Hi Pete,
I am working up a spread sheet for a similar problem.
My quick and dirty back of the napkin spread sheet says
that the Rohn 25 should be okay in terms of bending moment
with the parameters you gave. I don't know about torque though.
I need to figure out how to do this calculation (I think the
torque capacity of the tower depends on how you attach the top
guys since this determines how much torques is transferred to
the guys and how much is left to "twist" the tower structure).
My concern would be with the side load on the rotator.
If you are using a point top with no thrust bearing, and the
rotator only a couple of feet below the gib sleeve, my guess
is that you will be subjecting the rotator to a pretty nasty
side load. Imagine a 12' lever arm working against 2' lever with
the gib sleeve as the fulcrum - this is a 6:1 force multiplier.
I am planning on using a flat top with a TB3 and a second
bearing 6' or 8' below the top. The rotator will probably be 10'
below the top bearing. This should ensure that the all the side
load gets taken up by the bearings instead of the rotator.
I'll let you know when I get my spreadsheet done. It will be
interesting to plug in your numbers once I get this thing checked
for errors (I will probably need a legal disclaimer too - I am not
a registered P.E. - strictly amateur hour here)
Regards,
Mike, W4EF.....
----------
From: Pete Smith[SMTP:n4zr@contesting.com]
Sent: Saturday, November 21, 1998 11:52 AM
To: towertalk@contesting.com
Subject: [TowerTalk] limits of Rohn 25G with tall masts
I live in 70 mph territory, and currently have 100 ft of Rohn 25 with a
Force 12 C-3 (5.6 sq. ft., 280 inch/lb torque) just above the 25AG3
pointy-top, and a Force 12 EF-240S (3.8 sq. ft., 210 inch/lb torque) six
feet above it, each with an 18-foot boom. The top set of guys are at the
95 foot level, about 2 feet below the top of the 25AG3.
I am concerned that there is still sufficient interaction between the two
antennas to somewhat degrade the performance of the 40-meter yagi, whose
pattern still seems not up to par, even though it is mounted at 90 degrees
to the C-3. One solution could be to replace the current 2-inch Rohn mild
steel mast with 16 or more feet of chrome-moly of appropriate wall
strength. Then I could move the 40-meter yagi another 6 feet or so up the
mast (I know that MARC or similar calculations can tell me how good the
mast has to be).
An experienced big-antenna guy here thinks, however, that doing this might
overstress the 25G. I have no idea how to calculate or judge this. Seems
to me that the torque load is the same regardless of the height of the
mast, so what we are really talking about is bending moment applied to the
top section where the mast passes through it on the way to the rotator,
which is mounted about 2 feet down inside it. Would moving the small
40-meter antenna up another 6 feet put too much load on the 25AG3 section?
It would be a fairly mammoth job to replace the mast, and I'll know more
about antenna performance after next weekend, but is it even feasible?
Thanks in advance, gang.
73, Pete Smith N4ZR
n4zr@contesting.com
Loud is good.
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