I'm not familiar with the Rohn HDBX 48 ft tower, but the only advantage that
I ever saw to having your rotor located at the bottom of the tower was just
the fact that you didn't need to climb the tower to work on it or replace it
if it became damaged. Placing it at the bottom would require that you also
install a couple of plates so you can use thrust bearings to help hold the
large amount of weight off the rotor. If you want to lower your rotor down
inside the tower another 8 ft then just make certain that the plate the
rotor set on is secured to the tower so it has no movement, I suggest
welding it in place.
However, damage to the mast from wind usually occurs above the masthead and
I have found that using a high quality thick walled mast provided as much
protection as one could have from ever coming out to see your mast bent and
your beam pointing to the moon. One thing that I highly recommend is that
you mount your beam at 3 ft from the masthead and no higher. If you mount
any higher than in high winds a much larger load is placed on the mast and
the chances of it bending will be increased.
I also run my mast about 20 ft above the antenna and mount a porcupine on
top for static discharge.
Hope this helps.
73
John, KA9OFM
On 7/15/06, Paul Ferguson <Paul@paulferguson.us> wrote:
>
> I am putting up a Rohn HDBX 48 foot tower. I will put a 3 element
> SteppIR on it.
>
> The SteppIR specs are 42 lbs, 6.1 square feet of maximum wind load,
> and 16 foot boom.
>
> The HDBX 48 antenna load specs at wind pressure of 20 psf (70.7 mph)
> are loads of area = 18 square feet and thrust lbs of 360. There is a
> note for all BX type towers: "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."
>
> At my location, there are tall trees surrounding the tower that will
> offer some wind protection, but there is no way to quantify this
> factor.
>
> I remember hearing some have located the rotator at the tower base so
> that the torque is transferred to the larger tower legs. I do not
> want to use a 50 foot mast, but I am considering lowering the rotor
> plate for my Ham IV rotator. The rotor plate supplied is located
> about 3 feet from the top of the top 8-foot section.
>
> Would there be much advantage to lowering the rotor plate down to the
> next lower 8-foot section?
>
> Has anyone done this and have any tips on fabricating a new larger
> rotor plate?
>
> 73,
> Paul
> K5ESW
> Raleigh, NC
>
> _______________________________________________
>
>
>
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