[TowerTalk] Double shield

[Jim Brown]

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