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Beef Up Your Antenna - Cheap Trick!




 
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Author Topic: Beef Up Your Antenna - Cheap Trick!  (Read 12113 times)
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K1JJ
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« on: September 27, 2005, 12:17:51 PM »

I came across a great way to use common, smaller diameter, cheap aluminum tubing found at many scrap yards for Yagis.

I'm making up a pair of 50' boom, 15M Yagis. I ran out of aluminum for elements and went to the scrap yard. I found they had only smaller 5/8"  and 1/2" 12' tubes left. Common sense says you can't make 22' elements out of 5/8" tubing... it will fail.  Usually you need to taper the lengths from 3/4" to 5/8" and 3/8" or whatever.  Putting a single length of 5/8" out there would collapse in a 45 mph wind according to YagiStress software..

So, since I had no larger 3/4" or other pieces to taper the elements, I experimented with the software, YagiStress, to come up with a solution. I found that the stress and failure point of a full length 5/8" 22' element came right at the center, as you would expect. By adding a SHORT 1/2" piece INSIDE the 5/8" piece, thus doubling the wall thickness, it strengthened the element to withstand 83 mph or 0.5" of radial ice! 

So, for a 22' element, just insert a 4' piece of 1/2" tube [slides in tight] at the center. Now the Yagi can be built very simply and cheaply using common 5/8" and 1/2" tubing. 

I picked up a scrap yard load of 45, 12' lengths of 5/8" for less than $100. This is enuff for the two 15M Yagis and enuff to build a few more antennas later on. That's a LOT of aluminum [540'] that would cost probably $600+ plus shipping at the venders.

Just wanted to pass this info along. This technique can be SCALED up or down to suit your needs on other bands. ie, There's no need to have to taper your elements if you are short of stock. Plus, the wind loading is often less with the thinner constant diameter element vs: larger tapered, depending upon taper.  Plus the constant elements do not need the "taper factor" correction for accuracy.

Normally I like to see 110 mph with tapered elements, but 83 mph is still OK for central CT.  Always verify your choices with YagiStress before building..

73,
Tom, K1JJ
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« Reply #1 on: September 27, 2005, 01:00:56 PM »

Tom,
That was exactly what I did to my log. After it was all assembled the first time, I noticed a lot of sag in the new elements I built to fix the L/D ratio.
They were a heavier than stock so sagged more. I didn't want to rebuild all the elements so slid tubes inside the existing ones to fix the sag.  I didn't go down the whole length, just enough to fix the sag problem.  Some required 2 pieces inside and always had overlap of about 6 inches at any joint  fc
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« Reply #2 on: September 27, 2005, 01:22:56 PM »

Good infolmation. Another trick is to double nut boom to mast and other U clamps instead of relying on lockwashers which sometimes require too much tension of the fastener to work in the first place.
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« Reply #3 on: September 27, 2005, 02:26:52 PM »

Dave,
 I also did that. I laminated the mast with inner sleeves down to 1/2 inch ID
from 2 inch OD.
I put a 1/2 inch bolt through the mast and antenna mounting plate. I can send the whole antenna boom vertical on the mast if I ever need to repair an element. We brought it up the tower vertical on the gin pole. The slid the pin in to allow us to pull it horizontal and mount the 4 U clamps.
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K1JJ
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« Reply #4 on: September 27, 2005, 02:51:22 PM »

Good on the double nuts, Dave.

Yes, lock washers are sometimes useless for fragile spots.  ALSO, especially when joining thin walled elements together. The torque needed for the lock washer to work will crush the element, usually .058" wall.

The solution is to use the Stainless steel locking nylon insert type nuts. I use them for ALL element joints and have never had one fall off. That is a common problem using anything else. Use SS hardware - it's cheap in small sizes used for elements.

Lastly, when splicing booms together... if the elements are long, with big winds you can sometimes get enuff longitudinal? torque [twisting of the boom] to break the bolts that hold the boom sections together. It happened to my old 40M beam. The sections almost pulled out and split in half before I took it down. You could see the two halves of the boom/els spinning in the wind. I now use 1/2" diameter bolts for all boom splices. No more 1/4" bolts there.   Wink

T
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
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« Reply #5 on: September 28, 2005, 09:25:07 AM »

Nylon does not like UV. I used them then found my Jar of mil SS lock nuts,
They have distorted sleeve above the nut to grab the bolt. This is the best way to go if you can buy these nuts. I've removed them fron dead units I have stripped over the years. They use some nice hardware on jet engine
cases. 
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K1JJ
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« Reply #6 on: September 28, 2005, 10:43:25 AM »

Nylon does not like UV.

Yes, good point, Franz. I've seen some nylon open wire spreaders turn to a white powder years ago, so know what you mean.

Well, guess one idea would be to mount the screws vertically so that the nuts are always on the bottom. Then they will never see direct sunlight. That's how mine are mounted. Would that work or does indirect sunlight UV still cause a problem?

I've had antennas using these nuts up for about 5 years now with none failing yet, but will take a close look at the log periodic nuts I removed recently from service.

At least with standard steel screws/nuts, they will rust and lock themselves in, unlike SS...  Roll Eyes

T

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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
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« Reply #7 on: September 29, 2005, 11:12:55 AM »

Tom,
I read somewhere vertical screws weaken the element so mine are horiz.
I guess you could paint them. Mine have been up since '97.
How about Aircraft Hardware on Rt5 in East Hartford? They are near Pratt Whitney. The Mil hardware is all metal. I kick myself for not using it.
Maybe someday I'll put up a log at the new qth.   
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K1JJ
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« Reply #8 on: September 29, 2005, 11:50:53 AM »

Tom,
I read somewhere vertical screws weaken the element so mine are horiz.
I guess you could paint them.    

Yeah, painting the nuts/nylon with a spray is a good idea to protect it from the sun. I usually spray all hardware anyway, but will include these element nuts before I put everything up.

As for lining up the eles or boom sections so that the bolts are horizontal vs: vertical... well, I used to think that too, but remember there is an overlapped double thickness wall where the eles join together where the holes go. I usually go in at least one diameter from the end for holes, so I doubt there's any difference to vert vs: horiz. I'll bet if the el breaks, it will not be at the joint, but out somewhere else unrelated to the joint/bolts in the single thickness areas..  On many of my booms and eles, they are TRIPLE thickness cuz of the reducing sleeve.

FWIW, I noticed that M2, for one, doesn't bother to put them horizontal on their ants.

T

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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
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« Reply #9 on: September 29, 2005, 07:45:29 PM »

Tom said:
Quote
Yeah, painting the nuts/nylon with a spray is a good idea to protect it from the sun

You might want to becareful with what you are spraying them with. Anything containing solvent will soften the nylon up and you won't be any better than with the UV rays. I would use something along the lines like the blue Servicable Loctite. It does pretty good in the weather and seems to be a little firmer the older it gets.
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K1JJ
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« Reply #10 on: September 29, 2005, 10:30:52 PM »

Tom said:
Quote
Yeah, painting the nuts/nylon with a spray is a good idea to protect it from the sun

You might want to becareful with what you are spraying them with. Anything containing solvent will soften the nylon up and you won't be any better than with the UV rays. I would use something along the lines like the blue Servicable Loctite. It does pretty good in the weather and seems to be a little firmer the older it gets.

Thanks for the great suggestion, Mike!

I will do that with the blue Locktite soon.

73,
Tom
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
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