[TowerTalk] Lighting

[Tom Rauch]

 Hi Jerry,

I really do understand where you are coming from. I'm just
trying to understand why it would be a popular statement
among Hams.

<<<Your example describes the cloud as already over the
tower. My "gradually and constantly bleeding off the charge"
scenario takes place before that, before the storm arrives.
In my scenario, the tower area rarely gets enough charge
built up to be attractive. When the stormcloud arrives, the
area around the tower is already "discharged", maybe even to
a greater degree than other surrounding areas, so it looks
less attractive to the charge in the cloud. That's what I
call a sort of "prevention", but it's really just making the
tower a less likely target.>>>>

I understand what you are saying, but the voltage gradient
is between the clouds and earth. The only way to discharge
the clouds is to exchange charges between the clouds and
earth. If you think a charge has a tough time moving through
wet soil, why does it move easily through insulating air?

It's already been proven physically impossible for corona
discharges from anti-strike devices to affect cloud charge,
and as quickly as a charge leaves the end of a conductor
another moves in to replace it.

There is one way some reduction in strikes might occur. If
we have a bunch of  horizontal elements it could make a
blunt point. Blunt points spread local charges around
evenly, and that reduces voltage gradient at any small
point. This is why HV air tuning capacitors have polished
rounded edges on plates, and spark plugs have sharp edges in
gap areas. (It's also why plugs need replaced and not
regapped.) The change in voltage breakdown is very
noticeable when comparing a burr or sharp edge to a wide
blunt area.

A second effect could occur. When we look at a distance, we
see a wide azimuth and depth of area. Lightning five miles
this way and ten miles that way all appears as one "area".
When the storm is close enough to be considered overhead,
only lightning within a mile or two would be considered "in
our area".

I face this ALL the time with my wife. She claims lightning
rarely gets near her. Where she goes, lightning does not. If
she wasn't so sweet I'd ask her to sit on my tower in
storms. I can't convince her that when she looks around
close she only counts things in a one mile radius and when
she looks in the distance she counts hundreds of similar
areas, so the percentage always looks much higher far away.

Now if someone claims to live in an area where the storm
clouds part and lightning vanishes storm after storm, I
won't argue that point. But it could be local terrain and
not the tiny insignificant bump and little bit of charge
leaked off by a few insignificant antennas. Rain tends to
not hit Atlanta GA because of the heating effects and
updrafts of all that concrete and tar, or so says the
weatherman on channel 2.

Your scenario has the ground charge moving to a point
beneath the cloud "at will" and then discharging through the
tower because it represents a shorter, easier path.>>>

Of course it does. That's what conductors allow, and the
earth is a wide thick conductor when you consider large
cross sections.

If that were true, all towers would always be an easier
path, and we'd have towers being struck every time a storm
passes overhead... which we know, from observation, doesn't
happen all that much.>>>

Mine gets hit significantly more often than any ONE tree or
pole around this area does. Sveral times a year is the norm.
Of course it is significantly taller than any one tree.

So, how would you account for the well-grounded tower that
doesn't get hit much if at all? >>>

I've never seen that to be true. You wouldn't believe the
pits and holes in things on the top of tall towers caused by
lightning. As a matter of fact, it is well documented as
height goes up percentages of hits goes up. If the tower is
"invisible" because of extra grounding why do power lines
get hit so frequently? They are, after all, grounded to a
much larger lower impedance conductive grid than any of our
towers. Why do trees with higher conductivity get hit more
than trees with lower water content?

I think it is a perspective thing. Our towers aren't all
that tall (as a general rule), they are barely above tree
level. They are one small target out of ten's of thousands
and some other targets are better grounded, taller above
average surroundings, or have sharper points.

When we ground the tower properly we don't change the charge
distribution in a near-static condition. We simply make the
system more immune to damage from local hits.

I will say this in closing. My wife had deer whistles on her
last car. She drove ten years, eventually hit a deer last
year, installed the whistles, and never hit another since.
Sometimes we just can't argue with statistics. The whistles
obviously work.

73 Tom