[TowerTalk] Tower grounds

[Jim Lux]

On 6/14/12 4:05 PM, Gary Schafer wrote:
> It has often been said, by Polyphaser, that "a good lightning ground also
> makes a good broadcast band antenna ground".
>
> This implies that all of the things that make a good antenna ground are also
> necessary for a good lightning ground including radials, low AC resistance
> and low inductance leads.
>
It's more the reverse of that.. a good broadcast band antenna (500-1600 
kHz) ground makes a good lightning ground.

I wouldn't say "necessary" for a lightning ground .. for example IR loss 
in a lightning ground isn't all that big a deal, as long as it's low 
enough that the grounding conductor doesn't melt.  A fairly simple 
calculation shows that the voltage drop along the grounding conductor 
for lightning is dominated by the inductance, much, much more so than 
the resistance.

For a 30kA strike on a AWG10 wire (0.1 inch diameter)
	about 1 uH/meter inductance
	about 0.003 ohms DC resistance/meter
	 (0.008 square inches cross section)
	Skin depth is (65 microns)2.5 mils so effective cross section
         is about 0.0008 square inches.. 1/10th that for DC
	AC resistance at 1 MHz is 0.03 ohms/meter

di/dt is 30kA/microsecond
Voltage due to inductance is about 30kV/meter
Voltage due to AC resistance is about 1kV/meter.

Going to strap reduces the 1kV, but not the 30kV.



You can have a good lightning ground that is a terrible BC band antenna 
ground.


In the BC antenna case, the inductance doesn't hurt (it's small in 
comparison to that of the antenna itself, and it's lossless, anyway), 
but the resistance hurts a lot.
And, when it comes to amateur bands, except maybe for 160, the 
difference diverges even more.