In a message dated 96-10-06 12:22:10 EDT, you write:
>Has anyone had experience with 48ft HBDX Rohn freestanding tower? I have
>a light tri-bander (A3S) up there now 2 feet above the tower. I am
>contemplating adding a 40-2CD on a new mast.
>Cushcraft advised that the 40-2CD should go above the A3S by at least 8
>feet but I am considering reversing that and putting the heavier 40-2CD 2
>feet above the tower and the A3S 8 feet above that. Has anyone had
>any experience with this combo?
>The Rohn catalog indicates that the HBDX is not designed for antennas
>with boom lengths greater than 18 feet. My tower has one set of guys
>which I believe will reduce the twisting effect.
>
Hi, Mike --
Pushing the envelope again, eh?
Rohn BX-series was designed for TV antennas and lacks the structural
design that would make it more useful for ham HF yagi systems. While the
HDBX (heavy-duty BX) is a pretty strong tower, Rohn gives it 18 square feet
of antenna capacity but limits the boom to a maximum of 10 feet "and the use
of such boom lengths is not recommended". Your installation would have 37
feet of boom.
Nonetheless, many BX towers do not meet the Rohn specs and have been up
for years. Since you have it guyed somewhere, you have probably given it
some additional help but it depends on where the guys are attached on the
tower. Don't forget that it wasn't designed to be guyed.
For a large-ish mast and stacked antennas on your HDBX, one of the crux
bits is the bent tin plate that is supplied as the top plate of the tower; it
probably isn't up to the task of holding your proposed system against big
side forces. If you beefed it up, that would help. Are you going to install
a thrust bearing?
As far as your proposed configuration, I would mount the 402CD at the top
of the tower (not 2 feet up; the moment increases significantly when you do
that) and the A3S 6-7 feet above that.
According to my SAM callsign database, you're in King County (I thought
Mill Creek was in Snohomish). Anyway, King county is rated at 80 MPH wind
zone and Snohomish is 75 MPH. The "typical" calculations in the Rohn catalog
for BX64 uses a wind pressure of 20 psi which I think is around 64 MPH. You
would have to decrease the loads to make your installation compliant for the
higher wind speed. A PE would probably croak at your proposed installation.
Cut to the chase, you could go ahead with your project and "probably" be
okay reliability-wise but it certainly exceeds the manufacturer's specs and
engineering. You pay your money and take your chances.
Options? How about going for a Force 12 C-4S? It's got 40M capability
in addition to 12M and 17M coverage, it's all on one boom so you don't have
to worry about the stacking problems and the boom is only 12 feet.
73, Steve K7LXC
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