The definitive book on this is by Tsaliovich -- "Cable Shielding For
Electromagnetic
Compatibility."
FWIW -- lots of the high futility folks use double shields and quad shielded
cable because
they are trying to make RF cable do double duty for baseband video, and the
quad shield
simply provides lower resistance (and thus lower loss) at low frequencies. And
often they
don't KNOW why these cables work better, but that's why. :)
Many years ago I worked on MATV systems in high rise buildings along Chicago's
Lake
Shore Drive, in the shadow of the transmitters for all the local TV stations
those systems
were carrying. There, cable shielding was VERY important -- that difference
between 60
dB and 80 or 90 dB of shielding made the difference between serious ghosting or
a
relatively clean signal.
Like any other engineering spec, it all depends on your application and
understanding
what the higher spec buys you IN YOUR APPLICATION. That's why there are a
hundred or
so different coax cables in the Belden book. Of course LMR400 is serious
overkill at 40
meters -- I wouldn't use it there unless I happened to have 3,000 feet in my
garage that I
bought for $100 a spool. A run of the mill RG-8 or RG-11 is fine for a full KW,
and any
decent RG-59 or RG-8X is fine for a few hundred watts. In fact, the smaller
cables are
better -- there will be less sag and the antenna will be higher! And if you
think that
difference is small, so is the difference in loss.
Jim Brown K9YC
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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