On 11/16/11 9:04 AM, Wayne Willenberg wrote:
> Hello,
>
>
>
> I am in the planning stages of a new antenna project. It is going to be a
> simple, center fed, 40-Meter dipole (using a 1:1 DX Engineering Maxi-Core
> balun) connected to RG213. (My county has a maximum wind speed of 100mph.)
>
>
>
> The supports I intend to use at each end are Rohn H50 telescoping masts,
> raised to their maximum, about 44. I am following Steve’s (K7LXC) first
> rule – follow the manufacturer’s directions. So, the stays I am using are
> 25 guage-galvanized wire sold by Rohn and the radius of the anchors for the
> stays will be per Rohn’s specs.
http://www.rohnnet.com/rohn-telescoping-masts has a link at the bottom
to "guy wire kit information"
The manual for the towers calls out 18ga 6 strand wire for guying and
that's what they have in their installation kits.
>
>
>
> Steve’s second rule, “don’t do something the manufacturer doesn’t tell
> you,” creates several issues. First, the instructions for the Rohn masts
> don’t indicate the length or type of the anchors that should be used. So,
> I plan to use the recommendations from the ARRL Antenna handbook. I plan
> to use 1” OD galvanized pipe driven 4 feet into the ground. QUESTION 1:
> Are these anchors sufficient?
Considering Rohn recommends a screw in anchor 3' long with a 1/2" shaft
and 4" diameter auger at the bottom... why not use that? It's designed
to have high pullout strength.
Your pipe will "probably work".. it's like a tent stake.. You could just
as easily use a standard steel fence T-post, which might be easier to
drive and cheaper, as well. I assume you're driving it at an angle (top
pointing away from the antenna) and anchoring your wire at the point
where the stake enters the ground? But bear in mind that it will tend
to lever out, or the pipe/stake will bend.
It's not like a H50 is a 200 foot stick of 45G with a full size 40m
yagi on top.
A lot depends on what happens if the anchor pulls out? Is it
catastrophe, or just inconvenient. If the latter, you can play pretty
fast and loose with the manufacturer's recommendations (which are for
the nifty augers)
>
>
>
> The second embellishment I plan to use is ¼ inch galvanized thimbles at the
> anchors and at the collars of each section of the mast. It seems to me it
> is better to have the guy wires wrapped around a thimble rather than going
> through the rough edges of the collars and anchors. QUESTION 2: Is this a
> good idea?
You might have a tough time getting the thimble through the hole in the
guy collar. The holes aren't all that big. But certainly, you could do
that. (presumably, though, the mfr designed it to just have the wire
passed through).
>
>
>
> At each point where a guy wire goes through a thimble, I plan to use a
> series of 3 -- 3/8 galvanized wire clamps (i.e., the type that has a U-bolt
> that goes around the two cables. Each end of the U-bolt is threaded. The
> U-bolt extends through a flat part, and nuts are tightened on the threaded
> ends of the U-bolt and thereby clamp the wires together.) QUESTION 3: Is
> this form of attachment correct?
You'll have to go tighten the darn things periodically. Why not use
gripples, specifically designed for this (again, they're in the standard
Rohn guy kit for this antenna.. but they're available separately from
lots of sources)
Also, this is a light duty mast with light duty guy wires. The loads
aren't all that high, even in a howling wind: a few hundred pounds at
most. Mostly what the guys are for is to keep it from bending too far.
The static guy tension is probably almost zero.
>
>
>
> The ends of the dipole opposite the antenna wire from each feedpoint will
> be connected to H1200 Phillystran. The Phillystran will go through a small
> marine-grade stainless steel pulley at the top of the mast and then
> continue to the bottom of the mast. The center of the dipole is heavy
> because of the DXEngineering balun (rated at 2,000 W) and the RG213
> connected to the balun. My reason for using Phillystran for the dipole
> halyards is the strength vs weight ratio. I know I can’t make the dipole
> horizontal to the ground, but I hope to avoid an acute angle on each side
> of the feedpoint by exerting significant horizontal force on the halyards.
>
How significant a force? A droop won't make a huge difference in
performance (and will actually improve the match to 50 ohms). That
horizontal force will need to be resisted by a guy opposite (which you
have planned) but that puts a big compression load on the mast.
The Rohn diagram shows 33.5' out to the anchor for 44 ft up. That's an
angle of 37 degrees at the top of the mast. So, for every pound of
horizontal load you'll need 1.66 pounds of guy tension. That translates
to 1.3 pounds of downforce on the mast. Those masts aren't designed for
huge loads beyond their own weight.
RG-213 weighs about 12 lb/100ft and yours will be 40-50 feet in the air,
so figure about 6-7 lbs, including your balun. You've got a 40m full
size dipole, 33 ft long on each side, roughly. If you let it droop by,
say, 4 ft, that's an angle of about 7 degrees. The horizontal tension
to hold 6 pounds is about 8 times greater (roughly 33/4), or call it 50 lbs.
So your "back stay" guy tension is going to be about 80 pounds, and
you'll be putting an additional 65 pounds of downforce on the mast.
Since you have a big ol piece of 1/2" diameter coax coming down in the
middle, what about putting up another mast in the middle?
>
>
> I plan to tension 3 of the sets of guy wires just enough to take the slack
> out and to make the masts plumb. However, the fourth set of guy wires
> opposite the each end of the dipole will have turnbuckles for adjusting the
> pull in a direction opposite the dipole wire. I intend to have the guys
> exert significant force to try to take some of the droop out of the dipole.
> I know some of this force will be directed downward through the mast and
> try to buckle the mast. QUESTION 4: Is this additional force on the fourth
> set of guy wires a problem?
yes.
>
>
>
>
>
> My final general question: What is the primary reason for causing
> telescoping masts to fail?
#1 Bending over in the wind
#2 Buckling from vertical load
The rohn brochure doesn't give a vertical load spec, and they talk about
not using more than 2 square feet of antenna, but don't give a wind
speed to work with. Let's be pessimistic and assume that they're
specing that for something like 70 mi/hr.. that's about a 25 lb
horizontal load at the top of the tower.
Am I setting up a failure situation by planning
> to have more force in the guys opposite the dipole wires?
yes and no.
People have used pushup masts like this for TV antennas and dipoles for
decades and most stay up. Some bend, some don't, some collapse, some don't.
This is a difficult problem to "engineer" because it's really not
intended for that kind of critical application. For instance the join
between sections is basically a giant set screw... when compression
loads get too high, the mast will just telescope, or fold at that point.
If you're in a situation where if the thing comes down, it's not going
to hit someone on the head, or fall across a power line, you can pretty
much do what you like. If you over tension it, and it collapses, you
run to get out of the way and you've learned something useful for a $170
investment. Next time, you leave a bit more sag in the dipole. You'll
get to reuse the guy wires, the anchors and the gripples. If something
pulls out, pound it back in or buy something better.
I guess the point is that you can't overanalyze this.. It's really a
"put it up and try it" sort of thing.
BTW, if *I* had to put up a 40m dipole, I'd put the big mast in the
middle and make it like an inverted V, and use very light weight masts
at the ends to hold the ends up 30 ft or so.. you could probably use
something like electrical thin wall conduit.
And, I'd just accept that the thing will fall over in a 100 mi/hr wind
and make sure that when it did fall down, nothing bad happens. Heck, I'd
assume it would collapse in a 50 mi/hr wind: using lightweight
materials means that when it does collapse, there's not huge spears of
steel flying through the air and coils of guy wire tangled in all the
bushes and trees.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|