This brings up a related question. We lost a lot of towers in southern Haiti
during Hurricane Matthew. Most of them were newly built 100-120 ft Rohn 25
towers with adequate guying and torque brackets but not torque arms.
Here is a question for the tower professional. If you are not using a rotator,
but only some VHF antennas at the top, how important is it to add the torque
arms. We use a baseplate with pier pin so we are allowing for some movement at
the base.
Our tower in Dame Marie was hit by 145 mph blunt force winds and I dont know if
anything would have survived. But several towers folded over, with the guy
anchors and guy wires intact. We are thinking that we need to move up from Rohn
25 just to survive the hurricanes, although the antenna wind load is minimal.
To include torque arms or not, that is the question. (for tower without ham
rotator). Will the oscillations be reduced by the torque arms?
Dale - N3BNA
From: Jim Thomson <jim.thom@telus.net>
To: towertalk@contesting.com
Sent: Saturday, January 7, 2017 2:44 PM
Subject: [TowerTalk] Datasheet for Rohn TA-55
Date: Sat, 7 Jan 2017 11:10:24 -0500
From: Guy Olinger <k2av@contesting.com>
To: john@kk9a.com
Cc: Towertalk <towertalk@contesting.com>
Subject: Re: [TowerTalk] Datasheet for Rohn TA-55
On Fri, Jan 6, 2017 at 5:57 PM, <john@kk9a.com> wrote:
. I remember when K4JA's rotating AB105 tower started twisting
back and forth in heavy wind before breaking apart. I would assume that a
star guyed tower is less susceptible to destructive harmonic oscillation.
Believe this was during hurricane Isabel in 2003. The prevailing yak
afterwards was that if he had cut the lines at the bottom and unpinned the
rotator, let the tower and yagis "windvane", that he would have weathered
Isabel with only minor damages. Maximum sustained winds were only 52 mph
with gusts to 66.
73, Guy K2AV
### How tall was the rotating AB105 tower ? 52 mph wind with gusts to 66 mph
isnt what I would call hurricane force. I call that a real heavy wind.
## On paper it appears that a fixed tower with one or more star guys will
result in
minimal to no torque at all. 2nd best would be a fixed tower with no star
guys, and just the
usual 3 guys per level. 3rd best appears to be a rotating tower, which
provides no torque
reduction at all.
## I just cant fathom a 150-200 ft tall rotating tower with the usual myriad
of yagis on it, in a
100 mph wind. A go pro camera at the base of a rotating tower in high winds,
pointed straight up
would be an eye opener.
## Torque balancing the yagis would go a long way to minimizing the torque on
the tower.
I designed a torque compensating plate for a local fellow years ago, using yagi
stress. Without
the tq comp plate installed, it would eat his T2X rotor. He ripped 2 of em
apart. With the plate
installed, we lucked out, and torque was reduced to almost zero. It was
tested on a temp lower tower,
at the 30 ft level, with just a pair of thrust bearings installed, no rotor,
and no coax. It was free to windmill.
In a 40 mph wind, it did not budge or windmill. He climbed up the 30 ft tower
during the windstorm, and could
rotate the boom by hand, and it would stay put, pointed in the new direction.
We thought that was good enough
and re-installed the yagi on the taller tower. I factored in the coax and
balun etc, plus the offset between the 2 inch
boom, and the 2 inch od mast.
## dunno why yagi makers dont include a TQ comp plate with their offerings,
its essentially a non cost item.
Done right, it would take a huge load off the tower. If yagis mounted on a
mast, I also alternate sides of the mast.
Jim VE7RF
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