[TowerTalk] Ground system design, RF vs AC

[Jim Brown]

On Fri, 25 Feb 2005 14:40:36 -0800 (PST), Martin AA6E wrote:

>Potentially (no pun), that's a good thing
>if you want an isolated system.

You do NOT want an isolated ground system in the sense you seem to 
be thinking of. All grounds MUST be bonded together. The manner in 
which you tie them together is important. Remember -- wire has 
inductance, and the peak of energy in lightning is in the AM 
broadcast band. Wire SIZE does not matter (except to the extent that 
bigger wire takes slightly longer to vaporize when lightning hits 
it, and thus might last long enough to discharge your hit, depending 
on how close and how big). INDUCTANCE is the only thing that counts 
in determining the impedance to earth. Dale, WA9ENA, who is an EMC 
engineer at Collins, gave some excellent advice on the voltage 
gradients established in "ground" under lightning hit conditions. 

See also my White Paper on Power and Grounding to understand what an 
isolated ground system really is, why it is good, and why what you 
are thinking of is bad. 

http://audiosystemsgroup.com/SurgeXPowerGround.pdf

Also see some of the references in the paper, especially Morrison 
(Grounding and Shielding in Facilities), the IEEE Green Book and 
IEEE Emerald Book. 

And I fully subscribe to Tom Rauch's advice on facilities grounding. 

Jim Brown  K9YC