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)
installation.
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
dish.)
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.
Doug
K4AC
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Steve, W3AHL
Sent: Saturday, September 12, 2009 3:56 PM
To: kr2q@optimum.net; towertalk@contesting.com
Subject: Re: [TowerTalk] how much mast out of the top?
Rather than rely on gut feelings and others' anecdotal experience, calculate
the mast load. There is a fairly simple spreadsheet in the companion files
section for the Physical Design of Yagi's book at:
http://www.arrl.org/notes/3819/pdya.exe
The MAST.WK1 file will need to be converted to Excel (you may need to
download an add-on from Microsoft since Lotus support is no longer included
in recent upgrades).
Cell H8 will give you the total load transferred to the top of the tower by
the mast and antennas. You can get the tower manufacturers spec for this
from their stress analysis calculations.
This program calculates the maximum wind speed that the specified mast can
survive with the given load. To see what the tower load is at YOUR target
wind speed, just reduce the material yield strength in cell C5 until the max
wind speed in H5 is what you want, then see what the tower load is.
There is also good info and a spreadsheet in the July/Aug 2001 QEX on Tower
and Antenna Wind Loading as a Function of Height.
The article is available at: http://www.arrl.org/qex/1123.pdf and the
spreadsheet is at:
http://www.arrl.org/qexfiles/Travanty.zip
This allows you to figure out how much to lower your freestanding crank-up
tower to survive a given wind speed, what is the maximum wind it will
survive fully nested, etc.
Steve, W3AHL
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