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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Topband Reflector
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