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Re: [TowerTalk] Horizontal tower movement at the top

To: W7CE <>
Subject: Re: [TowerTalk] Horizontal tower movement at the top
From: David Gilbert <>
Date: Sun, 06 May 2007 16:04:28 -0700
List-post: <>
I'm not a tower expert, but mechanically that sounds like a very bad 
idea.  If the Rohn sections are spaced out from the tree as you propose, 
the wind against the tower and antenna is likely to put a lateral load 
on the angle iron braces and twist the tower around the tree like a 
corkscrew.  It might not fall down, but you probably will wish it had.

The thing would be much stronger with the tower pinned right up against 
the tree, but then you might as well forget the tower.  Just fabricate 
some steps that suit you and mount those to the tree.   A mast could be 
fastened to the top of the tree for whatever you want to put on it.

Dave   AB7E

W7CE wrote:
> Does anyone know how much horizontal movement is considered acceptable at 
> the top of 100' tall Rohn 25G, 45G and 65G towers?  I don't see any mention 
> of it in the Rohn catalog.  I know that the tops move, I just don't know how 
> much is considered normal.
> I'm considering a rather unusual tower installation at my QTH.  Because of 
> large trees that occasionally lose branches during wind storms and property 
> line constraints, a guyed tower isn't practical.  For years I've had my 3 
> element yagi on top of a 95' Doug Fir tree and it has worked well there.  I 
> watched it during our record setting wind storms last winter and the top of 
> the tree was only moving about 6-7" (less than the diameter of my Ham IV 
> rotator).  This is a big tree with a 42" diameter base and an 8" diameter 
> stump at the 95' level where the antenna is mounted.  The problem with this 
> installation is getting up to the antenna to do maintenance, plus I'm 
> planning on replacing it with a new SteppIR as soon as I figure out how I'm 
> going to mount it.  What I'm thinking about doing is installing 100' of 
> tower as close to the tree as practical (less than 3') and attaching it to 
> the sides of the tree with angle iron at 20' intervals.  With this approach, 
> I don't think the tower could ever come down unless the tree itself did 
> also, which is unlikely based on our soil type and the excellent health of 
> the tree.  I would then mount the antenna on the top of the tower.  This 
> would give me a much easier way to climb up to the top.  I can use 25G, 45G 
> or 65G for the installation, whichever would be best.  25G would be the 
> easiest to install because of it's weight and I suspect that it is the best 
> choice.
> I'm looking for opinions on the viability of this solution.  Will the 
> movement of the tree in the wind stress the tower too much or will it about 
> the same amount of movement that a guyed tower would normally experience in 
> a big wind?  Opinions?  Suggestions?  Am I thinking a little too far outside 
> the box?
> 73,
> Clay  W7CE 
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