[TowerTalk] Guyed tower stuff
Michael Lamb
n7ml@imt.net
Fri, 14 Aug 1998 13:55:32 -0600
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From: Kurt Andress[SMTP:ni6w@yagistress.minden.nv.us]
Sent: Friday, August 14, 1998 1:38 AM
To: Michael Lamb
Cc: towertalk
Subject: Re: [TowerTalk] Guyed tower stuff
Thanks Kurt:
I would much rather lose the rotator and some coax than the tower, so maybe I have things the way I should. On one tower I have six stacked C3-D tribanders spaced every 30 feet up the tower. On the other tower I have 3 stacked F-12 EF-340D 40 meter shorty's plus 4 each M2 9 element six meter yagis on 40 foot booms. The M2 antennas are not mechanically balanced like the F12 and do represent a much greater angular torque in a wind storm.
Before I put the six meter antennas up I survived a 120 mph micro burst that lasted for about 20 minutes. The towers are guyed every 30 feet with 3900# Phyllistran up to the top guy set that is 6800# as spec'ed by Dick Webber, k5ua.
By the way, I am using the HDR-300 rotator and it gets hung up in wind storms
with just not quite enough uuuumph, but that has saved me more than once when lazy vee antennas have hung up in the yagi elements. I look at the slightly weak rotators as a safety device. I have heard too many stories about prop pitch motors that spun around for eight hours before being discovered.
73/Mike, N7ML
Hi Mike,
Don't lay awake at night worrying about this one! You wisely chose 55G, which is a really strong section. Here's my 10 minute explanation on your tower.
Your tower is going to behave just like a non-rotating tower, buried in concrete, with one exception. That exception is that the rotating guy rings prevent the tower from getting any anti-torque support from the conventional torque bars.
This has been a point of contention from John, W0UN, regarding how effective the torque bars can be. John is right (IMHO), that the torque bars can't and don't do anything until the tower rotates to some point where the guy is able to exert a resisting force on it. I believe that the torque
bars are effective, and empirical experience from others supports this.
The op hits the antenna rotator switch, or the wind blows at the right angle. The tower starts to wind up. At 1 Deg of rotation not much happens. At 2,3,4,5,6...degrees of deflection the guy has more beneficial influence. John mentioned that his experience at K0RF indicated something on the
order of 45 deg of tower rotation at the top, for a non torque bar installation, with a full sized 40 meter antenna.
I'll throw in a note here that the "W" braced Triex tower will also wind up. Since all of the Triex bracing is diagonal, it will simply wind up less. Glenn, said as much in his post.
So, my opinion is that any torque bars are going to result in less tower rotation than no torque bars. Even if the absolue difference is 45 Deg to 30 Deg. This will result in lower torsional loads at the tower base. That's usually where the rubber meets the road, or the steel meets (or hits)
the dirt.
Back to Mikes question. The rotating tower will wind up freely, generating the max torque at the base, or rotator. If the offset rotator drive system is weaker than the tower (for torque loads), then something will fail in the drive and save the tower (my choice, easier to fix). If the
rotator and drive are stronger than the tower section, then Mike gets to call the crane company to help him fix it, or pick up the pieces.
It may be that, the loads on this tower never approach failure of either the drive or tower components. Since, guessing is popular, I'd guess that 70-80 Mph winds are not a problem with this installation. 90-120 Mph would make me really want to think about it!
If I knew what was on the tower, I' know more about how much to worry! Stacked 30 SqFt 80's result in more sleepless nights than stacked 6 SqFt tribanders.
Oh, BTW, forgot to mention the difference between tower torque caused by antenna (or tower) rotation and tower compression. They are two completely different sources for tower torque.
Antenna/tower rotation torque loads are developed by the acceleration of the rotating device and the antenna mass.
The tower torque caused by the vertical components of the guy reactions are defined by the wind speed and associated antenna & tower loads. They are only caused by the tower bracing configuration.
My info says 55G doesn't come down very often, unless the trees that Mike doesn't have, take out the guys, or overload them to to simply pull the tower over and fail the sections in lateral bending, as in K8DX's case.
73, Kurt
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