Roger (K8RI) wrote:
I think many have misunderstood the original post, or I phrased it
badly. My concern is only whether this "structural steel tube" will have
enough resilience to handle the 200# plus antenna with out oscillation.
The only information I have is "structural steel" so it almost has to be
mild steel. I'm successfully using a much more massive...ah... mast
that is only about half that length with the rotator mounted well up in
the tower..
I understand the nuances of mounting the rotator down in the tower or at
the base. BTW when turning large antennas or arrays, unless the tower is
really massive a rotator mounted at the base should have an independent
mounting as it can put horrendous stress on the tower legs over a short
distance. This particular tower does have the rotator mounted to the
concrete with a massive steel base. He has built or had built a very
nice mounting system for the rotator that also serves as a base mount
for the tower. The tower is also sturdy enough to hold the antenna as
it's done so for the last 12 years or so with winds toping 70-80 MPH at
times.
Yes, the tube becomes a very long torsion bar, or tube. IOW a linear
spring where the danger is resonance.
As a side note: When mounted on a pier pin mounting the rotator at the
bottom of the tower does not translate into shear forces, but uses the
tower is a more or less reverse fashion with the torque being
transmitted from the bottom of the tower. In either case with pier pin
mounting the torque is absorbed mainly by the guying system. With one
set in concrete the force with the bottom mounted rotator translates to
a shear over a short distance which is a very good reason for using the
pier pin arrangement. This also gets up into a comparrison of a deep
catenary seen with EHS guys compared to the very shallow catenary seen
with Phyllistran of the same strength.
One comment I saw referred to using couplings to reduce shock loading,
but "to me" wind induced loading is not shock loading unless the antenna
is mounted right at, or very close to the rotator or there are some
loose joints with a bit of slack. Shock loading (again to me) is abrupt
as when the bake is engaged on a ham M style rotator of the motor is
used to stop rotation. My rotator is on the bottom end of 42' of 1
1/2" steel tubing inside 2" steel tubing. (This is structural steel,
each with a 1/4" wall, so I basically have a 2" tube with a 1/2" wall.
It is springy enough there is no shock imparted to the rotator which is
a PST-61. Substantial forces? Yes, but not shock. Actually rubber flex
joints at the end of a long torsion tube may *increase* the stress on a
rotator by letting the antenna move farther and gain more speed before
it is stopped. Yes, the rubber flex joints slow the stopping and spread
the stress out over time, but in this case they can let much greater
stress build and they lower the mechanical resonant frequency of the
entire (rotator, coupling, shaft, and antenna) system. Remember most
rubber will store the energy resulting in a rebound that still contains
most of the original energy. The flex joint needs to have a material
that dampens the motion, rather than being springy like rubber. That is
why shock absorbers use a fluid filled cylinder (or disk) with a bleed,
plus a return spring, but they have to be tailored to the specific
load. There are also solid and foam materials that absorb energy, but
that energy gets turned into heat so the choice of material can be
important from more than just a damping approach.
I spent many months with vibration damping in industry working with
various materials and devices. One thing I found was that over a wide
range of frequencies, elasticity was not usually your friend. But
vibration and vibration damping is a very complex task. The same is true
for antenna systems and particularly with long shafts to the rotator.
Any time we add *give* to the system we are playing with a mechanically
resonant system. USUALLY we stay out of resonance, but at these low
frequencies it is very easy to make things worse with the wrong choice.
If you've ever seen a system with a large wind load go into resonance it
can be scary. About the only resolution at that point is to release the
hold of the rotator on the antennas and let them windmill with the hope
that things can be repaired. Generally when they go into positive
feedback resulting in resonance there is no time to do anything except
get out of the way. I saw one system literally come apart with the
rotator being about the only thing usable that was recovered. As was
mentioned in one post, the shaft needs side-to-side support (thrust
bearings) to keep it centered, otherwise it can start to slap from side
to side and there is a lot of energy released when that shaft impacts
the inside of the tower.
Back "in the old days" I borrowed the idea of the wood frame from the
ARRL hand book and modified it to support a wire antenna. We lived on
top of of glacieral moraine (which is a very shallow hill) out in the
open farm country. Some extreme, straight line winds put that thing into
oscillation. I heard a loud hum and looked out just in time to see it
come apart. It sounded like a gun shot and the largest piece of 2 X 4
left was about 3' long. I didn't lose much in the way of money, but the
example above was expensive for the owner.
My system is a very large wind load with lots of leverage
http://www.rogerhalstead.com/ham_files/Tower29.htm and the antennas
swing about 15 degrees. (That's an eyeball measurement, not physically
measured) and that part of the system has held up quite well.
It's important, or even essential to remember that due to the immense
number of variables there is no "cook book" approach to this topic and
what works for one person in one instance may not work at all or even
make things worse in another unless the systems are identical. With most
large antennas or arrays we are moving into experimental territory with
most installations unless duplicating an existing system so they should
be approached in a methodical and cautious manner. Even with identical
systems, differing wind and terrain conditions may result in "all bets
are off".
73
Roger (K8RI - ARRL Life Member)
www.rogerhalstead.com
N833R (World's oldest Debonair)
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