Once you get above tree-line on a hilltop it is a completely different
ballgame. Average tree height at my qth is about 50 – 60’. On a heavy wind
day there are some good gusts near the ground with occasional rattling of the
windows and screens. At the 50’-60’ foot tree-line, the trees sway and rock
quite a bit, but the top branches of the bigger trees (80’-90”) begin to
approach a 45 degree angle. On the tower, anything above tree-line becomes a
“wind-sail”.
Many years ago, a heavy pull rope got snagged in the reflector of a yagi at the
120’ level and it hung vertically from there for years. When we had 20 – 30
mph winds near the ground, the rope would fly out and occasionally almost
approach horizontal, like a flag. Attempting to get that pull rope near
horizontal at the ground was almost impossible unless someone held the other
end.
The other issue above tree-line is that there is nothing to protect you from
wind-blown debris. After hurricane Sandy I found a 20’ crown of a tree
embedded at the 80’ level in my 130’ Rohn tower. The 110+ mph hurricane gusts
tore off the top of the tree and blew it ~70’ into my tower where it remained
for five months until removed by a climber!
Bob KQ2M
From: Bill via TowerTalk
Sent: Tuesday, May 16, 2017 12:42 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Winds
My place out in KH6 is an example of how much stronger the wind is up top.
I live on a very steep slope facing the tradewinds, which normally blow at an
average of 12-15 MPH. When it is blowing this hard it is impossible to raise
or lower antennas. So any raising or lowering must be done at lower speed
levels.
Why, when the wind is only 5-6 MPH it is usually closer to 15 MPH at the top of
a 90 foot tower. It is amazing how at the base you feel almost no wind but it
quickly picks up once you get about 20 feet or so. I've had many 200 ft towers
on the mainland but have never experienced such a big difference in surface and
top of the tower speeds.
Being on a steep upslope is the reason, I'm sure.
73 Bill KH7XS
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