In a message dated 97-07-09 09:14:51 EDT, k1my@nlis.net (Bruce Makas) writes:
> 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.
If you read Note 4 in the General Notes, you'll see that Rohn has
already deducted 8.0 sq.ft. for a symmetrical commercial antenna mount. It's
my understanding that if you take the 12.3 and add the 8.0 back in that the
total round-membered tower capacity is 20.3 sq.ft.
>
> 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?
>
I have interpreted the tower capacity as the total antenna load
regardless of where it is installed on the tower. Obviously it's not always a
load on top of the tower (called head-load).
Some engineering types might choose to split hairs with this
information. Fine. If you use the Rohn data and do everything to spec, IMO
you'll be building a safe, reliable tower and antenna system that is within
the manufacturer's limits.
>
> 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.
>
The 55G as well has the aforementioned Note 4 so you can add 8.0 sq.ft.
to its specs.
> What am I doing wrong?
>
Well, you must not be overloading it like most hams do!
73, Steve K7LXC
TOWER TECH -- professional tower supplies and services for amateurs
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