Leeson in "Physical Design of Yagi Antennas" says on page 4-39, "A common
failure is in the tip section. This process can be prevented by adding damping
to the resonance of the tip section, typically by inserting a piece of
waterproof rope inside the length of the tip or by weighting the tip to reduce
the mechanical resonant frequency below the vortex shedding frequency shape.
Knot the rope or glue it in place with silicon caulk at the inner end so it
can't slide out if the tips sags under heavy ice."
He says nothing about the size of the rope relative to the element ID but I
would suspect bigger is better.
Wes N7WS
On 8/29/2022 7:48 AM, Lux, Jim wrote:
On 8/29/22 7:36 AM, Steve Maki wrote:
Back in the 70's we were building what we thought were heavy duty yagi's.
Constant tapers with 1/2" or 5/8" tips. They buzzed like crazy in light winds
and shed pieces of the tips on a regular basis.
Someone suggested the rope-in-element trick, which cured the problem
absolutely. Every element got the rope, which I believe was usually 3/8" poly
- the cheapest rope we could find. I've always believed the mechanism was
simply the rope slapping the tube if the flutter tried to start. If that is
it, then it would make sense to use something that has some minimum amount of
mass, but not so thick that there is no room to move.
Pretty much - you're looking for an increase in damping - You want something
that will turn motion into heat, i.e. frictional losses, whether from the rope
flexing or the rope sliding on the internal surfaces.
And since it's inside a pipe, it's not subject to UV, nor other weathering
forces, so cheap is the name of the game - and besides if it did get all
ratty, it wouldn't make any difference as long as it stayed in roughly the
same places.
I note that people have done damping by things like sand and rubber, or things
like automobile undercoat. There's definitely sticky paper based damping, but
there could well be "rope and glue" products too.
One could probably fill the tube with some sort of foam - although one would
need to think about what properties of the cured foam you'd want. Probably
not something super rigid.
Lately I've been buying commercial OWA Yagi's; they seem to have solved the
issue with judicious taper scheduling.
-Steve K8LX
On 8/29/2022 7:25 AM, Paul N1BUG wrote:
I'm trying to get a better sense of how the rope in elements to stop
vibration thing works. I've read that it works because as the element swings
in one direction, the rope slams against the opposite side, providing a
counter-force. What confuses me is that I don't understand how the rope is
free to move around. Nylon or polypropylene seem to be recommended but those
ropes come coiled or folded and they have a memory effect, so they don't lay
flat. When put inside an element, they are going to be pressing against it
in multiple places and directions. It seems like that would limit the rope's
ability to move to do its job. What am I missing? I have seen braided rope
with extremely fine strands that is soft and doesn't have the memory effect.
Is that what I should be using?
My 6 meter yagi was having a problem. The element tips were oscillating at
about a ~200 Hz rate, and the tips were moving at least two inches! It took
four trips up the tower to rig and get it down. It will take another four
climbs to put it back up, and more if it still has a problem and needs to
come down again. I'd like to try to get this right on the first try to avoid
all that extra climbing. The four foot center section of each element is
.750" OD, with .625" tips. Any practical tips or experiences with similar
elements would be appreciated.
Paul N1BUG
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