use 100% coverage is your cable TV wiring.
You don't want that stuff radiating or picking up your RF. If you wire
your house with RG59, the probability of getting RF into your cable
system is high.
As for your sucess by changing to 9913, I am a bit surprised. I would
have not expected to see any difference. I use RG213 (which is not
100%) for most everything and have seen those kind of problems. It
could be that you replaced some bad coax or bad connectors.
I don't have a dB number to give you on the difference between 85%
coverage and 100% coverage, but from experience, for ham use I don't
think it matters.
Jerry, K4SAV
>
>
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