> From: K7LXC@aol.com
To: <towertalk@contesting.com>
> Date: Sat, 7 Dec 1996 19:08:25 -0500
> To: hartley@smart.net
> Cc: towertalk@contesting.com
> Subject: Re: HDBX help
In messages dated 96-12-07 it was written in part:
> >I was wondering if you had any ideas on how a Rohn HDBX-40 could be extended
> >in height to maybe 48ft? I would like to use mine as a top loaded vertical
> >and
> >a little extra height would be welcome.
> > It seems that Rohn used to sell straight sections, but no more.
> Hi, Chuck --
>
> The Rohn BX series consists of eight 8-foot sections. I'm not sure what
> an HDBX40 is since I thought all they marketed was the HDBX 48. You should
> have the Rohn catalog. Then you could measure the sections and see which
> ones you have. Then all you have to do is get the next smallest section and
> you'll be up to 48 feet. The total wind loading depends on which sections
> you're using.
>
> HDBX48 sections 8 thru 3 18 sq. ft. wind load capacity
> HBX56 sections 8 thru 2 10 sq. ft.
> BX64 sections 8 thru 1 6 sq. ft.
>
> TOWER TECH has the Rohn catalogs for $5.00.
>
The HDBX40 is merely sections 8 through 4, also rated at 18 sq ft, if
memory serves (IMS?). There is also an HBX48, which is sections 7
through 2, and an HBX 40, which is 6-2. There are also BX towers for
those heights, consisting of the top 7,6, and 5, respectively.
Be wary of using straight sections. The engineering and physics that
makes the BX type (or any self supporting tower) work is the
distribution of lateral forces at the top to a constantly increasing
cross section of support; hence the "tapered" tower.
By inserting straight sections, you significantly decrease the
ability to distribute those lateral loads to the base, thereby
creating a weak point.
I am not an engineer, so I don't know what percentage of capability
one decreases the tower rating by when adding a straight section, but
my experience with BX type tower suggests that getting 18 square feet
of antenna on a 10 foot boom requires a lot of imagination.
By the way, the 10 foot boom length limitation is a relatively
recent addition; Rohn, in an attempt at limiting liability has been
ever increasingly derating their products. It makes perfect sense
from an actuarial or management stand point, but it makes our jobs as
amateur antenna system designers difficult.
Every ham's first question when it comes to towers is, "what'll it
REALLY take?"
A professional installer has no choice but to trumpet the company
line.
I agree with the comments on the worth of adding one 8 ft section.
73, Rod N4SI
The DXer formerly known as N9AKE
(c) 4 November, 1996
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