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## [TowerTalk] stacking antennas & Rohn specs

 To: [TowerTalk] stacking antennas & Rohn specs K7LXC@aol.com (K7LXC@aol.com) Wed, 9 Jul 1997 10:02:28 -0400 (EDT)
 ```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 -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search.htm ```
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