[CQ-Contest] Re: [Towertalk] More on Lightning protection

[Tom Rauch]

Even lightning doesn't move around as much as this thread.

> I have heard these sorts of anecdotal accounts (including John's) from
> time-to-time. I wonder if all the tall towers are somehow increasing
> the capacitance between the cloud and the ground. Based on the
> relationship Q = CV, the earth-to-ground potential V would drop given
> a fixed charge in the cloud, if the cloud to ground capacitance were
> suddenly increased. Perhaps this is too much of an oversimplification,
> but it does seem to me like a plausible explanation for these
> anecodotal observations. What say folks?

Since the cloud is the primary concentration point of charge, there 
is very little you can do about it except to have something discharge 
the cloud. 

The other "plate" is the entire earth, and the little bit of corona 
you get just leaks off into the air around the points...it never 
makes it back to the cloud.

The capacitance change would be about zero, because in the scheme of 
things you are adding a tiny tiny point on a big blunt surface. 

The problem is better thought of as voltage gradient. Corona and 
perhaps an eventual arc forms when the voltage gradient is high. 
Gradient increases with concentration of an electric field. That is 
why two capacitor plates will have a much lower voltage breakdown 
when they have microscopic points on the plates than when they are 
finely polished. That's also why spark plugs and other intentional 
spark gaps have the lowest voltage breakdown when they have sharp 
points. 

Many points spread out over a large area can form a cloud of ionized 
air, and that cloud of ions can decrease the voltage gradient at each 
tower. That could increase the voltage required to have a breakdown.
 
> I wonder if this could be measured by current sensors placed around
> the various towers in big installations like these?

I've tried to measure current into my insulated 300 foot tower, and 
it is microamperes unless a bolt flashes in the distance. Then it 
ruins the meter, and knocks me on my rump. The very small leakage 
current simply goes to replenish the ion cloud formed at points 
near the top of the tower, it never really makes it back to the 
clouds. Even if it did (without causing a lightning bolt), the amount 
of charge change would be insignificant.

It seems reasonable to me that dozens of towers with big pointy 
antennas and wires spread all around would look somewhat "blunt" as 
an overall picture to the electric field, compared to a single tall 
sharp structure all by itself. That could reduce the voltage gradient 
around the towers, but not between cloud and earth.73, Tom W8JI
W8JI at contesting.com