Personally, at my QTH which is prominent but not quite hill top. I have 2 -
70 foot towers with 14 feet of extended steel mast. They are 150 feet
separated. I also used to have a 60 ft tower with 10 ft of steel mast that
is now down. The trees are generally at the 50 - 55 ft level with a couple
of trees in the 70 - 80 ft level that are about 150 - 200 ft away from any
tower. The ground soil in Vermont is about as bad as it gets for
conductivity. This is significant in my opinion.
I have had 5 known strikes in the 10 years I have had this station. The
only known hit to the top of a tower was the short one that is now down. It
showed top and bottom evidence with the classic vaporizing of coax for about
20 ft away from the base. This tower was ungrounded as it was a 160
vertical with an isolated base. There was no damage to the tower other than
the fried 40M feed at the top for a south facing beam and the vaporized coax
at the bottom.
The second strike that actually hit a tower I believe hit the tower 20 ft
above the ground by striking my 10M yagi fixed to Europe. It fried
virtually all stack matches on the tower but the only vaporized trail was
the 10M coax feed which lost 20 ft or so before its charred end appeared. I
saw no rotor, rotor cable, or control cable damage. When I have observed
top of tower strikes in Texas, it was common for the entire length of
control cable to melt to the tower or vaporize completely.
The other 3 known hits have been proximity hits that either hit trees or the
ground and couple significant energy to coax and control cables resulting in
minor damage in the shack (blown diodes etc).
My experience has been that strikes are very random and do not hit the
eyeball reference of the highest point, at all. And amazingly are as likely
to strike a nearby tree or the ground as my towers. I think if my towers
were 200 footers or something, that would be a different kettle of fish. Or
if they were in open fields without trees for a mile. But in many areas,
the towers being 10 - 30 feet above the averaged tree canopy is not uncommon
and I think we are kidding ourselves on what is going to be more likely to
get hit in these circumstances. Personally speaking.
One last thing I have noticed living in Florida, Texas, and Vermont. The
occurrence of cloud to ground lightning in Texas and Florida is WAY more
frequent than I observe in Vermont. Cloud to cloud is the norm here.
Ground strikes the minority. I remember feeling the opposite in Texas and
Florida. My suspicion is that it's a combination of storm intensity and
ground resistivity. I think that cloud to cloud charge release is much more
attract in Vermont than in states with much better average ground efficiency
than here.
Fascinating and extremely important topic for all of us.
Ed N1UR
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|