[TowerTalk] High VSWR

[jimlux]

On 9/1/19 11:41 AM, Doug Ronald wrote:
> I would like to talk to you about that vane idea when I get further along on
> the repair of this LP. Those back 4 elements are really heavy, so placing
> the boom-to-mast mounting plate in the physical center of the boom would
> have required massive counter weights on the high-frequency end of the boom.
> There are already two counter weights there now with about 240 pounds weight
> total.
> The wind torque on this antenna is so much that I have to remove the drive
> chain from the rotor in a storm to prevent breaking the shear pin in the
> rotor.
> If the vane idea will neutralize the boom's torque, what about the 104 foot
> long back elements? In a steady wind, how will the antenna orient itself if
> free to rotate 360 degrees? Of course now in a steady wind, the butt end
> always faces the wind direction.
> Doug, W6DSR
> 


I would also wonder about the elements flexing in the wind load. Both 
from an oscillation standpoint (what's the first mode frequency?) and 
from a scenario where the element bends, which changes the "projected 
area" relative to the wind, so the drag changes, etc.

Of course, in some cases, there is "previous installation experience" to 
look at - if there aren't any design changes, and they've got live 
experience with the antenna not failing in a wind of X mi/hr.

But the whole resonant excitation thing is a challenge to model 
properly, especially if the element is tapered (since the stiffness 
changes with position along the element, as does the aeolian excitation 
frequency).