Hi Jim,
This gets discussed on here periodically.
jim Jarvis wrote:
> K5PGW Wrote:
>
> I do have one additional "question of the day" and that is if you
> have had any experience in installing a tailtwister rotator near
> ground level by running a mast from the top of a Rohn 25 tower about
> 70 feet tall so that the rotator might be changed or worked on from
> near ground level? I know it has often been done, but I am not sure
> of the type mast, thickness of the material to use and how much
The long mast works like a torsion bar. The flexibility depends on
length, wall thickness, diameter, and material. The longer it is the
more torque will tend to cause it to move laterally so with 60 to 70' of
mast I'd expect to use at least one thrust bearing (can be a sleeve
mounted on an accessory shelf) although personally I'd prefer pretty
much evenly spaced.
IIRC the tail twister is designed to support substantial weight, so I'd
not use any method to remove the vertical load from it unless the load
exceeded the ratings.
The only drawbacks to the bottom mounted rotator are hitting a resonance
point with the combined antenna wind movement, torsion flexibility, and
rotator play. Unless you put something big "up there" or use a wet
noodle for a mast I'd not expect that to be a problem. The other
*potential* problem is the tower normally flexes or torques, but
mounting the rotator at the bottom of the tower and rigidly to the tower
changes that to a shear force on the legs which is the weakest axis of
the tower.
>
> bellying of the mast might occur. I have large thrust bearings that I
> can place near the top and strategically down the tower to prevent
> sway and bellying of the long mast. That would also take all weight
> off the rotator as well as as control the sway and bellying of the mast.
>
>
Preventing the bellying or lateral movement is good, taking the weight
off the rotator is not.
> You may see this as an ambitious project for a 70 year old, but I
> DESERVE IT after over 50 years as a HAM. Your support and
> encouragement is greatly appreciated. 73, John, K5PGW
>
> -0-
>
> John,
>
> W3GM had six 100' Rohn 25g towers, all of which used a ground mounted
> rotor.
> Ham 4 and Tailtwister, as well. They were turning things like a 3
> element full sized
> 40m yagi. The "mast" was 2" irrigation pipe. Wasn't particularly
> heavy, nor thick
>
I hope he had the rotators mounted independently of the 25G. IOW, the
rotator mount affixed to the concrete and not the tower (It did say,
ground mounted). A full size 3L 40 could likely over stress the 25G in
shear with a base mounted rotator attached to the tower. In addition
the 25G is a pretty light tower for that much wind load
I'd not put a 3L 40 on anything smaller than a 45G
73
Roger (K8RI)
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