Topband: Antennas, Guy Ropes, and High Winds

[Larry Molitor]

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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