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[TowerTalk] stacking antennas & Rohn specs

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Subject: [TowerTalk] stacking antennas & Rohn specs
From: (Stan Griffiths)
Date: Sun, 13 Jul 1997 02:09:57 -0700 (PDT)
>I am installing several Rohn towers and want to stack antennas on the
towers. Being a "good" amateur engineer, I intend to follow the specs in the
Rohn catalog. My question is about the allowable projected area that is
called out by Rohn. For example, if I were to put up 130 foot of Rohn 45G in
a 90 mph area (coastal Maine), the book says that 12.3 sq ft of projected
area of round elements is allowable (C870478R1). I assume that is at the top
of the 130 foot tower. 
>Now, I want to stack antennas; say one at 60 feet, one at 100 feet and one
at 130 feet. How do I calculate what is the allowable sq footage? Is it as
simple as summing the moment arms (height x sq ft of each antenna compared
against 130 ft x 12.3 sq ft) or is it more complicated?
>For example, a 205CA specs out at 9 sq ft, I would hope that I could put at
least three of these, and 3 155CA's (5.2 feet each) on the tower, but it
doesn't look it. 
>       ((60x9)+(100x9)+(130x9)) > (130x12.3) ........... for just the 205CA's 
>If this is true, how can anyone stack more than 3 large antenna's per
tower? Even a 55G installation only allows 14.8 sq ft @ 130 feet.
>What am I doing wrong?
>Thanks in advance for your ongoing support of my antenna project.  73,  Bruce

Hi Bruce,

I have yet to meet the guy on this reflector who will admit he knows how to
properly answer your question.  Rohn can answer it for you but it will cost
you lots of money, somewhere in the thousands of dollars.  It takes an
experienced structural engineer to really engineer a project of the kind you
are describing.

But, you say, you know other hams who have put up such installations.  How
did they determine they were safe?  I'll bet they didn't.  Most of them have
looked at similar installation that have been up a long time so they used
them as good examples.  They have also studied all the data they could get
on failures so they would know what to avoid doing.  You either have to step
up to a large engineering bill or take some risk.  Studying what has worked
what hasn't will minimize your risk.

Good luck.


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