[TowerTalk] Grounding mast?

Roger (K8RI) K8RI-on-TowerTalk at tm.net
Wed Jul 30 13:17:20 EDT 2008

jim Jarvis wrote:
> Been following the SteppIR v. Stepper.IT thread while travelling.
> Had a few minutes to browse both websites, this morning,
> and came across a short commentary about lightning and ESD protection  
> for the steppIR.
> It raises the question of static charge buildup due to wind motion,  
> in (one assumes)
Static charges build up in ran, snow, or just a dry wind.
> drier climates.   Points out lack of conductivity across rotors,  and  
> asserts 300-500V per meter
This is highly unlikely. Measure the resistance from top to bottom of a 
rotator just sitting there.  It should appear as zero or close to it. 
With the rotator supporting weight as it is designed to do the contact 
resistance is even lower.  OTOH a 100K will be enough to bleed off the 
slow static charge build up. There is so little current during the 
static charge build up it takes little to bleed it off.

As to the lightning, if the rotator is down in the tower it's pretty 
well protected.
At an average of 3 *verified* direct strikes per year over the last 6 
years (give or take a tad) the only damage to any thing on the tower has 
been to the coatings on the coax connectors with the exception of the 
repeater antenna (vertical on top of everything) which was prior to the 
installation of the present ground system.

Lightning will *GENERALLY* take the shortest route which is going to be 
from the mast to the tower at the top sleeve or bearing.  The thrust 
bearing shouldn't be supporting any weight so at most I'd lose the 
ability to clamp the mast when taking out the rotator. OTOH there is a 
second one about 5' above the rotator.
I take no special precautions at the top of the tower to protect the 
rotator which is between 12 and 20 feet down inside the tower and figure 
it's pretty safe there.  I also figure that extra work is a waste of 
time. However where lightning is concerned there are no guarantees so if 
putting something around the rotator makes any one feel better, then go 
for it.   If it takes one of the rare "super strikes" it probably 
matters little what any of us have done. 

For reference here  is a shot of the antennas and rotator which I have 
posted a number of times.  The rotator motor can seen be sticking out 
the left side of the tower near the bottom of the photo. 


Roger (K8RI - ARRL Life Member)
N833R (World's oldest Debonair)

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