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Author Topic: Antenna Mast  (Read 593 times)
AC4R
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Posts: 18




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« on: January 11, 2007, 07:24:47 AM »

Looking for good ideas in making a pair of 40 to 50 ft high antenna masts, to hang a 535 foot dipole from.
I will be moving soon and want to have everything ready so I can get back on the air fast. Comments on using 2 inch OD chain link fence top rail. Any help is appreciated.

AA4FH
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WB2WIK
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Posts: 20595




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« Reply #1 on: January 11, 2007, 08:38:18 AM »

Should work if you guy it every 10' in three directions for each mast.  I've used it myself, for supporting wire antennas.  Not strong enough for a rotator and a beam, but should be good for a wire.  

With a 535' wire and a feedline to support, the horizontal "pull" on each mast can be substantial -- like 100 lbs.  You might need to back-guy the mast very well, pulling the masts back vertical with guys and turnbuckles after the antenna's up.

WB2WIK/6
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N3BIF
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« Reply #2 on: January 11, 2007, 09:55:08 AM »

  50 -40 feet of this stuff will be  heavy and will need much physical help in getting it upright. Bad thing is if it starts to bow while raising you are done as it will never straighten out.
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WT0A
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« Reply #3 on: January 11, 2007, 02:14:42 PM »

535 foot dipole, interseting, what band(s)?
Glen
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AC4R
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Posts: 18




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« Reply #4 on: January 11, 2007, 03:30:30 PM »

It would be a full wave 160, which would also serve as an all bander as  I use homebrew balanced line made from 14awg stranded copper with 7 inch spacers drilled 1 inch from each end giving 6 inch spacing for the line. I run balanced line as it makes a dipole an all bander, something you cant do with coax, and the fact that the loss is nill compared to coax.
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N3BIF
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« Reply #5 on: January 12, 2007, 09:34:13 AM »

Most dipoles are half wave so you really only need 260 feet or so of wire , half on each side the feed point.
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K4SAV
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Posts: 1840




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« Reply #6 on: January 12, 2007, 12:08:47 PM »

I would suggest that you do some rough calculations of the forces involved.  You can estimate the force required to hold a cable up with a given amount of sag by using the on-line calculator at
http://www.spaceagecontrol.com/calccabl.htm?F=100&a=500&q=.02&g=32.1850393&Submit+Button=Calculate
Example: For #12 hard drawn bare copper (20 lb/1000ft), you would need 71 lbs of tension to hold up 535 ft of wire.  But this doesn't include anything attached to the center of the antenna. For every lb you add there, assuming a 10 ft sag, you would have to increase the tension by 13.4 lbs. (Easy geometry calculation for a rough approximation). If you allow only 5 ft sag, this number increases to 26.8 lbs tension for every lb you add at the center. If you have a 50 ft mast with guys anchored at 50 ft away, the vertical load on the mast would be the same as the tension in the wire (another easy geometry calculation).  So with a 10 ft sag, if 40 ft of your homemade ladder line weighs 4 lbs (it sounds heavy),  the tension required would be 54 + 71 = 125 lbs.   (WIK's seat of the pants guess was very good, but he probably forgot about the next item).

Now you need to calculate wind loading.  If you look at
http://www.vk1od.net/rigging/sag.htm
you will see that hard drawn copper is probably not a good choice.  It breaks at about 200 lbs.  Copperweld would be a much better choice.  Its stronger and a little lighter. Then you can look at the example at
http://www.iaei.org/subscriber/magazine/02_c/other_code.htm
for load calculations due to wind. From this you will discover that for a 70 mph wind (not considering gusts) that the additional loading on the  3.6 sq ft of wire is 45 lbs.  Since this is distributed linearly, just go back and add it to the wire weight and plug it back into the calculator referenced above to get an approximation of the load.  That will give you a tension of 370 lbs for a 10 ft sag. Now we are almost done, but we have to add something for wind loading on the ladder line. Not knowing what this looks like, but making an estimate based on the size of your spacers, 40 ft comes out to be about 1.5 sq ft, or a wind load of 18.8 lbs at 70 mph.  If you assume half of that is supported from below, that adds 9.4 lbs to the antenna.  Since that is at the center of the element it get multiplied by 13.4, so it adds 126 lbs of tension. We now have a final answer of  370 + 126 = 496 lbs for 70 mph wind.  That's 70% of the breaking strength of #12 copperweld, so maybe you need a larger wire and repeat the calculations.  It's more than any simple mast can handle too, so maybe another plan of attack is in order.  You could allow a much larger sag which will reduce the forces involved, but the antenna will be getting very low in the center.  Maybe three supports?  Inverted vee?  Shorter antenna?

Jerry, K4SAV
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