> Also incorrect. A case in point on a grand scale follows.
>
> The nuclear carrier USS Carl Vinson CVN-70 had a broadband
> 5-450 mHz local
> area network installed while still in the shipyard.
> Designed by the company
> I worked for as a R&D Broadband Development Manager the
> installing
> contractor had a very detailed set of cable
> specifications.
<Snip>
> We quickly discovered that the installing contractor
> ignored the specs and
> used cheap unbranded RG-11 with what looked to be around
> 75-80% shield
> coverage. After our grabbing a few hours sleep Captain
> Martin was informed
> that the complete cable plant would have to be replaced
> and why. Boy, was he
> pissed! But not at us. A few late phone calls were made
> and we got a few
> more rolls of the good RG-11 on board before getting
> underway. The next few
> days was taken up with having ships working parties
> stringing cable and us
> cutting and connectorizing. Testing network sections at a
> time and then all
> of it we had ZERO ingress, even from the ships 5-10KW HF
> transmitters whose
> feedlines ran in the same cable trays for considerable
> distance.
That all makes sense Carl, and I certainly don't dispute
what you saw, but respectfully this is what gets many of us
into trouble in understanding how systems work.
Without really measuring anything, including or especially
cable leakage, the entire cable and all the connectors on
the cable with a different type of cable and fresh
connectors and a problem went away. All we really know from
that is the entire cable system was replaced and after it
was replaced the problem went away.
We don't really know if the cable was defective, what the
leakage levels were, if it was the connectors, if the shield
was broken or damaged, or if the cable was manufactured to
be junk.
The problem really comes in when we apply that single
situation or even a few unknown situations to every other
case in the world.... like a 1000 foot feedline of unknown
problems to a 30 inch cable in a balun.
Factually Carl I can wind a 1:1 balun out of twisted pair,
with NO shield at all, and not have flux in the core from
differential currents. I can have immeasurable leakage
through a screen I can see through, people do that in screen
rooms every day of the week.
The truth is unless the shield or screen has a slot,
insulated gap, or hole that is a reasonably large fraction
of a wavelength long nothing significant gets through. We
might not want to believe that, but a little careful thought
will show it is true. Many of those screen rooms are single
shield, and they all have holes. They almost all have less
than 50% shield coverage.
I can do the same thing with coax, or screens in amplifiers.
If you look at the relay switching box for Beverages here,
the wiring is all twisted pairs of enameled wiring. It isn't
even shielded coax, yet the ingress is insignificant. It's
all about equal and opposite currents and close spacing, and
without current balance at both ends of the shield even
twenty shields won't be enough. The current simply can't be
allowed on the outside, and a hole isn't how that happens.
Common mode current is the issue.
73 Tom
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