On Thu, 8 Jun 2006 15:17:49 -0500, Stone, Gary R. wrote:
>I could put a cooper plate panel to the right side of the shack
Every piece of wire connecting things together is an inductor that
stands in the way of effective lightning discharge. The longer the
wire, the greater the inductance. When lightning hits, is is quite
likely to arc from one point to another than to follow those long
wires.
The most dirty words in lightning protection are LONG and
INDUCTANCE. More copper plates solve nothing except acting as a
place to bond all the antenna ground connections together, and
those connections are only effective if they are very short.
The ideal architecture is to have the antenna coaxes hit an earth
connection outside the shack, have the power system bond go it
that same point, have the telco arrestor go that that same point,
and have any other lightning protection go to that same point,
have all of those connections be VERY short, and try to keep the
wires connecting them together outside the house.
Unfortunately, we inherit houses and installations of power and
telco where they already are, sometimes on opposite sides of the
house, and the antennas are somewhere in the middle. When that
happens, we need to remind ourselves of what "ideal" is, and try
to get as close to that as possible without tearing the house
down, and pray that a lightning hit thinks our best compromise is
good enough.
Another point. When you tie the coaxes to a common ground bus or
to your RAT switch, the shields are now grounded together. The
Polyphaser or equivalent simply adds protection for whatever is
connected to the center conductor (that is, it shorts the center
to the shield with a lightning strike).
And remember that the nature of coax is to act as a common mode
choke at RF, so when you short the shield to ground, the center
conductor is pretty close to ground too. It isn't a PERFECT common
mode choke, so perhaps 1% of the lightning strike is present
between the center conductor and ground. So if the lightning is 50
kV, the center conductor might be 500 v above the shield. Enough
to blow something connected, but probably not to arc across the
switch.
73,
Jim Brown K9YC
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