Tom Rauch wrote:
>
> > When I put up my 90 foot tower a number of years ago, I stacked a 5
> > element 15 and 5 element 10 Hygain Yagis. I did not put ropes in the
> > elements. We have a fairly consistent breeze off of the gulf here, but
> > it isn't really windy. I had immediate problems with the tips on the
> > elements breaking on the 10 meter yagi. I was amazed and irritated
> > about the element breakage on these new antennas. I called Hygain and
> > they shipped me new elements. I replaced the elements and put rope in
> > each. Within 6 months, I had three elements broken again...rope
> > hanging out the ends!
>
> Like all resonant circuits, it's a matter of Q.
>
> What kind of rope did you use? You really must use a soft loosely
> woven rope that still has some mass, or dampening is not optimal.
> Laying a stiff rope or a hard rope inside the element won't help
> much, if at all.
>
> As for the people who say the "design the elements so they don't
> vibrate and break", please tell me where I can find details on how to
> design an element that way. Better still, tell me what you did to
> have that design.
>
> I'd be very interested is seeing the calculation for wind causing
> vibration, or even for element mechanical resonances. Certainly
> someone who designs an element not to fail from vibration would
> have that data, or could explain the process.
A rigid tube in space in the presence of a directional airflow will resonate.
Ask any flute player, beer drinker, or aeropsace engineer. It doesn't have
to be a tube either - ask the designers of the original Tacoma Narrows bridge!
Tom is right that it is a matter of lowering the Q. A rope inside causes
friction against the tube and damps out the oscillations. It also
disturbs the internal airflow, preventing the standing waves from forming.
If the rope is too small or slides to easily, it won't help enough.
(Try playing the flute with a piece of rope inside it...;-)
There are other ways well known in aircraft and bridge design. The trick is
to break up the airflow: if the airflow is turbulent, destructive oscillations
never have a chance to form. Your taper schedule has an effect - every
transition
is a little turbulence generator both inside and outside the tube.
One possible reason why some F12 antennas do not have a problem is the "linear
loading"
creates turbulent airflow for all of the elements. I would be curious to
see if the antennas that have shown failure have any linear loading elements -
or if they do, where the loading is relative to the failed elements.
My C3SS does not vibrate in the wind, though it does wobble around a lot. Since
it is located in an urban setting, the nearby buildings create turbulence,
preventing the airflow from becoming directional.
You could create enough turbulence by simply adding small fins or
ridges to your elements. Aircraft designers do this all the time. Next time you
are flying, look at all the little fins and ridges on the wing - they are
there to create useful turbulence and prevent the wing from vibrating. Some of
these fins and ridges even have the net effect of reducing drag by improving the
laminar flow of the wing surface - a similar solution for an antenna therefore
need not
increase the wind loading.
My point is only that there are other solutions. Rope actually seems like a
pretty elegant solution, but it isn't the only one!
hope this helps! - jeff wk6i
ps, Tom: I don't know that you will ever see a neat calculation for this sort
of thing.
There are equations that describe simple shapes, but when you get complex shapes
like bridges and wings (or multi-element antennas) the usual design
methodologies
include rules-of-thumb, computer modelling and/or good old fashioned "cut and
try".
--
jeff stai
radio stuff: WK6I in DM13
rocket stuff: NAR #21059 TRA #3356 Level 2 Cert.
email: jstai@home.com or wk6i@arrl.net
ROC web page: http://www.rocstock.org/
LDRS web page: http://www.ldrs20.org/
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