Very interesting , why can't you post like this on other threads?

Now back to this post. I have never seen spiral windings on a chimney
and certainly would not put it on an antenna. Ice formation is the
worst enemy for any antenna and spiral windings increases the adhesion
of ice to an element. I will say it again that the best solution is to
make the surface as aero dynamic as possible and to reduce the adhesion
of ice which means no clamps on the element. One must also design
connections of element tubing such that added weight to the element
increases tension that is used to hold the element sections together
such as a telescopic type taper connection which helps regarding air
flow. Making the element lighter also helps because it allows for the
willow tree effect like bending in the wind instead of stiffness that
provide for occillation and fatigue failure.
Putting all these facts together and since I live in the Midwest known
for its wind and ice storms I decided a long while ago to change the
method of manufacture of elements which especially helps today as
aluminum costs go sky high.
First I chose a foundation design which was inexpensive, light,
and telescopic
The 20 feet telescopic fishing pole available for less than $10 on ebay
was the final choice
( I did not use the thin portion at the end) I then placed glass
reinforced tape along on one side to counteract distributed weight
sagging and then drilled small hole along that side
so that urathane foam could be squirted inside while at the same time
exuding air to
provide a long homogenous section of the closed cell type.
Now we get to the conductive surface addition.
Hardware stores sell sticky back aluminum tape for air sealing purposes
which bearing in mind that only material to allow for twice the skin
depth penetration is required is then placed length wise along the
element length thus overcoming joint resistance change and with a rub
down of the surface to remove air bubbles you have a radiating element
that is superior to the present state of the art.
One more point before closing, imagine a 20 pound block of ice hanging
on the end of the element, would the ice hang on to the clamp or
aluminum without breaking the element? If the same ice load was on a
non clamped form of element I just described would it slide off, bend
like it was holding a fish or...... immediately break?

One other point, these telescopic poles made in China appear to be
standardised and thus intechangeable also when home brewing you just
add or remove the added foil at the ends without messing with aluminum
section joints. I changed over to this design years ago when I
homebrewed a long boom yagi with 13 elements so my fold over tower
could handle lifting it
and to this day I use this method for making elements. I have mentioned
this before but I got a lot of hoots and cries from old timers who
object to change and fully expect a repeat of the cat calls.

When hiking just take the sections as is and assemble at site as for
short term closed cell foam is not really needed.
Hopefully some of the younger hams will be attracted to this approach
as the cost is way less than the $1000 required for a manufactured
antenna.
Cheers and beers
Art




and nothing li
Brian Kelly wrote:
art wrote:
Dan Andersson wrote:

But David isn't that the object of the exercise? The object is to
dampen the occillation such that metal fatigue does not take place You
can remove or redirect the causes of occillations
but energy cannot be created or destroyed. Recognising this some
manufactures place rope inside the elements and it works even tho it
costs the manufacturer. To change the surface
of the object receiving these stress reversals only hurts what you are
trying to achieve unless what you are adding delays the effect of
stress. With foam insulation I only followed the path that is generally
used except I also wanted to isolate the inside from environmental
effects such winds from other directions as well as providing
stiffness. The bottom line is to make the elements as aerodynamic as
possible and dampen the occillation that are left. Making things less
aerodynamic only serves to worsen things. Are you sure that these
additions to chimneys is to dampen occillations as opposed to
strengthening the structure?

Looks to me like you're overconcerned about fatique problems. Given a
2M antenna the elements would have to deflect +/- some large amount, an
inch or so, probably more before the bending stresses would exceed the
endurance limit of the types of aluminum alloys normally used to build
yagis. If the stresses are below the endurance limit the element can
deflect "forever" without suffering a fatigue failure.

Further the clue in the description of the original problem indicates
that the deflections are generating oscillations with frequencies in
the audio range. If you do a freebody diagram of the "system" you'll
find it's a rather classic spring-mass with excitation vibration
problem. If you crunch the numbers and calculate the resulting time
period on the basis of the mass of a half-element, the spring constant
of the element and the observed frequency of vibration I suspect you'll
find there's no way the element can deflect +/- fast enough to generate
stresses on the element as high as the endurance limit.

Stuff rope through the element and plug the ends with some heavy goop
to add a bit of inertia out where it'll do some good and call it a day.


Cheers
Art

Brian w3rv