Formula for lenght of mast verses distance of anchorpoint from mast...

[tallship]

There is a formula, and I believe I read it years ago in my ARRL operators manual, or perhaps my tech or general study guide, that has to do with the angle of an inverted V, as well as the distance from the mast that the anchor points of your di-pole will be, depending upon the height of the mast.

In my case, I'm using an MFJ G5RV which has each leg of the di-pole cut to 51 feet.

My question is : what is the formula to determine how far out the anchor points will be from the mast, given a particular height of mast?

Also, what angle will this inverted v be, given a particular height of the mast and distance of the 51 foot anchored elements?

Thanks a lot, coz this will help me to determine how high I want to have my mast, in order to acheive an optimum angle for my inverted V, on the roof of my commercial building.

Kindest regards,

Bradley D. Thornton
kd6ncg




.:headbang

 

[tallship]

Also the formula for cutting lengths of guy wire

My question is : what is the formula to determine how far out the anchor points will be from the mast, given a particular height of mast?

Also, what angle will this inverted v be, given a particular height of the mast and distance of the 51 foot anchored elements?

.:headbang

Okay, this is also the same formula used to determine how long to cut your guy wires, provided you know how far the anchor points should be and the heights of the guying points on the mast.

 

[W5LZ]

Quick-n-dirty...
Triangles, finding the lengths of each side.
Sides: A, B, and C. 'C' is the long side.
C = SQR(A^2 + B^2)

- 'Doc

 

[Moleculo]

The Pythagorean theorem. A = height of tower (or height to guy points), B = distant to anchor points. Calculate for C like 'Doc showed you.

As long as you want to calculate this with the tower or mast on flat ground, the formula works perfect. If you're installing on the apex of a non-flat roof, it won't work.

 

[rfoverlord]

Well i remember reading somewhere that is was always 80% of the height of the tower away. I will try to find it and update this. here is some actual info if this helps.
http://www.tpub.com/steelworker2/88.htm

 

[W5LZ]

How 'steep' the guys are, or how far from the base of the tower those guy posts are, depends on a number of things. Average wind, consistency and make up of the dirt, total height of the tower, and just how much of a 'safety factor' you want to build into the thing. The 'best' angle for those guys is 45 degrees. But, that just depends on how much room you have before getting to the "Can't get there from here" point in tower height. The "you better watch that!" angle, or where you'd better quit is something like 60 degrees for those guys, and that means with a not so tall tower, kind of. The best practical guide is the examples given by the various tower companies in relation to their towers. But unless you have one of 'their' towers, it's only a -very- rough guide. There's a huge number of variables in that equation so it's always better to error on the safe side. Another good piece of advice is to over-build to some ridiculous extent. It costs less in the long run.
- 'Doc

 

[Captain Kilowatt]

There is no hard and fast rule that states how far an anchor should be from a mast. It all depends on how steep of an angle you want or what the design of the mast can handle. The general rule of thumb however is that the anchor's should be 2/3 the height of the mast so if you had a 40 foot mast then the anchors should be about 26 1/2 feet away.You have some leeway with however and 25 feet would be fine.

 

[Beetle]

The Pythagorean solution (a² + b² = c²) only works accurately if the vertex angle is 90°. After that it becomes less precise unless you use a bit more complex math.

 

[W5LZ]

Beetle,
A slightly more complex kind of math, or just smaller and smaller right triangles untill you get sick and tired of the whole thing. I've found that the easiest way for me is to get close with that Pythagorean solution thingy then add a few feet just 'because'. I usually end up with some left over, but that comes in handy to use to beat myself with because I got too much. Beats not having enough, cuz then you have to beat yourself with your imagination, right? The one time I got so close that I only had a tiny bit left over, I dropped the @#$ thing in the grass and couldn't find it! Now THAT is the pits! Oh well...
- 'Doc

 

[tallship]

Wow, so many fantastic answers, each providing a bit more information that I both needed, and was afraid to ask for, since - well, as pointed out there are a lot of factors.

Fortunately, the roof is a flat roof on a commercial building w/raised edges that have strong beams to facilitate mounting the eye-carriage bolts.

My daughter's been challenged w/coming up with optimum placement of the guys, mount, and driven elements based on degrees and the dimensions of the roof. She's taking algebra so it's a kewl, and applied (as opposed to pure) problem.

One of those, "Yup, I told you we use this stuff everyday - so get good grades in math!"

um... she's also 12yrs old

 

[Jay in the Mojave]

Hello Tallship:

Yeah what those other guys said!

Have you ever nailed your Calculator to the wall in the Garage because there's just too many numbers or your just slow. I have!

Heres a good rule of Thumb way of doing that in your head. If your mast heigth is say 30 feet high, then the reqired guy wire length at 30 feet from the mast will be 1.41 times 30 Ft, which equals 30 + 30 x .4 or 12 + 30 = 42 Ft. Which will be for 100% of the mast heigth, which should be long enough for a 80% guy wire length of the mast.

So when buying guy wire rope you will have a good idea as to how much to buy. Nothing like having a antenna raing party to be short on guy wire, then all the guys get their beek deep into the Goon Juice.

Jay in the Mojave