Re: [TowerTalk] Fiber rods as guy insulators

[Pete Smith <n4zr@contesting.com>]

At 11:36 AM 2/27/04 -0500, KD4E wrote:
> For what reasons might one select fiber rods & steel guys vs 
> Phillystran?  (Both eliminate the shunt-feed problem and the
> resonance problem with nearby wire antennas.)
>
> Any concern that the fiber rods may be more vulnerable to vibration
> failure in a severely gusty storm?

K7NV has an analysis on his Yagistress web site (Google for the new server 
location) of the three materials -- fiberglass rod, phillystran and EHS 
steel.  His analysis didn't assume the semi-rigid fiberglass power pole 
insulators (18-20 feet long), but rather the use of continuous "pultruded" 
fiberglass rod such as that made by Polygon.  As I recall, the chief 
downsides of Philly are that it is about twice as elastic as steel, is 
prone to failure through abrasion, and can be easily cut or burned, which 
argues for a stretch of EHS at the bottom of each guy to put the Philly out 
of reach.  The fiberglass, if I remember correctly, is only a little more 
elastic than steel, would be hard to cut, and is more abrasion-resistant.

Another point -- Originally, I had the 21-foot fiberglass power pole 
insulators on the top guys of my tower, with grounded EHS for the remainder 
of the length.  A 40M yagi above the tower top maybe 6 feet was a mediocre 
performer.  When I replaced the top guys with Phillystran, the 40M yagi 
suddenly "woke up."  This said to me that 20 feet of insulator, even 
sloping away at 45 degrees,  may not be enough to eliminate guy wire 
interaction with yagis, at least at 40m.... YMMV, needless to say.

Some local Polygon rod users invested in some vibration dampers for the 
material, thinking that would be a good idea.  I';m not clear on whether it 
turned out to be necessary or not.  I do know that another local just put 
up a 195-foot tower with stacked yagis using pultruded fiberglass without 
dampers.  I think if he thought they were necessary he would have used them.


73, Pete N4ZR
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