>
> In many guyed tower configurations, a major component of stress on
> certain tower sections during periods of high winds is a *bending*
> stress (in contrast to, say, a purely compressive force). This bending
> stress originates in the *elongation* of the guy wires under increased
> tension from the wind loading on the tower and antennas. Flexibility of
> the guying material is of little import here -- only its tendency to
> stretch under the applied loads. If you are trying to obtain maximum
> utilization of a guyed tower (i.e., "more stacked beams!!!"), relative
> elongation for a given length and a given wind loading is the
> specification you should be concerned with.
You have to remember that a deep catenary in a high mass guy is the
equivelant of stretch as far as movement of the tower is concerned.
Having been "up there" on some pretty windy days, I've found towers that
have deep catenaries in their guys move a lot more than say something with
several times the stretch of EHS.
OTOH the "frequency" of the movement is much higher with something like
Phyllystran as there is a very shallow catenary and low mass.
The important items are how far does the tower move and is it any where near
a mechanical resonance in any of the antennas, the over all system, or part
of the system.
Typically the resonance of the Kevlar guyed systems is higher than any
mechanical resonances normaly found while that of the EHS is *usually*
lower.
Grab a properly tensioned EHS line out about 10 feet from an anchor and pull
up and down on it. You should be able to get a resonant movement going on
towers of a 100 feet or so. This should result in sympathetic movement in
the other guy lines.
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com (Use return address from home page)
Do the same thing with Phyllystran and you can "shake" the top of the tower.
Neither movement is desireable, but normally neither one is a problem even
on very windy days.
>
> K7NV has some good modeling studies on his web site; it's been a while
> since I've gone through them, and as I recall his thrust was primarily
> to analyze the relative impact on tower loading from low beams versus
> high beams, but the message is clear -- low stretch guy wires allow you
> to do much more with a given tower.
>
> Bud, W2RU
>
>
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