On 8/20/2012 8:53 AM, iain macdonnell - N6ML wrote:
> How do you "ground" the coax shield at the top of the tower? What do
> you connect it to??
Not a dumb question at all, and it arises because "ground" is the wrong
word, although we commonly use it. The right word is "bond," and the
connection point is the tower. The intent is to limit the potential
difference between the coax and the tower, both of which are conductors.
This matters both for RF and ligntning (and lightning is RF, not DC).
From the point of view of lightning protection, I would run the coax
all the way down the tower, bonding top and bottom as noted, and then
take it to the shack, where I would place the entry panel and lightning
protectors. it's also critically important that the tower have a good
ground system, and that the tower's ground be well bonded to the power
system ground, CATV ground, TELCO ground, the ground for the shack, etc.
An important point. Lightning protection for the BUILDING and stuff in
the building, is provided by all that bonding, including the bonding of
the shield. Lightning protection for the ham gear connected to your
antennas is provided by the Polyphasers, and they do it by limiting the
voltage between the center conductor and the shield. To be effective,
they MUST be close to the equipment they are protecting.
The potential issue with bringing the coax through the attic is that
lightning current below the point where the coax splits off divides
between the tower and the coax. Yes, the path down the tower is shorter,
straighter, and more robust, and thus morel likely to carry most of the
current, but there can be huge currents in lightning, so "less" of
"huge" is still a lot. :)
73, Jim Brown K9YC
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