On 18 Oct 2006 01:46:45 -0700, wrote:


Francesco

Do you receive my email?

Wes N7WS

Hello Wes,

didn't receive anything, I sent an email to you.

73

Francesco IS0FKQ

Wes Stewart ha scritto:

On 18 Oct 2006 01:46:45 -0700, wrote:


Francesco

Do you receive my email?

Wes N7WS

Dan Andersson wrote:
wrote:

snip Don't make it complicated! Complicated equals heavy!

You most probably suffer from the effects of turbulence combined with
harmonics from the aerial design itself.

The immediate solution is to do what you do with high chimneys and car radio
aerials - you wind a thin spiral round the full ( most? ) length of the
aerial rods. This will break most of the turbulence. You choose lightweight
and thin material for this. Try a couple of materials until you are happy.
Also, bear in mind that it might only be one part of the aerial that
generates the problem!

By adding ropes etc, you just contain the result of the turbulence and the
harmonics!

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?

Cheers
Art





By adding the spiral around the antenna rods, you have a good
chance of eliminating the source of the problem which is the rods being
susceptive to air turbulence, created by the environment as well as the
aerial construction itself.

The reasons for adding spirals on high rise chimneys are twofold, first to
create a controlled turbulence as an added up draft as well as eliminating
the possibility of creating a self oscillation due to the air
pressure/turbulence and the harmonics generated in the design.

By adding these spirals, your aerial will be susceptive to icing at cold
weather! Check so that you don't get a crashing aerial because of heavy ice
coating!


Cheers

Dan / M0DFI

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

wrote in message
ups.com...

Hi Gene,


wrote:
Hello all,
I build one year ago an 8 element 50 MHz yagi. Boom is made with 50 mm
square aluminum tubing with
a second square tube on the center of the antenna to strenghten the
boom. Elements are made with 20 mm
round aluminum tube in electrical contact with the boom. The antenna is
a good performer but it has one
problem: it sounds even in light wind and vibrations come down the mast
and are very annoying in the whole
house. The worst problem is that resonance succeeded in unscrewing the
rotor bolts. I'm going to use a
special for bolts and screws but I'd like to find a method to avoid
mechanical resonance of the antenna.
Does anyone know a good method? Would it be better to add two insulated
pulling wires at both ends of the boom (even if probably also elements
produce udible resonance)?
Thanks in advance

Francesco IS0FKQ/I8


Hi Francesco,

You did not describe the vibrations, so this may be off base. If it is a
"sound", then perhaps the tubing is acting like a flute. Plugging the
ends of the open tubes would stop the sound. It may be necessary to
leave a small hole for drainage.


The sound is low frequency, I'd say less than 100 Hz with amplitude
modulation of 1-2 Hz probably
due to boom vibration. I thought about flute effect but I think it
would be an higher pitch in that
case. I read many interesting suggestions though, I hope to be able to
try some of them in the next
week end.

those vibrations are quite common in 10m and 6m yagis where the elements are
made of aluminum tubing. they are from the elements fluttering in the wind
(usually just in light wind).

to put a few of the misconceptions to rest...
First this is not a 'wind over the bottle' or flute effect. those are much
higher frequency and are easily stopped with caps on the ends of booms and
elements. so forget the solutions about plugging the ends of elements and
booms for this one.

This problem is caused by 2 properties of antennas about that size coming
into phase with each other. the first is the vortex shedding around round
objects like tubing or wire. this is an aerodynamics/fluid flow problem
that is caused by air flowing around the tubing creating a low pressure area
behind the tubing, this sets up two vortices behind the element. at a
critical wind speed these vortices start to slip away from the element, and
when they do they create an unbalanced force... i.e. if the top vortex sheds
first the element gets pushed down, this often disrupts the flow causing the
bottom one to shed which then pushes the element back up, which causes the
top one to shed again, etc, etc, etc,... the effect alternately pushes the
element up and down at a rate mostly determined by the wind speed. the cure
for this is the spiral wrappings that you often see on chimneys and power
lines. the spiral wrap is a way of breaking up the smooth flow around the
object so that the vortices don't have a chance to form in the first place.
they have nothing to do with strengthening the object, in fact they add to
the wind load a bit, but the breaking up of the vortices is their main
purpose.

This vortex shedding problem is there all the time on all types and sizes of
antennas, but on most antennas it does not cause the damaging type of
vibration you see on some of them. why is this??? as long as the shedding
frequency is not close to the element's or boom's mechanical resonance there
is no problem, the forces occur but they are out of step with the natural
response of the element so the vibration can't build. however in 6m and 10m
yagis with commonly used tubing the combination of tubing weight and
stiffness happens to have a mechanical resonance about the same frequency as
the vortex shedding frequency at low wind speeds around that size tubing...
everything is wrong. to make it worse, the added motion from the mechanical
resonance makes the vortex shedding worse since the element moves more,
causing it to move even more, and eventually fatigue and break in extreme
cases. note, that the combination of very stiff larger diameter tubing used
in later design telrex yagis down to 15m and 20m also had this problem, i
found this when i started picking up pieces of elements and eventually whole
elements from the ground under the towers from this effect.

if you don't break up the vortices the other option is to change the
resonant frequency. there are 2 ways to go, you can increase the frequency,
which is very hard... to do that you have to make the element stiffer or
lighter. you can't get much stiffer than the tubing being used without
extreme measures, and likewise lighter is generally not an option. so the
answer is heavier or less stiff. its easier to go heavier, just fill the
element up with something heavy. This can also effect the 'stiffness' of
the element, if the weight you put in the tubing is free to move it acts
like a floppy weight and reduces the effective stiffness of the element.
this is the combination that led to putting rope into elements. a good
heavy hunk of rope (this is not the time for polypropelene) smaller than the
inside diameter of the tubing adds weight and flops around effectively
absorbing the vibration before it gets going. this gets harder on small
diameter tubing usually seen on 6m yagis... on those try either aircraft
cable or the plastic coated wire clothes line, or even heavy guage insulated
wire.

there are also other ways to stop vibrations like this. i have some 20m
yagis now that have a long gap between the 1st and 2nd director, this gap
was fluttering in certain winds so i snubbed it by adding a second boom
brace rope in the middle of the section with just a little bit of tension to
keep the boom from vibrating. electric utilities also use dumbell like
spring devices that change the resonant frequency of large cables, these are
often seen in pairs on either side of the insulators on towers in flat wind
prone areas.

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

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

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

What?? You're new around here aren't you? Run a search in this group on
"w3rv"for the years 1995 thru 2005 inclusive then kindly reconsider
your comment.

Any of my online food-fight posts which have shown up in this group
recently were cross-posted from Rec.Radio.Amateur.Policy without my
intent or knowledge. RRAP was founded to specifically debate the
contentious code test issue and I've posted to it many times over the
years often in a bit of a combative mode. Which is normal, expected
practice in that particular group. Food-fights are *not* normal,
expected practice in this group and as your search results will
illustrate I *don't* indulge in them in this group.

The problem all the ham radio USENET groups is suffering involves the
behavior of a few members of the "ham community" who have no respect at
all for propriety and civilized behavior and they're destroying
valuable resourses like this group. They obviously come from levels of
society which the average ham would not like to have living anywhere
near their neighborhoods. For lack of a better way to put it . . It's
the same bunch which are turning the HF phone bands into
embarrassments.

It's a shame but that's the way it is these days. The only ways out
seem to be moderated reflectors and discussion groups. Sad.

Brian w3rv

I don,t think that is all encompasing. During the last couple of days I
have written two new postings regarding the academic side of antennas
which is what I thought members wanted but that is not the case, read
a few answers that I got which are very few. On the other side of the
coin there are more members responding to non the antenna issues with
slander
so as with any group the majority rules. Throwing mud is always more
enjoyable in the absence of applicable logic, but then it is safer to
say what Roy resorts to
"I don't understand "
The days that ham radio pursued the sciences are long gone and this
antenna newsgroup
is a perfect example of that We have to recognise that simple fact and
find a way to turn it around to our own advantage, or alternatively get
a gun and play fire with fire.
As far as being a short time poster that is not true, Over the years I
have seen more slanderous postings from the suedo experts of which
there is still a few around
At one time such comments were thick and heavy even racial with
respect to jews so many knowledgable hams left and were replaced by
posters who enjoyed that sort of exchange.

Go to the archives and pick out a name that applies to a self percieved
antenna expert bearing in mind that I am a learner and you will quickly
come across the worst that you will ever see. As I said earlier some of
those early combantants are still having a hayday.and more
silly logic is coming in from all sides to join them. Again count where
the majority postings have headed in the last few days to gauge what
antenna types really want.
I must exclude Walt from all others since no matter what politeness
rules in all of his postings as well as applicable logic. Sometimes I
have been driven to a lessor ind of response based on slander given.
Cheers
Art





Brian Kelly wrote:
art wrote:
Very interesting , why can't you post like this on other threads?

What?? You're new around here aren't you? Run a search in this group on
"w3rv"for the years 1995 thru 2005 inclusive then kindly reconsider
your comment.

Any of my online food-fight posts which have shown up in this group
recently were cross-posted from Rec.Radio.Amateur.Policy without my
intent or knowledge. RRAP was founded to specifically debate the
contentious code test issue and I've posted to it many times over the
years often in a bit of a combative mode. Which is normal, expected
practice in that particular group. Food-fights are *not* normal,
expected practice in this group and as your search results will
illustrate I *don't* indulge in them in this group.

The problem all the ham radio USENET groups is suffering involves the
behavior of a few members of the "ham community" who have no respect at
all for propriety and civilized behavior and they're destroying
valuable resourses like this group. They obviously come from levels of
society which the average ham would not like to have living anywhere
near their neighborhoods. For lack of a better way to put it . . It's
the same bunch which are turning the HF phone bands into
embarrassments.

It's a shame but that's the way it is these days. The only ways out
seem to be moderated reflectors and discussion groups. Sad.

Brian w3rv