[TowerTalk] Microbursts and flying branches

[Dick Green]

Wow. We had quite a little thunderstorm here yesterday afternoon.

It didn't look like all that big a storm as I watched it come over the
Connecticut River from Vermont --  just some gray clouds and some cloud
lightning. Since it was still a few miles away, I pulled the coax off the
radios and unplugged the rotor cable. I have an extensive lightning
protection system, but that always makes me feel better. As the trees
started swaying gently in the wind, I decided to lower the tower. But first
I wanted to see how it reacted to somewhat higher gusts than we usually get.
I thought the storm was moving slowly enough that I would have time to do
that.

My 70' U.S. Tower MA-770MDP, cranked down to 50', was hidden by the trees,
so I popped into the next room where I could see the element tips of the
TH-7. By the time I got there the trees were swaying enough that I could see
part of the tower, too. The antenna caught a gust of wind and rocked the
tower back a bit, but nothing out of the ordinary. Suddenly the trees
started doing a hula dance, some bent nearly in half, and my wind speed
alarm went off, indicating a 30 MPH gust. I raced back to the shack and hit
the DOWN switch for the tower. Running back to the next room, I saw the
tower struggling to retract. The positive pulldown worked and the tower was
down to 22' in a minute and a half. I couldn't believe how fast the wind
picked up, even though the nasty part of the storm was still a ways off.
Then the wind *really* picked up, punctuated by massive ground strokes of
lightning and torrential rain. It was a wild scene for 10 minutes or so, but
my wind speed indicator only reached a maximum of 40 MPH. Somehow the storm
seemed more violent than that.

When it was over, I inspected the antenna and tower, relieved to find no
damage at all. Then I glanced over at the 40M 4-square and saw that it had
become a 3-square! A huge, healthy, 20-foot long, 10" diameter limb had torn
off from the 50-foot level of an 80-foot tree, flown through the air about
ten feet and creamed one of the 36' aluminum elements. The element was bent
in half at the bottom of the next-to-lowest section, a piece of 1.875"
diameter drawn aluminum with .058" walls. The bend was right at the
insertion point into the bottom-most 2" diameter section, which was firmly
fixed to a pressure treated 4x4. The tip was touching the ground. Several of
the upper sections were slightly bent in the same direction, indicating that
the limb caught the top part of the element and pulled it over until it
bent. The limb must have bounced pretty hard, because it had pulled back ten
feet from the element back towards the tree, and the end of the branch that
had been attached to the tree was now several feet *behind* the tree.

Only a couple of sections of the element could still telescope all the way
in, and the bottom two sections were badly damaged. Luckily, I had a spare
bottom section (a defective piece that wouldn't allow the next smaller
section to telescope in all the way), so I was able to replace the damaged
one. Also, the next section up, the one that was bent, was quite a bit
longer than it had to be -- about 2/3 of it was nested inside the bottom
section. I was able to cut off the damaged part and reuse the part that had
been nested. Since most of the sections wouldn't telescope anymore, I
couldn't use my favorite technique of mounting the fully-telescoped element
to the 4x4 and extending each section. Instead, I had to put the bottom of
the antenna against the 4x4 and walk it up to vertical. The clamps I use
open to the front, so that worked out better than I had expected. The
element is now slightly bowed, but hardly noticeable. The measurement on the
reused section was close enough that I didn't even have to retune the
element -- the 4-square worked exactly as it had before. It took about an
hour and a half to fix the element, and I had time to cut up the limb with
my chainsaw and toss the debris aside. No radials seem to have been broken.
Whew, just in time for IARU this weekend!

Today's paper reports that the storm contained 60-80 MPH winds. The copper
flashing was ripped off of several buildings, including the town hall of the
next town over. Some gigantic branches were torn off big trees on our local
golf course and a power pole a few hundred feet down the road from my house
broke in half. Severe damage like that was reported all over the local area.
I thought it was interesting that my wind gauge only registered 40 MPH.
Judging by the localization of the severe damage around town, I have to
believe that the storm contained powerful microbursts that only took out
selected targets. I have a feeling that one of them tore off the branch that
hit my 4-square. Either the burst missed the tower, which was only about 100
feet away, or the tower was fully cranked down when the burst passed
through.

A few lessons I learned:

1. Watch the weather forecasts more closely! The morning forecast said
nothing about the severity of this storm, but the afternoon forecast was
accompanied by a severe weather alert. I hadn't seen or heard that forecast
and was just lucky that I was home and had time to lower the tower. I better
get one of those weather radios with the built-in alarm.

2. Don't experiment with high winds. Just lower the tower.

3. The nominal winds in a thunderstorm might not pose a threat to my tower,
but microbursts can. Lower the tower whenever thunderstorms are predicted or
sighted in the area. That probably makes the tower a less attractive target
for lightning, too.

4. Keeping the tower at 50' most of the time, instead of 70', is a good
idea. Even though there was some swaying when the wind got strong, the tower
looked pretty rugged in the face of it. I know it would have been in danger
had it been extended to the full 70'.

5. Be careful about judging the potential of branches to hit antennas. When
I put in the 4-square, I didn't think this particular branch could ever fall
on that element. That's because branch was tilted up and looked shorter and
farther away than it really was.

6. Branches can fly! Even horizontally, the branch did not reach from the
tree to the element. It flew there on the wind, probably helped by the fact
that it was in full leaf.

7. Branches can be very heavy. I'd say this branch weighed at least 500 lbs,
maybe more. If I had a guyed tower, and a branch like that fell on a guy
wire, the tower would be toast.

So, just because it doesn't look like any trees can fall on your tower,
don't forget about flying branches! Clear the trees back as far as you can.

I was lucky. I hope this helps someone avert a disaster.

73, Dick WC1M



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