We have seen numerous posts on TT about mast clamp slippage. The gist of the
posts is that we need to have a tighter grip on the mast and, hopefully, the
rotator will tolerate the torque without damaging it. I believe that this
problem can best be solved by looking at how the torque forces can be
reduced. There are 2 torque problems involved
1. The inertia of the antenna causes a torque when rotation starts and
stops. We have some control over this by using a rotator which starts and
stops gradually. This is not the primary problem.
2. Wind forces act upon a yagi which is not balanced for wind area on the
left and right sides. Most yagis are attached to the mast at the balance
point for gravity. Uneven element spacing or even the different weight of
directors and reflectors makes this problem occur. Dick Weber, K5IU has
written extensively about how to balance wind forces by adding a plate or
boom extension to one side of the yagi to balance wind forces.
3. I believe the mast slippage is greatly worsened by buffeting winds which
bump the unbalanced antenna periodically to pulse a sort of resonance
capable of slipping the mast in its clamp and applying possibly damaging
forces to the rotator. My ham shack has a window with a view of my Yagis and
I have seen this happen many times. My partial solution is to point the end
of the boom into the wind and I park my beams that way when not in use.
Since the elements are the same length left and right, there is no
rotational force developed by the wind. Obviously it would be better to do
the wind balancing per Webers ideas but, like most hams, I have not done it.
If you are using the beam during the wind storm you are vulnerable.
Fortunately most of our wind storms seem to occur at night and the wind is
almost always from the SW.
I have a KLM 6 element"Big Stick" yagi on a 131 foot crankup tower which
is totally exposed to serious wind gusting and it has not given me a problem
in 7 years of use. This is probably the most unbalanced antenna for wind
that I have ever seen. The boom is 57 feet long and the directors are wide
spaced. The rotator is a KLM 2800. I agree that the clamp design gives too
little contact area to the mast (2 7/8 chrome moly) but I understand the
present design is intended to clamp various diameter masts.
73, Dan, N5AR
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