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Re: [TowerTalk] Balun question shield leakage

To: "jeremy-ca" <>,"Jim Brown" <>,"Tower Talk List" <>
Subject: Re: [TowerTalk] Balun question shield leakage
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Thu, 14 Jun 2007 21:37:10 -0400
List-post: <>
> 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.


> 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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