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Re: AMSAT-BB-digest V2001 #578



Dave,

    I'm no mechanical engineer, but from what I've read, acrylic(polymethyl methacrylate)
is much weaker than fiberglass. Fiberglass is a composite material, in which the fibers
give extra strength to the material when in tension, as they will be on the upper side of
the crossboom.  In addition, acrylic is more susceptible to degradation by ultraviolet
energy.  This can be reduced by painting, but the strength won't be improved by
painting.  So pay the extra money and avoid the possible consequences of a broken boom
and damaged antennas, Get a fiberglass one. Incidentally, it's a good idea to paint
fiberglass booms, too.  Some Ops have had problems because the fiberglass is elastic and
smooth.  That makes it difficult to clamp antennas to it and have them keep their
rotational position on the crossboom.  A large, snug fitting wooden dowel is what some
use inside the fiberglass tube to help keep it's shape.  I cut my fiberglass boom in
half, and used a short piece of thick wall aluminum tubing passed into and clamped to the
elevation rotator.  The fiberglass pieces are pushed over the outside of the Al tubing
for a foot each and pinned to it with 1/4-20 stainless steel bolts and elastic stop
nuts.  I use 6" long wooden dowels as plugs in the outside ends of the crossboom and pin
the antennas to it to eliminate any tendency to rotate off boresight.  Hope that this
helps.

                                                                73, Bill AA6PA

Date: Thu, 13 Sep 2001 01:49:14 EDT
From: KB7ADO@aol.com
Subject: [amsat-bb] Cross booms:  Fiberglass vs. Acrylic Rod?

I'm looking into getting a solid rod vs. the hollow 1.5 in x 5 ft KLM type
that I have now for my cross boom.  One outfit called E-Com Plastics
(www.ridoutplastics.com/acprofrodtub.html) has both extruded and cast acrylic
rod available in lengths up to 6 ft.  The extruded rod is several dollars per
ft less than the cast rod.

My questions are:
* would an acrylic rod be better or worse than fiberglass, and which should I
consider, the extruded or cast rod?
* what are other sources for the solid fiberglass rods besides Max-Gain
Systems in GA?

Because Max-Gain's solid fiberglass rods are only available in 8 ft lengths,
UPS would charge the 30 lb. rate for shipping, which means I'm looking at a
minimum of $89 for one 8ft x 1.5" rod.

Thanks.
Bob, KB7ADO

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