Date: Wed, 21 Jul 2004 07:13:12 -0400
From: "James W. Fisher, Jr." <74237.2073@compuserve.com>
Subject: [Force 12 Talk] 5-year report on VE1JF C51XRN rivets and
XIM400
Yesterday we were able to share some boom truck costs with a local church
(steeple project) and had a brief opportunity to work on the lower 2 yagis
of our cliffside C51XRN/C31XR-H/C51XRN stack. Of particular interest was
the one C51XRN we could reach, which has been up for 5 years vs. 4 for the
others--we took off the 8 front elements and looked them over and made some
progress on Tom's recommendations to triple-row-rivet.
These antennas are all H-rated and have been subjected to real hurricanes
and numerous storms and innumerable gales, plus ice storms. I wonder if
some others' reported problems are really due to too-optimistic selection
of a regular vs. D or H. Going all H on my 48 F12 yagi elements was
cheaper here than one boom truck visit from hours away.
A few rivets (one of three in a row--I believe always the outboard one)
were missing. In all those places, we replaced the missing rivets and
added two additional rows of others at 120 degree spacing per Tom's
recommendation. We also put in 120 degree-spaced rivets on other joints
until time ran out.
The rivets had taken comparatively less of a beating than those on our
180BV-D 4square verts right by the cliff, which have been subjected to what
K7LXC calls our "antenna destruction laboratory" since 1998. We rebuilt
those a year or two ago after some rivet failures and first tried the
triple-row riveting (again, per Tom's recommendation)--they have held up
well since. If I were to do it again, I would order H-rated. Tom pointed
out that since winds come from any direction the stresses on a vertical are
different. If anything, in the bolts-vs-rivet argument, I would point out
that the bolts on our verts have caused some reaming and the rivets have
not. When we overhauled these verts, we also triple-riveted the bolted
connections and I doubt they are going anywhere. Our cliffside yagis also
are subject to lifting from the aerodynamics of the cliff, so they may be
unusual in that respect.
We were also able to reach and repair the 20 and 15M reflectors on the
C31XR-H. The 20M reflector has had the tips bent down at an angle (with a
pipe bender) for four years because they didn't safely pass under the upper
guy from our new tower (a replacement to be strong enough to hold the full
3-stack). This means that they are constantly twisting even when the
antenna is pointed into the wind. Not surprisingly, the rivets on one
joint had been worn through and the tip came down. Again, we put in three
rows of rivets and don't expect any other problems. The 15M was tip
actually an aluminum failure, so that is another topic, although Tom tells
me they have changed the placement of the outboard rivets to mitigate that
possibility (were pretty close to the end of the outer tube). This type of
aluminum failure also occurred as we took down the 20M director on the
bottom C51XRN.
For full disclosure, I have also lost some 10M tips on the top C51XRN, but
have not had a chance to see what happened. I have ordered new tips (the
others flew off to who knows where) and will replace them when I get a
chance, and also of course will triple-rivet.
This work also gave me a second opportunity to look at how the aluminum is
holding up in the salt environment. Per Tom's recommendation, I had
originally coated all elements with clear XIM400 primer (talk to your
independent paint distributor if you're not familiar with this). When we
took the C51 down a few years ago to make the (very worthwhile) conversion
to N-series elements, the aluminum still seemed shiny and new. Now it is
not corroded but I think the coat of XIM400 has been somewhat abraded by
five years in these conditions so it is not as shiny as it was. If
elements are down on the ground for some future project I may re-XIM400 if
time permits for continued protection.
As you see, these antennas have held up remarkably well for years of severe
conditions. If you are putting in H-rated elements, I suggest you buy
plenty of extra rivets (I got them simply by ordering them from the order
desk) and triple-rivet in the first place. Maybe Tom should advertise
these as a recommended option at least for H. If you are in a relatively
corrosive environment, I suggest you XIM400 each element when it is
complete and riveted.
73,
Jim, VE1JF
jwfisher@alumni.princeton.edu
http://www.ve1jf.com
_______________________________________________
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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