[TowerTalk] Rotor Location

K8RI on Tower talk k8ri-tower at charter.net
Sat Apr 15 13:35:17 EDT 2006

A tower is like a very large torsion bar.  It has lots of give for rotation 
with the guy lines serving as dampers that also limit the travel.  When the 
rotator is moved closer to the bottom of the tower the moment changes from a 
twisting to shear.  If this is not taken into consideration the system can 
end up with undue stress  on the tower legs.  In most systems which are 
lightly loaded this isn't a problem. However the mast will serve as an 
undamped torsion bar with only the rotator being a limit at one end.  Like 
any spring system you do have to be careful of resonance and the amount of 
travel allowed. A long mast should have thrust bearings, or sleeves to hold 
it *centered*. *One* thrust bearing may be necessary for support if the 
weight is near the limit of the rotator's ability to support.   With the 
more massive antennas this becomes a trade off between the amount you are 
willing to let the antenna swing in the wind and how much weight (and 
length) you are willing to put into the mast.

Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2

> I'm seriously considering moving my rotor from the top of my tower to a
> spot in the bottom section so that it will be easier to reach for
> maintenance.
> I'm not worried about the cost of the additional mast, but what other
> considerations are there?  Is there any reason not to move it?
> 73 de N8AU, Jim in Raymore, MO
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