[TowerTalk] Antenna element vibration
Eric Scace K3NA
eric at k3na.org
Mon Aug 11 01:05:19 EDT 2003
There appears to be some confusion on the topic.
The vibration modes under discussion are vibrations in an element usually associated with very LIGHT wind. I have personally
witnessed this close up. With a breeze of less than 5 knots (10 km/hr) I have seen a half-element on a 20m beam vibrating with a
standing wave containing a few nodes and loops along the length of the element. (This is just one of many examples that I have
witnessed.) The vibration rate was high: 5 Hz or maybe more -- enough for the element to be a blur at the loops. On this
particular antenna the displacement was about 1 cm in each direction. That's almost a half-million cycles per day. Aluminum,
unlike steel, will gradually lose strength when cycled millions of times within its elastic limits; the graphs I have seen show a
gradual but steady decline in elasticity. After many millions of cycles, when subject to a stress which it would have normally
easily survived earlier in its life, the element will simply snap off at one of these weakened points in a classic fatigue failure,
Simply reaching out and lightly touching the element with a finger at one of the maximum vibration loops was enough to kill the
vibration in the whole element. Remove the finger and the element would start oscillating within a few seconds.
In a stronger breeze this particular mode of vibration did NOT occur on this particular set of antennas.
A piece of rope in the element helped a little bit -- but not always. Standing on the ground, one could sometimes hear what
sounding like someone sending a continuous string of dits. It was the rope hopping up and down in a vibrating element! Obviously
the rope wasn't helping much at that particular moment...
I suspect, but can not prove myself, that an element design with an aggressive taper would be less likely to vibrate in this way
simply because there would be no simple eigenvector or vibration mode that would set up as a standing wave along the length of the
half-element. But more specifically, I would speculate that a change in material diameter or aggregate wall thickness in each
location that was 1/2, 1/3rd, and 1/5th of the distance between the boom an element tip which introduce a mechanical discontinuity
in the element such that it would have a hard time setting itself up to vibrate with a standing wave of 2 loops, 3 loops, 4 loops, 5
loops etc. I don't recall seeing any elements with more than 5 loops of vibration along its length. Under this hypothesis, it's
not the "aggressive" taper that prevents these oscillations, but rather the fact that there are many step-changes in diameter... and
some of these step changes are in the right places to dampen out specific vibration modes. If the hypothesis is correct, then one
could have relatively few changes in diameter but, if the changes were in the right places, the element would still be unlikely to
vibrate in this relatively high-frequency standing-wave form.
Hose clamps, rivets, etc have nothing to do with this. Rope inside the element could help but doesn't seem to be a cure-all in
I hope this helps clarify some of the situation.
-- Eric K3NA
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