Larry asked:
<< As I try to lay out possible guy anchor locations,I wonder, exactly how
much "leeway" do I have for
The angle with respect to each other? (nominal 120 degrees)
The distance between anchors and tower? (nominal 80%)
I mean, theoretically, I can get away with a steeper guying angle by
using stronger guys and supports, right? And if the angles were
110-130-120 instead, what's the harm? >>
As far as the angle with respect to each other, (nominal 120 degrees); When
I laid out my anchor placements, I used a tape measure to measure both the
distance from the base to the anchor (all three equal to each other, then
measured the distance between each anchor point (again, all 3 equal). There
is only one placement that satisfies all six measurements. If you're within
a foot or two of the exact spot, I think it should be fine. You could spray
paint arcs on the ground in the rough positions for your chosen
anchor-to-base spacing, then trial and error with temporary markers on the
arcs until all the anchor-to-anchor distances are within a foot or so. This
is all assuming relatively level ground. If you have sloping terrain, I
would do the geometery and vary the anchor-to-base spacing to keep the
angles between the guy and ground as equal as possible. Comments from those
who have done this?
As far as the distance between anchors and tower (nominal 80%) is concerned,
there is plenty of leeway, depending on your tower type, strength, and
loading. If you are going to vary from one of the pre-engineered solutions
in the Rohn catalog, I think the best practice is to calculate the windload
and guying forces and make sure they are compatible with your guying
hardware or have a professional engineer do this for you. The ARRL handbook
gives a general guideline of 60% to 80% of tower height, which I think is
still a good rule of thumb.
I would try to have all base-to-anchor measurements as equal as possible
(level ground), but if your tower plan is much shorter than the maximum for
your tower type (190' is the max for Rohn 25, and you are going up 40-60 ft,
for instance), variations of several feet shouldn't be much of a problem.
I've seen short towers (60 feet of Rohn 45, which can go far higher) go up
fine with two anchors out at 35 feet, and the third at 25 feet from the
base. The guys on the short anchor have more tension to have the same
horizontal component as the other guys. It can work, as a custom engineered
solution, but I don't recommend it for general practice.
As far as upgrading the guy hardware, I wholly recommend it! My tower was
Rohn 25, 110'. The Rohn catalog shows a pre-engineered solution using 3/16"
EHS. For my installation, the greatest danger is not wind, but another
nearby tree or a limb falling over on one of the guys. This appears to be a
common mode of tower failure. I chose 1/4" EHS, and the appropriately rated
turnbuckles, thimbles, grips, etc, for the larger size. I think it's
generally accepted that modest over-engineering is good. It certainly is
better than NO engineering, which is why we are all here!
That ought to get the gears turning, and I'm sure there'll be more comments.
Good luck!
--...MARK_N1LO...--
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