Topband: Antennas, Guy Ropes, and High Winds

[Eric Scace K3NA]

   Polyester and many other types of rope are quite stretchy -- and some loose weaves are horrible in this regard.  With the long
lengths involved, it is quite easy for the combination of sag in the guy plus stretch to allow considerable change in the
coordinates of the guy attachment point.  This can put the vertical out of column.  Once far enough out of column, it will crumple
over.  No guy needs to break for this kind of failure.  Changing the size of the polyester rope will not fix the problem -- it will
make it worse because the heavier rope will sag more.  The lighter twine looks "straighter" because it doesn't have the same weight
sag.

   Amarid and kevlar fiber ropes are the least stretchy, but also expensive.  These are used quite a bit in sailboat racing, where
stretchy ropes are usually undesirable (one wants to keep a sail exactly in position against varying wind loads).  Naked versions of
these lines are readily available, but check their UV resistance.  You may be better off with a covered version.  If you do not want
to use metallic guys or fiberglass rod, then use these lines.  This will improve, but not eliminate, the issue of stretch and sag.
Rope guying can not be used at some point, regardless of rope technology, because it will just stretch too much compared to metallic
alternatives.

   Most of the give in metallic guys comes from taking up the slack in the guy.  For structures of typical amateur radio heights,
the stack is small enough not to cause a relatively large cross-section tower to move far enough to come out of column.
Thin-diameter poles are less forgiving: (a) the guys can not be tensioned as high because of the downward load placed on the
vertical tubing; (b) a small displacement caused by taking up the sag is a much larger % of the column diameter for a piece of
tubing compared to a piece of tower section.

   These lines will not address your initial failure mode: the break in the steel wire.  You should also take a close look to see
why that break occurred.  Was it really a tumbleweed?  Or was the tumbleweed merely the proximate cause, and that the steel wire was
weakened by fatigue or construction technique?

-- Eric K3NA

-----Original Message-----
From: topband-bounces at contesting.com
[mailto:topband-bounces at contesting.com]On Behalf Of Larry Molitor
Sent: 2004 March 6 11:23
To: topband at contesting.com
Subject: Topband: Antennas, Guy Ropes, and High Winds


We had a "last winter storm" blow through here yesterday that destroyed my 90 foot irrigation tube vertical. Very frustrating, the
thing made it all through the winter snow and ice when I was only able to operate from the cold shop building. Now, 5 days after
moving into the new house with a nice warm and cozy hamshack, the stupid antenna falls over.

Anyway, the real reason for this post is to discuss materials used for guying this type of structure.

This 90' tube was guyed at four levels with four guys at each level. The lower three levels used double braided polyester rope from
Syntehetic Textiles, Inc. The top level was also my top load consisting of four 17 gauge steel fence wires with 650 pound baling
twine down to the anchor points. The baling twine is dirt cheap and so I used it temporarily while I optimized the top load
structure. I intended to replace it with the polyester rope after tune-up was completed, however it turned out that the twine was
the strongest component in the guy system.

The actual failure was one of the steel wires. It broke near the point where the twine was attached. I believe several things
contributed to this failure. This polyester rope has more stetch than any nylon rope I've ever used. I was constantly adjusting each
guy to keep the vertical straight. The twine did not stretch at all. Thus, in high winds, more of the side load was transfered to
the top guy. In this case, additional wind load caused by a tumbleweed flying 40 feet off the ground at 60+ MPH impacting the third
level guy was too much for the wire. After the top wire broke, the aluminum tube just folded over about two feet below the second
level guy point at 42 feet.

All guy ropes are intact. If it wasn't for the stretch of the polyester, I think the antenna would have survived even these extreme
conditions. Now the question is how to fix the problem. In the past I guyed permanent structures like this with steel wire of one
sort or another and insulators at intervals. For temporary structures I always used hardware store nylon rope. Never had a problem
with either of these methods.

Steel wire and insulators are not useable for this antenna because of the method of raising and lowering which would cause kinks and
tangles in the wires. Baling twine is cheap and very strong, but I would have to replace it at least once a year. Since it takes
almost two days of work to re-string the guys, I'd rather not do this. I suppose I could use a larger size of polyester rope, but
even the smaller size I used cost more than the aluminum tubing for the entire antenna did! What to do?

Comments, suggestions, and/or discussions are invited.

73,

Larry - W7IUV


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