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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