That is a good point, Roger. Search Tacoma Narrows Bridge for a visual of
harmonic oscillation destruction.
John KK9A
To:towertalk@contesting.com
Subject:Re: [TowerTalk] Ground level rotator mounting
From:"Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net>
Date:Fri, 06 Sep 2013 22:20:28 -0400
List-post:<towertalk@contesting.com">mailto:towertalk@contesting.com>
On 9/6/2013 9:38 PM, Doug Wetzel wrote:
When I put up my HDBX-48 in the spring, I'm thinking of mounting the
rotator at ground level. Has anyone on the list done something similar?
What did you use for mast all the way up the tower, and did you use
Be careful about mechanical resonances. A long mast is in effect a torsion
bar that with its flex, the inherent slight give in the rotator, plus the
mass of the antenna May result in a low frequency oscillation.
You may or may not end up with a resonant condition.. If you do, the
condition may or may not be destructive.
IOW you are now in unexplored territory unless you can find someone who has
use the same rotator, mast, and antenna combination.
Many years ago I built a simple, laminated, wood mast. It worked quite well
for a while, but one windy day I noticed the mast was oscillating. You could
easily see the node and nulls.
As the wind increased the nodes became larger and the speed was so fast that
the nodes looked like a solid mas several feet across. IIRC the nodes
appeared to be nearly 3 feet across before it let go. The longest piece left
was about 3 feet long.
Now this was a different material and a different mode of oscillation.. If a
mast went into severe oscillation I'd expect the destruction to be confined
to either the the rotator, or the antenna. Possibly both.
I've had relatively minor oscillations of a relatively low frequency with a
40 foot mast. The antennas were swinging about 2 feet in azimuth. It did
destroy a Ham-IV and an HDR300 before I installed the PST-61
multiple thrust bearings to align the mast?
Typically they only keep the mast aligned and do not support weight except
for one.
73
Roger (K8RI)
Thanks in advance.
Doug
K7IP
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