[TowerTalk] Rotator Placement Question

[Stan or Patricia Griffiths]

Bill Coleman AA4LR wrote:

> 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

My personal opinion is that when you want to use a long mast that extends out
the top of the tower more than 6 or 8 feet and stack another antenna up there
with several square feet of wind loading, this is the time to consider
putting your rotator lower in the tower.  It sounds like your installation is
conservative enough that putting the rotator that low is really not
necessary.

Stan
w7ni@teleport.com





--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com