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Author Topic: Aluminum wire loop?  (Read 1185 times)
ND1W
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Posts: 10




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« on: August 06, 2012, 05:42:13 AM »

I'd like to put up a large loop antenna but with the cost of copper these days I was thinking of using aluminum wire. The stuff used for electric fences is very inexpensive and comes in lengths up to a mile! Has anyone used this? How did it work and how did the calculations differ?

73
Mike
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WB6BYU
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Posts: 13045




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« Reply #1 on: August 06, 2012, 07:13:41 AM »

I have a couple antennas made using electric fence wire.  They work fine.  The
differences are insignificant - the height above ground makes more difference
than the wire type.

The main issues with solid aluminum wire are:

1) not being stranded, it isn't as flexible as stranded wire.  It will tend to
break under repeated flexing.

2) it is more difficult to get a reliable connection.  You particularly want to avoid
copper to aluminum joints due to problems with dissimilar metal corrosion.  One
simple solution is to use a stainless steel bolt to join them, with a stainless washer
separating the two wires so they don't touch.  A balun with stainless steel hardware
would also work.

On the other hand, aluminum is light, so you don't have to pull up as much tension
to reduce sag.

But my backup long wire antenna is made from the thinnest aluminum electric fence
wire and has been up for something like 15 years, running out to a walnut tree that
sways in the wind.  It has a counterweight at the far end, and is still in good shape.
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K0ZN
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Posts: 1538




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« Reply #2 on: August 06, 2012, 05:45:19 PM »

 Second the previous post.  Aluminum is just fine as long as you take into consideration the mechanical and corrosion/dissimilar metal issues.

 There would be no difference in resonance calculations assuming you use a "typical" wire gauge. Copper and Aluminum are both excellent conductors.

You absolutely want to avoid any sharp bends in AL wire. I would use an end insulator with a moderately large radius where the wire connects to the end insulator.
It would be adviseable to VERY SLIGHTLY stretch the wire before you assemble it into the antenna to reduce the chance for stretch and detuning once it is aloft.
Same applies to soft drawn copper wire too. Obviously, you want to be extremely careful to avoid any "kink loops" in the wire when you are putting the antenna up.
Make absolutely sure there are no kinks in it before you start pulling it up and putting the wire under tension. There are various ways you can mechanically secure
the wires at the center and end insulators but just be sure to avoid a direct copper or bronze to Aluminum joint. Since AL wire is softer than copper and you could create a weak point if you significantly over tighten a connection fastener, so be aware of this.

 Just my two cents, but if I was going to put up a suspended wire antenna using aluminum wire, I think I would go to the effort to have a pulley
and rope to a counterweight system (like W6BYU has) to keep the tension more constant and reduce bending due to wind movement.
Aluminum is less forgiving of cycling than copper, especially stranded copper.

The only significant down side to aluminum wire, especially a smaller gauge, is that it will be more susceptible to breaking under ice loading in the winter.
You can easily find/Google some tables to give you the breaking tensile strength of various wire types and gauges. I can't quote the numbers, but copper will be
somewhat stronger than AL would be in a given wire gauge. It would probably be worth the effort to have an idea of the strength of the wire you use.

Actually, as you note, with the cost of copper being what it is, it is surprising there are not more people considering aluminum wire for antennas.

73,  K0ZN
« Last Edit: August 06, 2012, 05:52:02 PM by K0ZN » Logged
ND1W
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Posts: 10




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« Reply #3 on: August 07, 2012, 04:47:46 AM »

Thanks for the replies!
I'm surprised that the calculations work out the same, although aluminum is a "good" conductor it's only 59% as conductive as copper and I expected with a long wire it would have to have been considered. Not that I'm looking to do more math!!
The physical/mechanical properties will be easy enough to deal with, a pulley and counter weight to keep it taught and stainless hardware is a great idea, much easier than soldering...I guess I'd better get the wire ordered before too many hams decide to use aluminum and drive the prices up! HIHI
 
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KG4RUL
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WWW

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« Reply #4 on: August 07, 2012, 06:09:30 AM »

Thanks for the replies!
I'm surprised that the calculations work out the same, although aluminum is a "good" conductor it's only 59% as conductive as copper and I expected with a long wire it would have to have been considered. ......

If you are working with DC or 60Hz AC, the resistance of the wire becomes significant.  When you are working with RF, the current travels on the outer surface of the conductor (skin effect).  At RF frequencies, the skin effect of copper VS aluminum are very similar so, the calculations are only minimally different.

Also, remember that the calculations are NOT exact.  Many other factors come in to play when actually installing and using an antenna: proximity to other objects; antenna supports; feedline placement - to name a few.  You often have to do a little pruning of antenna elements to get an optimum installation.
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KL3HY
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Posts: 117




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« Reply #5 on: August 07, 2012, 08:34:23 AM »

I'm looking into using aluminum fence wire for a horizontal loop too, and I was curious if icing or even rain might have any effects on the uninsulated wire.  Any odd behavior I should look out for?

Thanks,
Mike
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K7KBN
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Posts: 2766




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« Reply #6 on: August 07, 2012, 09:23:30 AM »

Shouldn't have any effect not detectable with several thousands of dollars' worth of instrumentation.  Basically the same effects with Al or Cu.

If you start to get some flaky results, check the Cu/Al joints for signs of corrosion first, then for tree branches that might be touching something they're not supposed to.
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73
Pat K7KBN
CWO4 USNR Ret.
WB6BYU
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Posts: 13045




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« Reply #7 on: August 07, 2012, 11:46:41 AM »

Quote from: KL3HY

I'm looking into using aluminum fence wire for a horizontal loop too, and I was curious if icing or even rain might have any effects on the uninsulated wire.  Any odd behavior I should look out for?



If you are expecting serious ice build-up, then you may need the strength of a steel core
rather than pure aluminum.

You might consider Teflon insulated wire to reduce ice build-up, as sleet tends to run off the
surface faster.  Otherwise I've heard of hams using car wax or other coatings (also to reduce
the changes in wet twinlead) though I haven't tried it myself.

Insulated wire might show slightly less detuning when covered with ice and snow, but
I don't know if it would be enough to make a difference.

At least with a loop you can run some current though it to help melt the ice: a low voltage,
high current transformer winding may do the job.
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KL3HY
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Posts: 117




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« Reply #8 on: August 07, 2012, 12:21:58 PM »

Great to hear--thanks!  I have a couple other questions, but I'll start a different thread so I don't hijack this one any further than I already have.   Cheesy

Mike
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