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[TowerTalk] Rohn HDBX Tower - Boom Length

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Subject: [TowerTalk] Rohn HDBX Tower - Boom Length
From: (Stan Griffiths)
Date: Wed, 4 Feb 1998 04:28:38 -0800 (PST)
W7NI says:
>> <<     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
>>   magazines, obviously saying they are intended for ham use.  What "ham use"
>>   is consistent with the 18 square foot and 10 foot boom specifications?

K7LXC says:
>     How about a nifty VHF/UHF installation? Since the BX series was designed
>for TV antenna applications, this isn't too far off.

W7NI says:

Yes, lets consider such a system.  A two inch 10 foot long boom has about
2.5 square feet of wind area.  This leaves 15.5 square feet for elements.
If it is a 6 meter beam, you need about 15 elements on it to come near the
15.5 square feet allowed for elements.  If it is a 2 meter or 440 MHz
antenna, you need a HUGE quantity of elements to get anywhere near a total
of 18 square feet on a 10 foot boom.  Like I said, it just does not compute
. . .

If you start STACKING antennas, you immediately run into another problem:
the 18 square feet is assumed to exist entirely at 3 feet above the apex of
the tower.  Rohn offers no assistance in derating the system if you spread
the load out vertically higher than 3 feet above the apex.

K7LXC says:
>    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

W7NI says:
If the direct experiences involve FAILURES of BX tower, I would pay very
careful attention to them.  "Conservative" is following the manufacturer's
recommendations, but that is admittedly damn hard to do in Rohn's case.
That was the point of my orignal post . . .


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