>
>Bruce, ZF2NT
Hi, Bruce--
Sorry about your antenna. I just HATE it when things are not very
well designed.
I didn't understand your comment about it NOT being metal fatigue
because of a lack of discoloration? I have seen a lot of metal
fatigue and I don't recall ever seeing discoloration. The aluminum
was definitely not damaged by high winds, but rather by low winds!
Constant winds that induced vibration into the aluminum and essentially
work hardened it until it failed. I consider this a design defect.
I had two identical 15M 8-element Yagis on 60 ft booms. One was
mounted at 80 ft on a well-guyed Rohn 55 tower and was more exposed
to the winds. The other was mounted at 56 ft on an unguyed, but heavy
duty (Class 1)cedar utility pole. The antennas had 1/2 elements of 6 ft
sections of 1 inch by 0.058 wall inboard and 6 ft 7/8 inch by 0.058 wall
outboard. These were connected by a hose clamp with a slotted inner
section. And both antennas had anti-vibration ropes installed in the
elements.
The antenna mounted on the pole has had four half-elements fail very
near the boom, and the tower-mounted antenna has had no failures.
What is even more interesting is that all of the failures have been
closer to the mast, rather than towards the end. And all of the failures
looked like your failure and I consider them all to be metal fatigue
failures.
It is my belief that the tower-mounted antenna did not have vibration
induced by the tower because it was well guyed and the tower could
not move. Whereas the pole-mounted antenna had additional
vibrations that were induced by the pole not being restrained from
movement. The booms are top guyed by a double, Vee-shaped set
of guys. All of the failures occurred BETWEEN the top guys and
there were no failure outside of the region that was guyed.
The inner elements received the worst of this small, but high frequency,
vibration because they were more tightly coupled to the vibrations
in the pole, because of the greater rigidity of the inner sections due
to the top guying. Whereas the pole-induced vibrations tended to
be dampened towards the outer elements because of the flexibility
of the 3 inch by 0.125 wall booms beyond the guyed points.
This is just my THEORY. But it is the only one I could come
up with under the circumstances of four failures on one antenna
and no failures on the second, identical antenna.
I think your antenna suffered a similar failure due to the aeolian
vibrations that seem to be the worst in steady, but relatively low
velocity winds. Your site may be the worst case for this sort
of problem because of the constant winds. I believe that just
getting a replacement section will only buy you another 12 days
and that a better solution by the manufacturer is needed.
73--John W0UN
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