[TowerTalk] Datasheet for Rohn TA-55

[Jim Thomson]

Date: Sat, 7 Jan 2017 11:10:24 -0500
From: Guy Olinger <k2av at contesting.com>
To: john at kk9a.com
Cc: Towertalk <towertalk at contesting.com>
Subject: Re: [TowerTalk] Datasheet for Rohn TA-55

On Fri, Jan 6, 2017 at 5:57 PM, <john at 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