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.
I also wonder about the ability of a thin foil shield to carry Amps of RF
without appreciable loss!
73,
Jim W8ZR
> -----Original Message-----
> From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Tom
W8JI
> Sent: Tuesday, May 07, 2013 8:03 AM
> To: Topband@contesting.com
> Subject: Re: Topband: Problem with compression F connectors on Quad RG-6
>
> >
> > What role does skin effect have in this situation - at 1.8 MHz?
> >
>
> Skin depth isolates the outside and inside, virtually nothing passes
through
> a conductor wall that is several skin depths thick. This is true for both
> the magnetic field and the electric field, and why "shielded loops" are a
> misnomer. Look at this link http://www.w8ji.com/skindepth.htm
>
> Current in the center has to be matched by current on the inside of the
> closest shield. Current always can get into the center three ways:
>
> 1.) The shield can be so thin it doesn't isolate the outside and inside,
but
> then that would not be a good shield
>
> 2.) Current spills over an edge
>
> 3.) There is a direct connection to the inside
>
> If we study connectors with an accurate picture of what current has to do
> (current, the magnetic field, nor the electric field can NOT go through a
> wall several skin depths thick), we can spot potential issues with shield
> connections. We don't have to have a connection to the side if there is an
> edge to spill over that is right at the same connection spot.
>
> What we do not want is the edge to be inches from the connection point.
For
> example, a braid overlay on top of foil that has a poor electrical and
> pressure connection to the foil is not a good connection point. This is
why
> LMR400 is problematic when the braid is soldered to a PL-259. The very
same
> cable is great with a crimp connector, if the cable is clean (not
tarnished
> or corroded) inside.
>
> 73 Tom
>
> All good topband ops know fine whiskey is a daylight beverage.
> _________________
> Topband Reflector
All good topband ops know fine whiskey is a daylight beverage.
_________________
Topband Reflector
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