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Re: Topband: Problem with compression F connectors on Quad RG-6

To: <Topband@contesting.com>
Subject: Re: Topband: Problem with compression F connectors on Quad RG-6
From: "Tom W8JI" <w8ji@w8ji.com>
Reply-to: Tom W8JI <w8ji@w8ji.com>
Date: Tue, 7 May 2013 22:55:09 -0400
List-post: <topband@contesting.com">mailto:topband@contesting.com>
I'm interested in your comment about LMR400 and soldered braids, Tom. I
understand the point about shield current flowing on the insde of the foil
or braid closest to the center conductor, but if the shield connection is
faulty, oxidized, or has high resistance, then it seems to me the outer
braid (presumed to be soldered to the PL259) would carry a potion of the
return current. It would be an interesting physics problem to work out the
relative return currents carried by a coaxial cable with two concentric
shields, each having non-zero resistivity, or a thickness comparable or
thinner than the skin depth..

I don't recall the skin depth at 1.8MHz, but my guess is it's probably
longer than the thickness of the coating on aluminized mylar. I don't know
what the foil thickness is on LMR400.


2 MHz skin depth of copper is .0018 inches.
2 MHz skin depth of aluminum is .0023 inches.

The LMR400 type cable I have is about .006-.008" foil. I chemically stripped the foil off, and the foil was two wraps thick around the cable.

LMR400 or any cable, in sensitive applications, requires a solid bond to the shield that carries the vast majority of return current. In the case of almost all cables on HF and higher, that is the innermost foil. Of course it is different at audio or lower frequencies.

One common connector problem comes from not forcing the woven shield tight against the foil at the connector, or having the foil or woven shield tarnish or corrode. The path to the inside of the foil is out on the braid to an eventual contact point, then back on the outside of the foil to the foil edge. At the edge current can go inside. This is like adding 2X the length of the path to the connection point in overall shield connection path length.

(Current can also "get in" across the edge of a longitudinal seam, if the seam's overlap is insulated. The problem with that is the seam can kill UHF performance.)


If you solder to the shield of LMR400, and put it on a network analyzer and measure the "stub" characteristics, many times (not always) it will move around as the cable is flexed. This is because the soldering heat contracts the dielectric, releasing pressure between the braid overlay and the foil. Now you have a crummy connection that changes electrical length of the connection to the "real" shield.

Even if you do things right, once the foil and braid develop an oxide layer the connection goes away. This can work its way out for several feet of cable length, really messing up a cable. This will not show with a single shield.

Cables with foil have to be installed and treated correctly. The more layers you add, the more careful we must be. Since the extra layers are pretty much meaningless, the best practice is to avoid them. Use a good shield against the center and connect to it at the connector.

I also wonder about the ability of a thin foil shield to carry Amps of RF
without appreciable loss!

RF flows on the surfaces nearest opposing (differential) current flow. You have the small outer surface area of the center conductor carrying the same current as all of that wide area of the foil. The shield has a great deal of surface area compared to the center.


73 Tom
All good topband ops know how to put up a beverage at night.
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