[TowerTalk] Mounting in the tower

Kurt Andress K7NV@contesting.com
Sat, 08 May 1999 23:03:21 -0700

Shane wrote:

> > I guess my question should have been....How is everyone mounting their
> >  rotors on these stacked HF beam setups.  I can't possible see anyone
> putting
> >  up two HF beams on a mast and placing the rotor 12" below the thrust
> bearing.
> >

Steve wrote:

>        A stack of HF beams on 25G? Must be small HF beams.
>        But you're right. Everyone is mounting the rotator in the one other
> spot where it can be mounted - ten feet down where the section joint is.
> Cheers,   Steve    K7LXC

This is good advice!
What "everyone is doing" is the better choice of the available solutions.

We may whine about the fact that the Rohn rotor plates only fit at one of two
places in the top section. I wished they only fit at one place, the lower one,
to prevent people from putting the rotor way up at the top of the tower!

Common sense should lead all of us to the same conclusion:

We put a bunch of mast and antenna loads above the tower top, and then expect
to counteract them with a very short couple, over 12" at the top of the tower?

Simple mechanics tell us that the short distance between the tower top and the
top mounted rotor will generate spectacular bending moments in the top of the
tower section!
These are the things that cause people to report weld failures etc.

Don't expect too much improvement by putting a pointy top section in place of
the flat top section. In many instances it can be worse than the flat top.

A longer distance between the tower top and rotor will reduce the local tower
top member stresses. The longer span allows more mast deflection inside the
tower, which presents more angular rotation at the mast/rotor interface. So
the rotor capability enters the equation.

Intermediate mast support, between the top and rotor helps the rotor but
increases the tower stress.

None of it is really easy to guess at, without going thru all the analytical
motions, which just leaves one with another layer of questions.

Like so many other things, the correct answer depends on the exact details of
what one is trying to accomplish.

Many other issues surrounding this discussion, but they will survive till
another day.

Today, the best idea is to spring for the longer mast that will allow you to
put the rotor down into the tower instead of putting it at the top.

Putting the rotor at the top is like taking a knife to a gun fight!

I think Rohn provided that option for people who wanted to put cheap rotors
and whimpy antennas to get the "Honeymooners", on syndication from outside
their area.

Don't think people on TT are after trying to accomplish that. But, it can be
an entertaining alternative.

73, Kurt, K7NV

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