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[TowerTalk] Elementary question - trig formula

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Subject: [TowerTalk] Elementary question - trig formula
From: (Bill Hider)
Date: Thu, 25 Jun 1998 15:54:21 -0400
There is no such thing as a dumb question.  The answer is also not as
simple as it sounds.  It is:

B is to (C+D) as ? is to C


B/(C+D) = ?/C
? = (BxC)/(C+D)

Just multiply B times C and divide the result by the quantity (C+D). 
The result is = ? Be sure to add a little safety margin for wind, ground
terrain, measurement error, etc.

Tower = 100 feet tall
Proposed antenna attachment point = 70 feet from ground
Guy wires are 80 feet from tower base (Measure this HORIZONTALLY from
the bottom edge of each corner of the triangular tower to each guy wire
anchor point or the point where this measurement intersects with the guy
wire or guy wire extended.  Choose the SHORTEST measurement of the

B=80 (shortest measurement of the three)

C + 70 = 100  Therefore ===> C=30
Substituting: BxC = 80x30 =2400
?=24 feet  (NOTE:  You must subtract any offset of a side mount used.)

If you use a ringrotor (, you must
subtract the distance from the tower edge to the boom cradle center
line.  The ringotor has no offset, unlike a side mount. 

Also include any additional offset due to the beam not being mounted in
the geometric center of the boom. This would be true for sidemounts or

I had to do this 4 times for my tower:  (and
many more times as I iterated the design before I actually installed

Bill, N3RR

Pete Smith wrote:
> At the risk of getting into the dumb question territory:
> If you know how far from the base of your tower your guys attach (on flat
> ground), and you know how high they attach to the tower, then you should be
> able readily to calculate how much turning radius inside the guys would
> exist at any given height on the tower.  Obviously, this would be useful
> for assessing a potential stack design.
>                                                  I
>                                                 /I
>                                         / I
>                                        /  I C
>                                       /   I
>                                      /----I
>                                     /  ?  I
>                                    /      I D
>                                   /       I
>                             -----/----B---I---------
> That is, if you know the lengths of B and C+D, then you ought to be able to
> figure out what "?" is for any length of C.
> Unfortunately, I've forgotten virtually all the plane geometry and trig
> formulas I ever learned, and I didn't keep any high school math books.  I
> think the two triangles involved are called "similar triangles," but that's
> as far as my memory goes.  Can anyone tell me offhand what the right
> formula is for this?  Once I have the fomula I can do the calculations, but
> what to calculate?
> Thanks!
> 73,  Pete Smith N4ZR
> In wild, wonderful, fairly rare WEST Virginia
> --
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