[TowerTalk] Sidemount yagis

[K7LXC@aol.com]

In a message dated 04/05/2000 1:31:53 AM Pacific Daylight Time, 
K7NV@contesting.com writes:

>  An important thing to note about the example cited here is that the
>   guinea pig was a catalogue Rohn configuration meant to have the entire
>   antenna load at the top. The guy cables on that tower are sized for a
>   concentrated top load. Placing part of the rated antenna area at a
>  lower
>   location resulted in the cable near the low antenna elongating more,
>  and
>   the upper one less. This caused the loaded tower to get out of column,
>   and close to the same limiting base stress was created. There is
>  another
>   example that shows that increasing cable sizes fixed the problem and
>   left some room to increase the loads and hence the antenna areas, to
>   arrive at a total that is more than the original.
>   
>   Remember, this is just what happened with this tower and these changes.
>   other initial configurations may behave differently.
>   
>   The one example just showed why the tower dudes said to use the same
>   total area regardless of placement. They sure know that they don't know
>   what the user is gonna do about changing guy sizes, so their statement
>   is safe and correct.
>   
>   There is a limited range of optimum solutions for every configuration,
>   so we've got to be careful about forming broadly generalized
>   conclusions.

    Another "important thing to note" is that the Rohn drawings (and 
calculations leading to those drawings) just represent ONE scenario. Guys can 
be at different ratios than 80%, guy wires can be bigger, antenna loads can 
be distributed all over the tower, you can install 3-foot off-axis 
sidemounts, elevated guy anchors, etc. It's just that additional calculations 
will have to be done for any other configuration. That's why when you ask an 
engineer a question, he'll most likely say that "It depends".

     The abuse that 45G in particular will take is well documented. How far 
over or close to the 'edge of reliability (sanity?)' these installations are 
probably hasn't been calc'ed (maybe you don't WANT to know - hi). That's why 
you see recommendations  from TowerTalkians to seek real engineering and 
PE-type help when you're contemplating doing something 'different'. 
"Hip-pocket" engineering is totally discouraged.

Cheers,   Steve   K7LXC
Tower Tech

--
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