Donald Hofmann wrote:
Concerning guy wire spacing, from what I have read
> 60-80% is ok. Mine would be right at 70%. The ARRL Antenna book
> recommends the guys at the top of the tower instead of 5' down as
> Rohn says.
The lxc prime directive is "do what the mfr says".. does the ARRL book
give a reason why and any analysis to support it.
The shifting soil problem is not fixable. Houses in this
> area crack and shift. A new house next to me has concrete piers 20'
> deep and their house still moves.
So that's what you need for your antenna. If folks are doing deep
piers, then there's probably an established infrastructure and vendors
around to drill the holes build rebar cages that fit, etc. (that's what
they do here in Southern California for things like traffic lights, etc.)
> Regarding guys: I would suggest taller guy posts--or further in the>
> ground than 3 ft w/4 ft out. Depending on the post material, I'd>
> certainly make certain I belled out the bottoms of those holes,>
> especially considering the tower has already shifted! Indeed, I not
> use> a round post, but an I-beam or channel instead--to help prevent
> such> shifting.
The cross-section shape of the post isn't going to make much difference
in the shifting, either it's big enough to not bend, or it isn't. An
engineer can calculate the loads and size it appropriately for whatever
you use, and round pipe is cheap and easy to work with. I-beam or square
tube, angle or channel will be orientation sensitive (not the same
strength in all directions), but does give you flat flanges to drill
holes in and attach stuff. Really, it all comes down to the cost of the
steel, and structural stuff is priced by the pound, whether it's round,
square, or I shaped.
Taller posts won't necessarily help. The load on the guy wire is really
determined by the angle between the point of guy attachment and the
point it ties into the ground. A taller post doesn't move the ground
point out, it just puts a "bend" in the guy wire. And, of course, a
taller post makes the "lever the anchor out of the ground" and the "bend
the post" problem worse. Take home point: the post doesn't reduce the
loads, it just helps with a guy-ground clearance problem.
If you do use posts, consider putting some rebar
> inside along> w/concrete.
There's not a good reason to put rebar in there. You're looking at a
bending beam problem, and the bending forces are maximum at the outside
edge of the beam. That's why a round pipe with the same metal cross
section as a smaller solid bar is stiffer and stronger. e.g., why tent
poles are tubes and not solid rods. If you need more strength, use a
bigger diameter pipe or beam.. you're paying for the steel either way,
so use it effectively. (On the other hand, if you've got a cheap supply
of rebar.. but why not weld it on the outside of the pipe, then..)
Concrete inside is popular for a couple reasons:
1) concrete is strong in compression, so if you're loading the post in
compression it helps
2) concrete fills the inside, and keeps the inner surface from rusting
(probably the best reason to do this).
3) in some applications, the extra mass helps, and concrete is cheap.
4) If the tube is really flimsy, the concrete adds some strength (e.g. a
mailbox post with which you want to surprise the local delinquents when
they come by with a baseball bat trying to knock it over..)
5) it reduces the odds of buckling if something bumps into and dents the
pipe.
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