[TowerTalk] Ground level rotator mounting

[Roger (K8RI) on TT]

On 9/6/2013 9:38 PM, Doug Wetzel wrote:
> When I put up my HDBX-48 in the spring, I'm thinking of mounting the
> rotator at ground level.  Has anyone on the list done something similar?
> What did you use for mast all the way up the tower, and did you use

Be careful about mechanical resonances. A long mast is in effect a 
torsion bar that with its flex, the inherent slight give in the rotator, 
plus the mass of the antenna May result in a low frequency oscillation.

You may or may not end up with a resonant condition.. If you do, the 
condition may or may not be destructive.

IOW you are now in unexplored territory unless you can find someone who 
has use the same rotator, mast, and antenna combination.

Many years ago I built a simple, laminated, wood mast.  It worked quite 
well for a while, but one windy day I noticed the mast was oscillating. 
You could easily see the node and nulls.

As the wind increased the nodes became larger and the speed was so fast 
that the nodes looked like a solid mas several feet across. IIRC the 
nodes appeared to be nearly 3 feet across before it let go.  The longest 
piece left was about 3 feet long.

Now this was a different material and a different mode of oscillation.. 
If a mast went into severe oscillation I'd expect the destruction to be 
confined to either the the rotator, or the antenna.  Possibly both.

I've had relatively minor oscillations of a relatively low frequency 
with a 40 foot mast. The antennas were swinging about 2 feet in 
azimuth.  It did destroy a Ham-IV and an HDR300 before I installed the 
PST-61

> multiple thrust bearings to align the mast?

Typically they only keep the mast aligned and do not support weight 
except for one.

73

Roger (K8RI)

> Thanks in advance.
>
> Doug
> K7IP