Re: [TowerTalk] Ground Radials Insulated or Not

[Jim Lux <jimlux@earthlink.net>]

At 08:18 AM 12/6/2004 -0800, Michael Tope wrote:
> From a performance point of view, I suspect it doesn't make
> any difference at all whether the wires are insulated or not.
> One consideration that I haven't heard discussed with regard
> to radials, however,  is lightning protection. Wouldn't it be better
> from a lightning perspective to have at least some of the radials
> in the system uninsulated? Perhaps a few heavy radials (6 to 8)
> made from #4 bare copper interspersed with the balance of the
> radial system made from whatever is cheapest. Just a thought.
>
> Mike, W4EF......................................

One might argue that you want to keep the "RF grounding" function of the 
antenna (which, after all, is connected to the coax shield) separate from 
the "lightning current discharge" function.

The junction from antenna/tower/whathaveyou to the lightning ground is 
going to rise in voltage pretty substantially, regardless of how good the 
ground is. A notional 5 ohm ground with 10kA lightning stroke current is 
going to go to 50 kV.

I'll have to look for the details, but I seem to recall that one of the 
analyses that NEC-4/NEC-3 were designed for was to look at whether 
lightning grounds should be combined with RF grounds (particularly radial 
fields).  I do recall that they came up with the surprising finding that 
with elevated radials, the performance with the radial field connected to 
the ground stake was worse than with them separate.

Jim, W6RMK 

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