> Has anyone used Rohn 25 section(s) instead of a boom pole on top of a
larger tower?
Please explain what a "boom pole" is. I don't know what that means. Do
you mean a mast, which is a round pipe sticking out of the top of the
tower where the rotatable antennas are mounted? Typically a boom is the part
of
an antenna that the elements attach to.
> I have a Tri-EX HZN 471 that the previous owner configured to lift to
66' instead of the factory 71'. He also used a 2" diameter boom pole
extending about 18' above the top of the tower with a large antenna on
top of that.
> I have erected the tower and discovered it would be better if I could
obtain some additional height. I am placing around 3 square feet of
antenna load on the tower.
Is that the total antenna load?
> I am considering the possibility of removing the top bearing,
installing
a Rohn 25 base plate then installing 3 sections of Rohn 25. I would
also add a second set of guys at the top of the Tri-EX and secure the
retractable sections to each other to limit rocking between them.
Bad idea. The load capacity of a crank-up is limited by the strength
of the cables. The cables have already been engineered for the tower as it
comes from the factory. Putting another 130 or so pounds of additional
vertical load is problematic. The vertical load is much larger with wind
pressure on the tower which translates into compressive load on the supporting
elements. Securing the retractable sections may help but you need to be very
careful about how the sections are secured.
You didn't mention where you're located since you have to take the
local windspeed conditions into account for any tower project. Also, even HD
crank-ups are hard pressed to be rated for 90 MPH or above.
> I believe the wind load produced by the Rohn 25 would be similar to
that
of the 2" boom pole and that my total wind load would be far less than
had been placed on the tower for the last 35 years. By securing
(clamping) the retractable sections and adding an additional set of guys
I believe the tower would be much stronger than the original design.
Well, you'd need to be pretty careful with your "back-of-envelope"
engineering, particularly since crank-ups are more structurally limited than
other fixed towers.
As far as the 25G section windload, it's got 3 - 1.25" OD legs plus a
bunch of 5/16" braces. I'm surmising that it's a bunch more of windload
than a single 2" mast.
If you're a clever engineering-type and a clever fabricator, you may
be able to get away with this but I would be very hesitant about pursuing
this.
Cheers,
Steve K7LXC
TOWER TECH -
Professional tower services for hams
Author of UP THE TOWER - The Complete Guide To Tower Construction
available from _www.championradio.com_ (http://www.championradio.com)
Cell: 206-890-1488
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