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[TowerTalk] An Engineering Question

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Subject: [TowerTalk] An Engineering Question
From: (Steve Sawyers n0yvy)
Date: Tue, 27 May 1997 20:56:50 -0500
David Clemons wrote:
> Hi,
>         I have a question concerning guy wire anchor placement.  I have
> two towers (one is 83 feet of Rohn 25 with a Hygain 155BA and 105BA, the
> other is 99 feet of Rohn 45 with Cushcraft 204-CD and 402-CD.)  A recent
> edition of the ARRL handbook says that the anchor distance should be
> between 60% and 80% of the tower height.  I have two anchors on each leg,
> with the lower two sets of guys on the inside anchor and the highest guy
> alone on the outside anchor.
>         Now the problem - the 13 acres of property behind me was bought
> last year and the new owners are putting up horse corrals for their
> boarding/training business.  We recently discovered that four of my guy
> anchors (both anchors on one leg of each tower) are actually on their
> property.  They have been nice enough to tell me it's ok, but two of them
> actually fall inside one of the corrals and I think it best to get them
> out of there.  I am not going to move the towers, so that means I have to
> put the anchors closer (the outer ones by 12 feet or so) and also
> probably shorten the towers somewhat.
>         The underlying question really is "how much can I cheat on the
> angle of the guy wires?".  Now there are those among you who will
> correctly tell me that any cheating is dangerous, and I understand that
> concept.  However, I am also aware that there is a different amount of
> danger associated with the anchor at 59% of the tower height than at 39%
> or 29%.  Also, my use of two anchors takes some strain off the anchor for
> the top guys - thus making the issue of the strain on the wire and tower
> legs more likely to be the critical problem.  The guy wire is 1/4 EHS,
> which I believe is rated for 6600 pounds.  The wires are broken up with
> insulators, and some of the wires are fastened with deadends while other
> connections are made using 3 cable clamps on each side.  (If it mattered,
> I could replace all cable clamps with deadends.)
>         Does anyone have any experience or idea of the reliability of an
> installation with the top anchor at 50% of the tower height?  How about
> as little as 40% (about 68% angle to the ground)?
> 73, Dave Clemons K1VUT
> --
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With some decent calculations, I could see going to 65%. 

For 40 and 50 percent you have to get creative. 

The thing that might be worth looking at would be to go to 6 (yes six)
guy anchors for the top of the tower. Bring the existing top guys into
the 50% mark or even 45% (don't know about 40%) and add three more guys
spaced around the tower at that point. Opposing guys would go to the
same leg ie, the new guy anchors would go between the existing guy
anchors on the ground and then go through the tower to the far leg. This
will then have vertical loading going into all three legs more or less
uniformly. Make sure the guy wire doesn't bend a tower rung. 

With the additonal guys to the top , I would investigate lowering the
tension on the 1/4" top guys to about 400 - 450 pounds rather than the
660 pounds normally.

You are going to be working the tower real hard in this application, so
some sound and detailed engineering analysis is in order. You will need
a guy bracket at the top rather than just looping the wire around a leg.
For this one the 'Devil will be in the details'.

With a little antenna modeling and the proper use of insulators, you may
even wind up with an HF Discone antenna using the yagis as a top hat.


de n0yvy steve

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