[TowerTalk] Rotator Placement Question

[Bill Coleman AA4LR]

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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