[TowerTalk] Ground resistance

[Jim Brown]

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