[TowerTalk] Trig Question

[Kenneth Earl Harker]

Thus spoke Rick Bullon:
> 
> 
>  I need some help doing some calculations for a tower. It has been over 
> 30 years since I took trig in school and I can't remember my formulas.
>   The tower is going to be 70 ft Rohn 25 guyed as per the Rohn book, 70 
> mph wind area guy  wires at 32' and 65' guyed at 56' from tower base it 
> will right now have a TH 6 tribander on it.
> My questions are
>  1. With 2 of the guy points on the property line how far is it from the 
> property line to the tower? ( I need this measurement so I can figure 
> the postion of the 3rd guy point to see if the proposed site is big 
> enough)

* The three guys form an equilateral triangle, with one side following your
property line.  In an equilateral triangle, each angle in 60 degree.

* The triangle formed by the tower and the two guy points on the property line
has two 30 degree angles and one 120 degree angle.  What you want is the 
bisector of the 120 degree angle.

* The triangle formed by the bisector of the 120 degree angle forms a right 
triangle with angles of 90 degrees, 30 degrees, and 60 degrees, with a 
hypotenuse of 56 feet.

* I always remember the trig stuff by the acronym SOHCAHTOA, pronounced 
kind of like some made-up Native American name.  For some reason, this
has always been easy for me to remember.  Broken down this is:
Sine Opposite over Hypotenuse, Cosine Adjacent over Hypotenuse, Tangent 
Opposite over Adjacent.

* We know the value of the hypotenuse, and we also know the value of both 
acute angles - let's use 30 degrees.  So, we want to find the length of the 
"opposite" leg of the triangle (the adjacent leg is the one following your
property line), so we use SOH.  Sine 30 degrees = Opp / 56.  Sine 30 degrees = 
0.5, so we get 0.5 = Opp / 56, or Opp = 0.5 / 56 = 28 feet.

* Thus, your tower center will be 28 feet from your property line.

* The guy point not on the property line will be 28 + 56 = 84 feet from the 
property line.

>  2. Will elevating the guy points make the guys wires longer or shorter? 
> and if so by how much? ( XYL requires elevaterd guy points)

Elevating the guy points will make the guy wires shorter.  How much is 
directly dependent on how high you elevate them.

>   I seem to remember reading on here that "I" beams were stronger than 
> round pipe for elevated guy points right or wrong? 
>   Do I need to use an equlizer plate for this setup or is that over 
> kill? 
>  thanks
> Rick Bullon
> KC5AJX
>   
> 
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-- 
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Kenneth E. Harker      "Vox Clamantis in Deserto"      kharker@cs.utexas.edu
University of Texas at Austin                  Amateur Radio Callsign: KM5FA
Department of the Computer Sciences         President, UT Amateur Radio Club
Taylor Hall TAY 2.124               Maintainer of the Linux Laptop Home Page
Austin, TX 78712-1188 USA            http://www.cs.utexas.edu/users/kharker/
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