[TowerTalk] Limits of 25G with tall masts

Steve Maki steve@oakcom.com
Mon, 23 Nov 1998 19:52:18 -0500

Pete Smith wrote:
> 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?

IMO, you are right to consider bending moment relative to the strength
of Rohn 25 top sections.  The pointy top section has a decided lack of
diagonal braces at the top.  A local ham here in Detroit managed to bend
his top section right at the rotator shelf with a 2 el shorty 40 at 10'
above the tower (the mast did not bend).  His top set of guys were BELOW
the rotator, and the rotator was mounted at the normal spot. And there
was a 4el 20 just at the tower top.

It was a mess to take down.  Don't do that.

When I reinstalled it, I put the rotator about 5' lower in the tower.
I think you gain an immense advantage with that modification alone.
For maximum strength, the top set of guys should be right at the top.

Steve K8LX

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