H John Kohl wrote:
> The interesting comment from the Rohn spec sheet "DWG. N). A-760001R1"
> for the BX towers.
>
> Note: Antenna types should be limited to those having a maximum boom
> length of 10 feet. No engineering data relating to the use of boom
> lengths in excess of 10 feet is available and the use of such boom
> lengths is not recommended.
>
> Nice disclaimer.
>
> 73 John KO4A@juno.com
>
> --
> FAQ on WWW: http://www.contesting.com/towertalkfaq.html
> Submissions: towertalk@contesting.com
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A friend of mine had a 2 element 40 on a 48 ft HBX. He mentioned that it
would wrap and upwrap about 15 degrees in torsion under certain wind
conditions.
One time I was up replacing the rotor for him (can't imagine why he had
brake failures on a Ham IV in this application) I decide to do a little
test.
DO NOT DO THIS AT HOME OR IN FRONT OF YOUR SPOUSE!! She may have you
committed.
The antenna was sitting with on the top thrust bearing and the
disconnected from the rotor.
I was at the top of the tower waiting for him to find some more rotor
mounting bolts (two had worked out and disappeared).
Being bored and curious - I had trouble believing the 15 degrees - I
grapped two tower legs, one leg in each hand, and started rocking from
left to right and back. After about five cycles, I noticed that I had
the tower twisting about 10 degrees with a minimum of effort. I was
really surprised at the lack of torsional rigidity of the tower. It was
extremely flexible in torsion.
After getting the new rotor installed and me down and off the tower, I
got to looking at the tower construction to figure out why it was so
flexible. My conclusion is that the legs are open sections, not tubes,
so you have almost no torsional rigidity there. The diagonal braces do
not fully triangulate the tower - pairs of braces are spaced about 6
inches above each other. But the most telling is that the braces have
almost no compressive load capability because they are flattened out
right where they cross.
The tower is fine as a vertical where the loadings imposed on it are
pure bending, but torsion from a long boom moving into and out of an
oblique wind really get it oscillating. So I really do believe the
factory when they say 10 booms - even for VHF and UHF antennas.
Another concern is a rusty HBX. How do you take care of the rust when
you have aluminum rivets holding it together? If you sand blast it, you
will destroy the rivets. If you use a chemical rust treatment it may eat
the rivets. And I think I heard it on this reflector, if you regalvanize
it, you will melt the rivets and poison the galvanizing tank with
aluminum. Ever buy 5 tons of zinc? You can bet it is expensive.
de n0yvy steve
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
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