HDBX help

[n4si]

> From:          K7LXC@aol.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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