[TowerTalk] Riveted Elements

[Mike Harris]

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