[TowerTalk] Rohn HDBX Tower - Boom Length

[K7LXC@aol.com]

In a message dated 98-02-03 12:46:35 EST, N6ZZ@aol.com writes:

> <<     While on BX tower, the HDBX is supposed to take 18 square feet with a
>   maximum boom length of 10 feet.  What kind of a ham antenna has those
>   specifications (other than a dish)?  But Rohn lists them in their "Ham
>   Catalog" and several well known ham dealers advertise them in QST and
other
>   magazines, obviously saying they are intended for ham use.  What "ham use"
>   is consistent with the 18 square foot and 10 foot boom specifications?
>    >>

     How about a nifty VHF/UHF installation? Since the BX series was designed
for TV antenna applications, this isn't too far off.

     The problem with BX is that the legs are stamped (not round like 25G,
etc.) and the X-braces do not overlap. This structure has little torsional
resistance, hence the "maximum 10 foot boom" warning.

    Don't forget that the top sections of the BX, HBX and HDBX are all
different (look it up in your Rohn catalog for more info). Obviously the BX-3
top section for the HDBX is sturdier than the BX-1 or BX-2 on the others. 
>  
>  Funny that Stan should comment on this anomoly.  I am considering the 
> erection
>  of a 48-foot HDBX tower.  In its self-supporting configuration, it is
indeed
>  supposed to take 18 square feet of antenna with maximum boom length of 10
>  feet.  I would like to put up a Force 12 C-4XL (9.6 square feet, 30 foot 
> boom)
>  or, second choice, a C-4SXL (8.4 square feet, 23 foot boom).  The square
>  footage doesn't seem to be an issue, but the boom length of either antenna 
> is
>  of concern.
>  
      An HDBX-48 tower is STRONG and I wouldn't be afraid of  putting the
aforementioned system on it. My experience is that it'll take medium sized
loads as well.

    The second caveat with BX is that the top plate and rotator plate are both
small gauge sheet metal. They won't take the constant torque from a large
system either. I've got a 4 year old HDBX in my bone yard that had a PRO67 on
it (NOT mine!). Both of the plates are cracked and need replacing. Beefing
them up will buy you some additional reliability but these towers have their
limits. 

>  As Stan comments, the value of this reflector is in gathering empirical
>  evidence.  So I seek specific examples of what antennas any of you have put
> on
>  an HDBX tower (square footage and boom length), what wind speeds have been
>  experienced, and how the tower stood up under those conditions.
>  
      Phil, Tarrant county is a 70 MPH wind speed zone, the lowest rating
given. That's the same wind speed used for the BX towers. For your proposed
system and wind speed, my recommendation is to go for it. You won't have any
problems in the foreseeable future. BTW, the reduced weight and windload of
the Force 12 antennas are beneficial to this and any tower system. 

    I would take any direct experiences with a grain of salt. Some people have
been amazingly lucky in overloading their system with no failure (K7LXC
visibly shuddering). Be conservative and you'll be fine. Exceeding the
manufacturer's boom length spec in this case of an HDBX-48 is one of the few
times where I recommend something other than following the manufacturer's
specs. In ALL other cases, DO what the manufacturer says!

73,  Steve  K7LXC

     TOWER TECH -- professional tower supplies and services for amteurs

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