----- Original Message -----
From: "K8RI on Tower Talk" <firstname.lastname@example.org>
> I like the tried and true method of torque balancing.
> If the rotor and/or tower aren't being pushed to their limits, I like the
> "It looks good to me" approach. IE, unless it's a potential problem with
> really big antenna, or antenna system, why bother?
Well in my case, I think we may be pushing the limits of the
rotor and tower. Its a long story as to why, but the two KLM
antennas in our stack (40M4 and KT34XA) have the 3" OD
x 1/4 wall "ICE" booms. Not really needed in Southern
California (at least not a 740 feet ASL). Because of the
heavy booms, the antennas have a very high mass moment
of inertia (300lbs total antenna weight with HD boom vs.
150 w/stock light weight booms). So high in fact that when
you run the OR-2800 rotator at its highest speed setting and
stop the stack, the whole stack oscillates back and forth pretty
violently (change your underwear kind of violently) like the
Tacoma Narrows bridge (we normally run the rotor speed
slow to avoid this as the oscillation is minimal at the lower
speed setting). It's like a big mass (300lbs of antennas with
lots of mass near the ends of the booms) on a torsion spring
(24' long 2" OD chrome moly mast). Although I haven't actually
seen this in person, I am wondering if when the Santa Ana
winds blow hard they aren't exciting the system similarly and
this is what is causing the mast clamp on the rotator to slip
despite the fact that its damn tight with 6 grade 8 bolts. I figure
if I torque balance the stack a little better, the wind will be less
likely to excite the system into resonance and make the clamp
Ultimately what we need to do is to take the whole stack
down and lighten the booms. While it might be prudent to
use thick tubing near the center of the boom, there is no
need for 1/4 wall thick x 3" OD aluminum on the outer ends
of this boom where the bending moment is minimal. All
it does is add to the mass moment of inertia of the system.
Taking those antennas down is a $%tch, so that is going
to have to wait until summer. In the meantime, I'd like a little
insurance so I can sleep at night after we pin the mast to
the rotor clamp.
73 de Mike, W4EF.............................
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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