[TowerTalk] wind on antenna vs bouncing
K8RI-on-TowerTalk at tm.net
Wed Jul 1 23:37:23 PDT 2009
Al Williams wrote:
> Much is often written on which direction to park antenna beams, however:
I always carefully park the big array in what ever direction I was last
using it. IOW I pay no attention to the wind direction.
> I have observed on my Steppir Monstr that the whole antenna bounces around quite a bit
> on windy days and I wonder if the weight crashing into a sudden stop might be more damaging than the constant (somewhat) wind.
It might be at times. Please note the weasel words in there.
> It would seem that a dampening spring or something would spread the stop and be beneficial but that method is rarely mentioned.
First, you don't want any play in there for the system to come up
against a stop, but rather you want it held against the stop.
ANY time and I do emphasize the "any", time you mess with the dynamics
of the system you are moving into experimental territory. Springs,
long masts serving as torsion bars between the load and the rotator, and
flex couplings can and have gone into resonance tearing a system apart.
The one thing you don't want is for the system to go into a mechanical
resonance. The exciting frequency changes with wind velocity. Adding
any elasticity may take some stress off the gears or break, but they can
add tremendous loads to the antenna, tower, and guys.
One given, is adding some give or elasticity will lower the mechanical
resonant frequency of the system when you really want to raise it. Some
years back I saw a couple of towers and antennas destroyed due to
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