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
----- Original Message -----
From: <kr2q@optimum.net>
To: <towertalk@contesting.com>
Sent: Saturday, September 12, 2009 12:23 PM
Subject: [TowerTalk] how much mast out of the top?
>I have a couple of crank up towers. I want to insert a 20' mast into the
>top section.
> I called US Tower since they sell 20' masts for this crank up (and other
> models). They
> replied that they only rate the tower, not the load on the mast. Thanks.
>
> Anyhow, a concern is that with a decent amount of antenna load on the mast
> (at the top)
> is there a chance (I know, always a "chance")..a GOOD chance that the top
> of the tower
> section would get so torqued as to bend (the top of the tower section)?
>
> My guts says NO because each telescoping section is held by only the top
> 3' feet (or so) of
> the next section down. By the time you get to the bottom, that is a LOT
> of bending torque
> at the top of the bottom section...and it does fine.
>
> I am not concerned about the mast bending...that is not the question. I
> am concerned about
> "bending" the top of the inner-most (top) tower section.
>
> The mast will be about 3' feet into the tower, the rest sticking out. I
> want to put an OB11-3
> at the top (9 sq feet) and a shorty forty just above the top plate of the
> tower. So in a 70
> mph wind, I guess this will be like 20x9 = 180# X the 17' of mast. The 40
> just above the plate
> should not add much additional torque at all, although it will add wind
> load for the tower. But hey, it's a crank up, so in winds, it cranks
> down.
>
> Another way to look at this is just laying the tower over (horizontally)
> seeing if I can handle
> 180# weight at the end of the mast without the top of the top tower
> section getting wrecked.
>
> Any comments? Has anyone done the calculation or is anyone actually doing
> this?
>
> Tower is TX472.
>
> Thanks!
>
> de Doug KR2Q
>
> _______________________________________________
>
>
>
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