> > Exactly that. A 100% coverage, perfectly uniform,
> > perfectly conducting shield which is of course not
> > possible in the real world.
> -0-
>
> So, answer me this, oh gurus...if I have a coax with a
> tight braid..98%, which is then covered by aluminum foil,
> how close to perfect is it?
Jim and all.
I ty wrapped two pieces of cheap Radio Shack cable, the kind
you can see white through the braid, together for 30 feet (I
think about that length). On looped from the analyzer to a
load, the other from a load to the analyzer receiver. Using
my network analyzer that has about an 80 dB dynamic range
all by itself and a 20dB amplifier on the source side, I
couldn't detect crosstalk at HF.
I have 2500 feet of F11 cable to some of my receiving
antennas. It has a thin bonded foil and a very sparse
aluminum shield over the foil. I can't hear anything to
speak of on those cables when they are open, shorted, or
terminated at the far end. Even S-meter pinning BC stations
are down just a few dB out of the noise.
We had a CATV/MATV system in apartment buildings next to a
5kW AM 50 kW FM station. The FM antenna was a low gain three
bay circular on the AM tower, so it had a whopping FS a few
hundred feet away at ground level, and the AM was so strong
you could get nasty RF burns off the CATV cables.
The company that owned the system before we inherited it had
installed triple shielded cable with very good shields all
through the system. They gave it up as a lost cause.
I had the system rewired using normal bonded foil and thin
braid cables so we could use bulkhead entrances and follow
the power line cables. We cleaned up the entire system
(hundreds of apartments) just by bonding power line grounds
to cable grounds, and threw all the special cables in the
dumpster. The only exception was two or three TV's that had
ingress right through the cabinets into the internal wiring.
I'm sure we could find some cases where thick double shield
is helpful, but my experience (even with repeaters) is
regular bonded foil or reasonable quality single shields are
good enough. I've had 250 watt RF output repeaters and never
detected any issues using RG-213 as TX to cavity and RX to
cavity cables.
The only time I've seen any issues have been with poor
connections or poor equipment to cable connector grounds.
For example some antenna tuners insulate the cover (by paint
or in one case a manufacturer actually intentionally
insulates with nylon!) from the chassis. This means back
panels have voltage differences on the ground between
connections.
You can definitely change the system when there are ground
loops over the cable shields, but I prefer to fix the
connections rather than mess with common mode impedances.
After 30 years of this (much of it trouble shooting problem
systems other people gave up on) I'm convinced common mode
currents at interface points are a far larger problem than
shield leakage.
73 Tom
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