One triangle, and the ratio that sets everything
A guy runs from a point on the mast to an anchor on the ground. The mast height to that point is one leg, the horizontal distance out to the anchor is the other, and the guy is the hypotenuse. With a 40 foot attachment and anchors 30 feet out, the guy is 50 feet, it leans 36.9 degrees off the mast, and it arrives at the anchor 53.1 degrees up from the ground.
What actually governs the design is the ratio between anchor radius and attachment height, because it decides how each pound of tension is spent. At a radius of 0.75 times the height, as in that example, every pound of tension puts 0.8 pounds straight down the mast and 0.6 pounds into the horizontal direction that is doing the holding. Pull the anchors in to 0.4 times the height and the split becomes 0.93 down and 0.37 sideways — you get less than two thirds of the horizontal restraint and you compress the mast harder for it.
| Anchor radius | Guy length (40 ft attachment) | Angle at the anchor | Down the mast per lb of tension |
|---|---|---|---|
| 0.4 x height, 16 ft | 43.08 ft | 68.2 degrees | 0.928 lb |
| 0.6 x height, 24 ft | 46.65 ft | 59.0 degrees | 0.857 lb |
| 0.8 x height, 32 ft | 51.22 ft | 51.3 degrees | 0.781 lb |
| 1.0 x height, 40 ft | 56.57 ft | 45.0 degrees | 0.707 lb |
This is why anchor radius is almost always the thing that limits a mast on a small site, and why moving an anchor a few feet further out is worth more than it looks.
Guys compress the mast
The vertical component is the part that gets forgotten. Three guys at 200 pounds each on a 40 foot attachment with 30 foot anchors put 3 times 160, so 480 pounds, straight down the mast — before the mast weighs anything, before the antenna is on it, and before any wind. Add a second level and the compression adds up down the structure.
The horizontal components cancel only if the guys are evenly spaced and equally tensioned. Three guys at 120 degrees with the same tension pull the mast nowhere. Three guys where one has been cranked tight because it looked slack pull it somewhere, and the direction is towards the tight one.
What the anchor radius has to be clear of
The layout occupies a circle. With anchors 30 feet out that is a circle 60 feet across and about 2,827 square feet of ground, and everything in it — the anchors themselves, the guys crossing it at head height near the outside, and the fall radius of the mast — has to be free. Neighbouring anchors on that circle sit 51.96 feet apart with three guys.
The fall radius is the constraint that overrides all the arithmetic. Anything raised has to be able to come down without reaching a power line, in any direction, with margin. That is a matter of looking at the site, not of calculating, and it is the single most common way this work kills people.
What is deliberately missing
Any verdict. This page does not tell you how many guy levels a mast needs, what tension to use, what wire to use, how an anchor should be built, or whether the thing will stand. If you enter a rated breaking strength it is printed back to you exactly as typed and is not divided into anything, because a ratio between a breaking strength and a working tension is not a design factor — the factor that applies, the load cases including ice and wind, the fittings and the ground the anchors sit in all belong to an engineer.
Level ground is assumed throughout. If the anchors sit at different elevations, every guy is its own triangle, the tensions no longer balance the way the even-spacing assumption claims, and the horizontal components stop cancelling. Measure each one.
And the failure mode is worth stating plainly. A guyed mast is held up by its anchors and by nothing else. When one anchor moves or one guy lets go, the mast does not lean over gradually — it comes down, quickly, across the whole fall radius. Erection, climbing and tensioning are specialist work with fall protection, and the permit and the design belong to the building department and to whoever engineered it.
Questions people ask
How long does a guy wire need to be?
It is the hypotenuse of a right triangle: the square root of the attachment height squared plus the anchor radius squared. A 40 foot attachment with anchors 30 feet out gives 50 feet exactly, before any allowance for thimbles, grips and turnbuckle take-up. This page adds a termination allowance you set for each end, defaulting to 3 feet per guy, so the buying figure comes out at 53 feet.
How far out should the anchors be from the mast?
That is a design question this page will not answer, but it shows you what the choice costs. The ratio of anchor radius to attachment height sets how each pound of tension divides. At 0.8 times the height, each pound puts 0.781 pounds down the mast and 0.625 sideways. At 0.4 times the height it is 0.928 down and 0.371 sideways — nearly a third less horizontal restraint and more compression to get it. What the installation actually needs comes from whoever engineered it.
Do guy wires push down on the mast?
Yes, and it is usually the surprise. Three guys at 200 pounds each, attached at 40 feet with anchors 30 feet out, put 480 pounds of compression straight down the mast before the mast, the antenna or the wind contribute anything. Bringing the anchors closer increases it: the same three guys with anchors at 16 feet would put 557 pounds down.
Does this tell me whether my guy wire is strong enough?
No, and it will not. If you enter a rated breaking strength it is printed back exactly as you typed it and is not compared with anything. Adequacy involves the design factor that applies, the load cases including wind and ice, the fittings, the anchors, the ground they are set in, and the mast itself. That is work for a structural engineer, the mast or tower manufacturer instructions, and the building department.
Does the arithmetic still work if my ground slopes?
Not properly. Every figure here assumes all the anchors sit at the elevation of the mast base. On a slope each guy becomes a different triangle with a different length and angle, the tensions no longer balance the way the even-spacing assumption requires, and the horizontal components stop cancelling — so the mast is being pulled somewhere even with every turnbuckle set the same. Measure each anchor separately and treat them as separate cases.