Doug Rehman wrote:
> I've reposted W3AHL's excellent email on mast loading below. I used the
> Excel spreadsheet several years ago to check out my planned (at the time)
> I've got an LM-354HDSP with 32' of mast- 8' in the tower and 24' out the
> top. The bottom 18' is chrome moly and the top 12' is aluminum. I've only
> got a single thrust bearing on the top. The rotator is a Yaesu G1000.
> I do get some flexing of the mast between the top thrust bearing and the
> rotator. The tower has been up several years without a problem.
> After seeing the flexing, I got a second shelf and thrust bearing, as well
> as a Yaesu G2800 rotator, that I planned to install, but never did. I'm not
> sure at this point if I want to stiffen the mast with the second thrust
> bearing or just leave well enough alone.
> With the exception of an 8 element HF log just above the tower top and a 40m
> rotatable dipole 12' up from the tower top, everything else is VHF/UHF.
> While there's a lot of mast out of the top, there aren't any antennas with
> significant wind load very far up the mast. (The HF log, 40m dipole, and 6m
> yagi are all going to be replaced with a 4 element SteppIR that's on order.
> This will give me some mast space for the 2.4 GHz grid dish and 10 GHz
> With 8' of mast in the tower and a 2 element 40M yagi on the top of the
> mast, you might see some pretty significant flexing in the mast. I think I'd
> be concerned about the rotator's ability to deal with the lateral forces.
With the VHF and UHF antennas as high as 24' above the top of the tower
the wind load from them may be as high as 8L Log. With the rotator 8'
down and the top antenna 24' above the thrust bearing that is a 3:1
ratio. IOW a 2' wind area up there would be the equivelant of 6' at the
rotator and the thrust bearing would see that load as the ft lb at the
antenna multiplied by 24. If some flexing is actually visible in 8' of
chrome moly tube (depending on wall thickness) I'd certainly be
concerned about the G1000.
I'd probably put the rotator farther down into the tower, add a thrust
bearing to be used for centering only with out supporting any load, and
go to the larger rotator. It's amazing how much 24' of mast can add to
the load at the top of the tower. I have 30' and with 2 relatively small
VHF and smaller UHF antennas (as I've often said) in the wind it looks
like a Bluegill fly rod that just tied into a record Bass.
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