[TowerTalk] Rotator Placement Question

[n4kg@juno.com]

For a single antenna just above the top of the tower,
placing the rotor in the open space near the top is 
just fine.

For long masts, it is best to place the rotor lower.
I prefer to place the rotor plate at the TOP of the
second section down.  You can cut ONE of the
horizontal braces at the bottom of the top section
for better access when installing / removing the
rotor.  Removing a closely spaced horizontal
brace in a non-tapered tower section has minimal
effect on strength vs. removing a diagonal.

de Tom  N4KG


On Wed, 4 Oct 2000  Bill Coleman AA4LR <aa4lr@radio.org> writes:
> 
> 
> I'm in the process of putting up a bracketed Rohn 25 tower of 45 
> feet, 
> with the bracket at 26 feet. I have a 25AG3 top (the pointy top), 
> which 
> permits me to place the rotator either near the top of the top 
> section, 
> or I can put it at the base of the top section.
> 
> I wonder about the tradeoff:
> 
> 1) At the top, this allows a much shorter mast, which presents less 
> wind 
> loading. However, the antenna torque is applied only a couple of 
> feet 
> from the top of the tower. With a bracketed design, this torque will 
> 
> cause twisting of the top two sections. (In a guyed design, the 
> torque 
> would be transferred to the top guys)
> 
> 2) At the bottom of the top section, the mast is longer, so it 
> presents a 
> larger wind load. But the torque from the antenna will be applied 9 
> feet 
> down from the top, which has better rigidity, since the top section 
> won't 
> twist at all.
> 
> Which is better? Rotator at the top, or bottom?
> 
> In this installation, the antenna is just a simple A3S, perhaps with 
> a 
> 40m kit. Cushcraft rates this antenna at 5 square feet (flat plate 
> equivalent). My calculations indicate this installation should 
> withstand 
> 75 MPH winds with a safety margin of 75%.
> 
> Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@radio.org
> Quote: "Boot, you transistorized tormentor! Boot!"
>             -- Archibald Asparagus, VeggieTales
> 
> 
> --
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