[TowerTalk] Rotator Placement Question

[K7LXC@aol.com]

In a message dated 10/4/00 10:46:28 AM Pacific Daylight Time, 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)

    Well, unless you're going to be really aggressive about the antenna array 
on top, you're not going to be able to exert enough torque to do any damage. 
Since your windspeed zone is only 75 MPH and the tower is rated at about 
10-12 sq.ft. in your scenario, I wouldn't worry about it. 

    While the antenna/mast array obviously exerts several different stresses 
on the tower, the vertical mast and antenna overturning moment is largely 
contained in the tube at the top of the tapered section and is spread through 
the welded flanges and the legs. My intuitive answer is that it'll take 
several times more stress than you'll ever subject it to. 
>  
>  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.
>  
    A 10 foot two-inch mast is about 1.7 sq.ft. of windloading. (Less if you 
consider the reduced windload of a cylinder.)

>  Which is better? Rotator at the top, or bottom?

    IMO it doesn't make any difference. You're splitting hairs (hares?) for 
something that is pretty inconsequential UNLESS you're really aggressively 
(over)loading the tower. If it was my tower, I'd go for what's easiest. I'd 
vote for the rotor mounted in the  tapered top section. 
>  
>  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%.
>  
    Since the tower manufacturers typically use a safety margin of 50%, I'd 
say that anything you do will be fine. This system will exert very moderate 
stresses on a properly installed tower system that's designed for much more 
than you're going to do to it. 

Cheers,   Steve   K7LXC
Tower Tech 

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