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Author Topic: Aluminum Tubing - How High Without Guy Wires?  (Read 4667 times)
AC2GS
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« on: January 28, 2012, 03:19:15 PM »

Here's a theoretical question:

Approximately how high can you build aluminum tubing before guying becomes required. I realize it depends on many variables. Say that the tubing supports a few pounds of dipole wire at the top (the dipole will act as guy wires 180 degees apart at it's top), and the tubing telescopes from 2" OD to 1" OD at the top, and say that it's supported at it's base by a weighed down tripod mount. I have read that anything taller than 10 feet should have additional guy wire support, and the simplest, least useful response would be "just put guy wires up no matter how high the tubing height". Would anyone throw out a specific number?

Thanks in advance.

73,

Roy, AC2GS


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KQ6Q
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« Reply #1 on: January 28, 2012, 08:27:32 PM »

10 feet above a tripod or house side mount is safe, anything more depends on how much wind, and how much ice and snow you can expect.
Also consider what else would be damaged if it came down accidentally...
and of course, do you have power lines in the area. If the mast came down, could it hit the power lines ?
If so your height should be limited so that can't happen.

Maybe I'm overly cautious, but I've kept my antenna installations limited and save since 1957...

Fred, KQ6Q
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K2DC
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« Reply #2 on: January 29, 2012, 07:47:12 AM »

Roy,

   Sorry, but with all respect I disagree.  The most useful response would be "just put guy wires up no matter how high the tubing height".  You are right that it does depend on many variables, few of which are covered in your post.  For that reason, anyone who would "throw out a specific number" might be giving you some very bad advice.

73,

Don, K2DC
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KL3HY
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« Reply #3 on: January 30, 2012, 09:37:48 AM »

Don makes a very good point.  That said, most instructions I've seen call for guys every 10 to 15 feet, with the Rohn mast instructions probably being the most conservative to guy every 10' section.

I think any way you look at it, it's going to be a judgement call--if you're in an area that gets windy you'll want more guy lines, and if you're in an area that gets very little wind you could probably get away with fewer.  Keep in mind that there's nothing wrong with overbuilding.
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WX7G
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« Reply #4 on: January 30, 2012, 09:57:48 AM »

The book "Physical Design of Yagi Antennas" will tell you what you need to know.

If you have not had physics you have a bit of learning to about Moments so that you can calculate the weight needed on the tripod legs.


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N3OX
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« Reply #5 on: January 30, 2012, 09:58:25 AM »

put up your mast and tripod on a calm day, (maybe with some friends on some temporary guys as "catchers" if it's a lightweight system).  Get a spring scale and a rope attached to the top and measure the force it takes to pull the mast to the tipping point.  That's roughly (to within some trigonometric factor based on the pull angle) the force on the dipole that will push your mast over.

You say you have a weighed down tripod mount and don't describe anything about it, so no one can estimate even the slightest thing.

A weighed down 5 gallon bucket with a 20 foot mast sticking out of it might fall over because you can't even level it or because the dipole wires aren't exactly aligned.  But I built a tripod with three ten foot pieces of 3/4 inch EMT as legs as a temporary support for a ~20 foot mast for a 15m monobander i built and that thing probably would take a 30-40MPH wind because it had such a wide base.

It all comes down to the force of the wind and how far away from the edge of the tripod the center of mass is.. how high you have to lift the center of mass of the system before it crosses the outer edge of a tripod leg.  But you haven't said anything about that.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W3HKK
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« Reply #6 on: January 30, 2012, 12:53:34 PM »

Army/Navy surplus stores sell 4 ft sections of used telescoping fiberglas or aluminum masts 1.5" in diameter.  You can  put 11 of them together and get to about 44 ft if you  add three guys about 25-30 ft up and  have two friends pull to prevent catastrophic bending in the middle.  We've done it 3-4 times and its not that hard.  Aluminum is better in the center as it resists bending a bit better than the fiberglas.  A mix of both is fine. 

44 ft is about as high as you want to go.  40 ft is a reasonable ht with a greater safety margin.

Then you need 3 guy ropes  at the 25-30 ft point to keep it up.  Digging a  3-6" hole for the bottom to sit in keeps it in place.  A pulley at the top  lets you raise and lower your wire. 

If the tension is high, a guy rope in the opposite direction may be needed to keep the top section from bending too much.
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KI4SDY
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« Reply #7 on: January 31, 2012, 05:57:53 AM »

Using telescoping electrical conduit with U bolts, a concrete anchor for the bottom pole, attached to the side of the house, I support or guy every 15 feet and have never had a problem through a lifetime of Florida's frequent hurricanes and or tornadoes. Make sure your guy wires are of adequate strength and anchored properly or they are useless!  Embarrassed

Out in the Midwest where tornadoes are really big, I wouldn't worry about the antenna pole, I would worry about the whole house!  Shocked
« Last Edit: January 31, 2012, 06:02:21 AM by KI4SDY » Logged
W5DXP
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« Reply #8 on: January 31, 2012, 11:23:50 AM »

Last time I was there, Texas Towers had a computer program that could answer that question.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
ES1TU
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« Reply #9 on: February 01, 2012, 01:24:56 AM »

For what it's worth - I have had a 17m (and 20m, 12m) moxon on top of similar diameter alu tubing for the last 2.5 years now.
This DIY mast is 21 feet tall.

https://picasaweb.google.com/lh/photo/D47Jj5bPd38cKnZ0fiWx2hVMjOt0VKCnQT8U_-azrBE?feat=directlink

As you can see - I decided to put 2 sets of guys. Each set has four 2mm kevlar ropes. This picture is a bit out of date - after taking it I decided to lower the bottom guy point. If you ask me wheter I would get along with a single set of guys, I would say - HELL NO! I'm really glad I was enough paranoid to put 2 sets. In my qth (and I believe in all others as well) the question is not IF the wind is going to blow, but rather WHEN it's going to blow. During those 2.5 years I have seen ca 20 days of 20m/s or more storm winds. Believe me - when I look at my installation during those days, I always think that I should add 3-rd set of guys.

Streetview of my 21ft alu mast https://picasaweb.google.com/lh/photo/pwhtqBOOQetjaf3y1wuFexVMjOt0VKCnQT8U_-azrBE?feat=directlink
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KI4SDY
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« Reply #10 on: February 01, 2012, 05:48:09 AM »

Good job! It looks scary thinking about climbing around on that slick tile, even with a ladder to walk on. I wouldn't have gone any higher either, but the guys look adequate. What did you anchor them to?  Undecided

When erecting homemade ground planes on roof tops, not more than 10 feet up, I use 12 AWG ground plane wires  and nylon parachute cord for guys. Double duty!  Grin
« Last Edit: February 01, 2012, 06:07:45 AM by KI4SDY » Logged
ES1TU
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« Reply #11 on: February 01, 2012, 06:07:09 AM »

Good job! It looks scary thinking about climbing around on that slick tile, even with a ladder to walk on. I wouldn't have gone any higher either, but the guys look adequate. What did you anchor them to?  Undecided

https://picasaweb.google.com/lh/photo/iCmnGhDawtKc_5QH_8T-AhVMjOt0VKCnQT8U_-azrBE?feat=directlink

Since it's my house and I'm a god in my property, I decided to drill 4 holes through the roof. And yes - everything IS waterproof. No leakage during 2.5 years. Back to the original question - I strongly suggest to add one set of guys too many, instead of one too few.
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K4SAV
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« Reply #12 on: February 01, 2012, 07:59:06 AM »

You are describing a dipole mounted as an inverted vee.  The wires will offer support in only two directions.  The greatest force on the support will not be from the weight of the wire but will be from the force applied by the wind (unless you anchor the lower ends to a tree).  You will have no support in the directions of maximum wind loading.  Of course the length of the dipole makes a huge difference. 

A 1 inch diameter section of aluminum tubing is not very strong, probably less that the 1.25 inch diameter EMT tubing I have supporting a birdhouse.  I have a 10 ft section with the bottom 3 feet clamped to a 4x4, so there is 7 ft of pole above the anchor point.  Every year when I take the bird house down to clean it, I also have to straighten the pole.  I suspect the birds don't mind the movement in the wind.  It's similar to having a nest in the top of a tree.  I also have a couple of Rohn H50 masts (46 ft) with 3 sets of guys supporting a 180 ft wire and that doesn't move at all in the wind.  I also have some unguyed 22 ft poles supporting the low ends of an 80 meter inverted vee.  Those are made from a 10 ft section of chain fence rail, a 10 ft section of EMT tubing and an 8 ft 4x4 post.  (Notice there is a lot of overlap used).  Those move in the wind but have held up without bending.

Jerry, K4SAV
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K3WEC
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« Reply #13 on: February 01, 2012, 09:02:37 PM »

I've used 2 10ft sections of the cheap conduit, total of 20ft, secured for first 3 feet or so by u-bolts, with no guying save the antenna wire itself.  This was an inverted vee.   I had it up a long time, sometimes experiencing healthy winds, and never had any problems.  I'd do it again.  This does not mean you won't (or I won't in the future) have problems, however.
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M6GOM
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« Reply #14 on: February 02, 2012, 03:16:28 PM »

I had a Cobwebb antenna on top of a 5 metre long ally pole with 1.5mm thick walls. A few days of 70MPH gusts saw it bend over like a banana.
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