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Re: [TowerTalk] Double shield

To: "" <>
Subject: Re: [TowerTalk] Double shield
From: "Jim Brown" <>
Reply-to: Jim Brown <>
Date: Tue, 12 Oct 2004 23:56:40 -0500
List-post: <>
The definitive book on this is by Tsaliovich -- "Cable Shielding For 

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 
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 
relatively clean signal. 

Like any other engineering spec, it all depends on your application and 
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:  for "Self Supporting Towers", "Wireless Weather 
Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
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