Mike-
You're exactly right. I put up a lot of microwave towers working for Harris
Corp. The structural analysis had to match the path design. If I recall
right, at 6 GHz, the 3 dB points on a 6-foot diameter dish were around 0.3
degrees. The tower had to hold that dish to +/- 0.3 degrees in a specified
wind speed, eg. 50mph. Those were hell-for-stout towers!
73,
Steve
N6SJ
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
Michael Tope
Sent: Sunday, January 08, 2017 10:37 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] torque arms or not?
I could be wrong, but I suspect one major application for star guying are
towers that support very high-gain microwave antennas that require very
precise and stable beam pointing. You don't want a microwave link to fade
because the tower is twisting in high winds.
73, Mike W4EF.........
On 1/7/2017 12:56 PM, DALE LONG wrote:
> 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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