[TowerTalk] Recommendation for mast on bottom mount rotator

[K7LXC at aol.com]

In a message dated 9/16/2006 9:02:29 A.M. Pacific Daylight Time,  
towertalk-request at contesting.com writes:

>> I'm going to move my rotator to the bottom of my 80 foot  tower and
> would like recommendations for the mast. I will support it  at the top
> with a thrust bearing, and at least one, possibly two,  non-thrust
> bearings part way down to prevent side to side  movement.
>
> My question is what would be a good mast material?  It would only have
> to support it's own weight and I'm wondering if 2  inch water pipe

>  Remember this mast has to do basically what  the top mast does. It is one 
long torsion bar.
That and with a thrust  bearing at the top the tower will end up supporting 
all the extra weight  of the masting.


        Umm, I'll go with the long  torsion bar. BTW, mounting antennas 
alternately up a mast goes a long ways to  minimizing torque since they'll cancel 
each other out. (Article in QEX or QST by  Dick Weber, K5IU, PE.)
 
        But the mast sticking out  of the top of the tower is naked and is 
subject to sometimes large forces. If an  antenna one foot above the top of the 
tower has x force on the mast, then  the antenna up ten feet will exert 10x 
bending moment on the mast; forces are  becoming significant. Your captured mast 
inside the tower doesn't have anything  like that so you can use more 
ordinary materials for this masting. 
 
        I'm not an engineer so  can't give you any calcs or specifics 
unfornately.
 
Cheers,
Steve    K7LXC
TOWER TECH