In a message dated 9/18/2005 9:39:30 PM Pacific Daylight Time,
towertalk-request@contesting.com writes:
>
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.
> 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
Uh, lots of people - especially where there's lots of wind; i.e. Texas,
Northern Ireland, etc. This has been a recurring theme unfortunately.
The problem is that there's enough room in the joint for the inner
element to work back and forth in the wind. Multiple rows of rivets cures this
problem. While IMO 4 rows is overkill, having 2 rows (180 degrees apart) or 3
rows
(120 degrees apart) is plenty.
Cheers,
Steve K7LXC
TOWER TECH -
Professional tower services for commercial and amateur
Cell: 206-890-4188
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See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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