How to Calculate Proper Guy Wire Distance? - towertalk Forum at eHam.net

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How to Calculate Proper Guy Wire Distance?	Reply
by KB9WIS on November 2, 2009	Mail this to a friend!
Pardon me if this question appears as a simple one, but it's been several years since I was in Geometry class, and I don't want my tower to come crashing down.. For 70' of Rohn 25G tower, Rohn only states to use 56' distance from the tower base to each guy point, with 120 degree spacing between each guy wire. If it was 90 degree spacing, it would be a lot easier to eyeball.. The single guy wire in the front is easy, however I'm worried that the 2 guy anchors in the back (one pointing southeast, the other southwest) are not properly spaced from each other.. I'm really not sure if they're too wide or narrow, of if there is a degree of tolerance alloed.. Anyone know what this distance should be from each other, or the actual formula to compute such, and/or some other way to calculate this distance between guy wires.. Right now I am eyeballing them, with a 75' distance between, from guy anchor B to C (Guy anchor A being the single anchor in the front). I'm surprise Rohn doesn't list this distance between these two guys.. Is eyeballing simply good enough, or is there very little tolerance allowed?

RE: How to Calculate Proper Guy Wire Distance?	Reply
by K9MRD on November 2, 2009	Mail this to a friend!
~97' between each guy point

RE: How to Calculate Proper Guy Wire Distance?	Reply
by WB2WIK on November 2, 2009	Mail this to a friend!
Not sure where the 97' came from. The distance between any two guy points if they are spaced equidistantly will be 117.3 feet walking around a perfect "circle" drawn around the tower spaced 56' away from the tower (this is a 351.86' circumference circle having a diameter of 112'). But if you measure the linear point-to-point distance between any two equidistant guy locations it should measure 79.2 feet. Simple geometry, where: A triangle is formed by the tower and any two guy points. Two legs of that triangle are both 56' long; the remaining leg, the longest one is the hypotenuse whose length will be the square root of the sum of the squares of the two shorter sides. The sum of the squares of the two 56' sides is 6272. The square root of 6272 is 79.196. So, I think the "linear" (straight line) distance between any two guys should be about 79 feet. I did not include the tower cross section in this calculation, since it's quite small compared to the guy lengths; so the error of omission should not impact the results by more than a foot.

RE: How to Calculate Proper Guy Wire Distance?	Reply
by KB9WIS on November 2, 2009	Mail this to a friend!
WB2WIK, That sounds about right.. The one I have in there now has a 75' distance. Do you think this is within a margin of safety, or should I remove and redo..

RE: How to Calculate Proper Guy Wire Distance?	Reply
by WB2WIK on November 2, 2009	Mail this to a friend!
It's probably okay.

RE: How to Calculate Proper Guy Wire Distance?	Reply
by K3GM on November 2, 2009	Mail this to a friend!
If you're looking for your anchor points, it's easy to locate them. It's all about equalateral triangles. All you need is 5 stakes, and in your case, about 230' of heavy mason's twine. 1) At the free end of the twine, make a loop in size that will pass over the stake. Now, make 4 more loops the same size every 56'. S now you have a loop at 0',56',112',168' and 224' 2) Locate your tower center, and drive a stake, drop the second loop over the stake. 3)Now pay the line out on either side of the stake so the 112' length of line is straight. This locates your first anchor point, and the common side of the triangle for anchor #2 and #3. Basically, you now have 3 stakes in a line 56' apart. 4)Loop the end of the line over the center stake, and stretch it out to form a triangle for each remaining point. Drive the 2 remaining stakes in at each location. In a matter of a couple of minutes, you can precisely locate the anchor points. If you don't like the anchor placement, you can easily "rotate" the layout. Just keep that first 112' length straight, and you can't mess up. I seen fellows use transits, lasers, and I'll admit to sighting down the tower legs on my first installation. I've found this to be the easiest, and most precise method.

RE: How to Calculate Proper Guy Wire Distance?	Reply
by WX7G on November 2, 2009	Mail this to a friend!
99'

RE: How to Calculate Proper Guy Wire Distance?	Reply
by K9KJM on November 2, 2009	Mail this to a friend!
Not only do you want the anchors 120 degrees apart, You also need to have the tower situated so each tower leg is "aimed" directly at an anchor so the guy wires are not trying to "twist" the tower. (If a tower is already in concrete and you want the anchors in a direction different from where the legs are pointing, Special guy brackets need to be fabricated.) As mentioned, Simply standing a tower section where you want to put the tower and "sighting" the tower from an anchor location will give a surprisingly accurate, Or at least general idea of where the anchors go. (Just look at the tower from an anchor point, Possibly through a spotting scope, etc, and make sure that there is equal "space" between the tower leg facing the anchor and the two rear tower legs.) Commercial tower construction crews simply set up a transit at the tower location and mark off 120 degrees each way.

RE: How to Calculate Proper Guy Wire Distance?	Reply
by WX7G on November 3, 2009	Mail this to a friend!
Take the 120 deg angle between guys. Divide in half for 60 degrees. We now have a right triangle and can apply some very basic trigonometry. The hypotenuse is 57'. (57')SIN60 = 49.36. To account for the second triangle double it to 98.7'.

RE: How to Calculate Proper Guy Wire Distance?	Reply
by WX7G on November 3, 2009	Mail this to a friend!
I see it's 56' and not 57'. Now we have 97'. WB2WIK you used the formula for a right triangle. A 120-30-30 deg triangle is not a right triangle. Bisect the triangle into two 60-90-30 deg triangles and do it again.

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