[TowerTalk] How To Avoid Telescoping Mast Colapse
W0MU Mike Fatchett
w0mu at w0mu.com
Wed Nov 16 10:15:37 PST 2011
You could also just buy or make the S9 31ft vertical with a 4:1 unun it
will work on 40-10 with a tuner. If you just want 40, then extend the
wire out the top a couple of feet. Toss some radials on the ground and
J6M CQ WW DX CW Contest 2011
J6/W0MU November 21 - December 1 2011
W0MU-1 CC Cluster w0mu.net
On 11/16/2011 11:06 AM, Jim Lux wrote:
> On 11/16/11 9:04 AM, Wayne Willenberg wrote:
>> 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?
>> 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.
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