G'day all,
A couple of days ago I noticed that one half of the rear element of my
18-30MHz log looked wounded, a bit of extra droop.
After taking it down for inspection I discovered that there was a fatigue
crack originating from the top rivet of the outer pair which are located
approximately one inch in from the 1st/2nd section junction. There are
three pairs of rivets (4mm dia) spread over five inches. The second
section continues as a doubler for the first.
Using the Leeson spreadsheet to investigate the taper it was discovered
that the failed section was good for 92mph, significantly less than the
other sections. Doubling up the wall thickness of the second section
sorts out that problem, also the third would benefit, however, because of
the riveting it will need a complete rebuild from tip to centre, not too
easy to tell where the existing holes are when they are covered up by a
new element section.
In the Falklands we don't see winds of 90+mph, however, we get a lot of
wind, the sort of thing that will cause fatigue failure.
Now, thinking about the location of the rivets. Perhaps it is better to
make the outer pair horizontally opposed rather than vertically, that will
ensure(?) that the upper part of the tube in tension is not weakened by a
hole. Maybe the vertical location is correct given that the element is
horizontally stressed by the wind.
I notice in the ARRL antenna handbook it shows rivet pairs with a 90
degree stagger.
Any mechanical experts out there? Anyone had the same problem with Force
12's
Regards,
Mike VP8NO
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