Doug:
Couple of comments:
1. Make sure your rotator will handle the added torque/twisting load.
Assuming everything on this mast will be a rigid connection (no loose clamps to
let the auxiliary masts pivot around the boom), then any wind load will be
applied four feet from either end of the main mast (at the outer ends of your
horizontal boom.) Could be a significant lever arm force.
This force might not be balanced on either side of the tower if the wind
comes in at the tower from a direction that shields one or the other
down-hanging array. In other words, there could be a net torque applied to the
main mast and rotator.
2. Wind force will be applied to the top of the tower just as if the
down-hanging masts had been installed upward instead. That is, if the wind
were to blow straight into the assemblage of antennas on this mast/boom
combination, your tower top would be subject to the total moment force from all
the antennas' areas, at whatever magnitude of moment they generate, given their
mounting distances from the attachment points to the top of the tower.
BOTTOM LINE: No such thing as a free lunch when it comes to wind loading.
73 de
Gene Smar AD3F
-------------- Original message ----------------------
From: "Doug Rehman" <rehman@surveil.com>
>
> I'm in the conceptual stage of adding a few antennas to my tower. At
> present, the 24' of mast above the tower is pretty well filled with antennas
> from 7 MHz to 1296 MHz. I plan on adding a few more UHF and microwave
> antennas.
>
> What I am contemplating is attaching a cross arm on the mast just above the
> thrust bearing. The cross arm will probably be about 8' long. On each end of
> the cross arm, a 10' mast will be attached downward. This will allow me to
> rotate the antennas 360 degrees with the other antennas and to retract the
> tower. Here's a poor ASCII art idea of what I'm talking about ("U"
> represents the tower itself):
>
> |
> |
> |
> |
> __|__
> | | |
> | U |
> U
> U
> U
> U
>
> Has anybody ever done this before or seen it done?
>
> Would the wind loading of the antennas on the "down hanging" masts actually
> offset against the wind loading of the antennas on the main mast? (This
> presumes that the antennas are somewhat balanced on the two down hanging
> masts to avoid adding an unbalanced lateral load on the main mast.)
>
> Thoughts, comments, criticisms?
>
> Tnx,
> Doug
> K4DDR
>
>
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>
>
>
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