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Author Topic: Power Limits for Aluminum Wire Horizontal Loop  (Read 4145 times)
KL3HY
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« on: August 07, 2012, 12:32:27 PM »

I've been thinking about using either 14 or 17 gauge aluminum electric fence wire to make an 80m horizontal loop, but I was curious how well the wire would handle power above 100 watts.  At this point I'll almost certainly use the 14 gauge wire, if for nothing else because of the mechanical strength of the wire.

I can't imagine there would be any problem running 100 watts through it, but I'm curious if I'd have any issues using my AL-80B with it?

Thanks,
Mike
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WB6BYU
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« Reply #1 on: August 07, 2012, 01:04:22 PM »

A full wave horizontal loop will have a radiation resistance around 100 ohms.  (It varies
with height, etc., but that is a good working number.)

At 100 watts, that would be 100V RMS at 1A.  That would be the maximum current anywhere
on the loop.

Current goes up as the square root of power, so at 1600 watts you would have 4 amps.

#14 copper wire is rated for 15A in conduits, more in open air.  I don't think that will
be your limiting factor.
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KL3HY
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« Reply #2 on: August 07, 2012, 04:51:55 PM »

Okay--that makes sense.  For some reason I had it stuck in my head that it's aluminum so it may not handle higher power.

Thanks,
Mike
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G3TXQ
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« Reply #3 on: August 08, 2012, 08:00:32 AM »

#14 copper wire is rated for 15A in conduits, more in open air.
Is that a DC rating?

At 3MHz the skin effect makes the RF resistance of #14 copper wire about 10 times its DC value, so for the same heating effect the current rating would need to drop by a factor 3.2 to 4.7A; then you need to consider the extra resistivity of aluminium compared to copper.

Probably still OK, but not as much in hand as appears at first.

73,
Steve G3TXQ
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W8JX
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« Reply #4 on: August 08, 2012, 08:38:28 AM »

#14 copper wire is rated for 15A in conduits

Actually is it rated at 17 amps but it is used with 15 amp circuits. It is also rated at about 20 amps in a cable and about 32 amps single wire open air. When you consider aluminum wire vs copper you general size up two wire sizes over copper for same circuit. So figure on about 20 amps max for 14ga aluminum single wire open air.
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G3TXQ
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« Reply #5 on: August 08, 2012, 08:42:43 AM »

Actually is it rated at 17 amps but it is used with 15 amp circuits. It is also rated at about 20 amps in a cable and about 32 amps single wire open air. When you consider aluminum wire vs copper you general size up two wire sizes over copper for same circuit. So figure on about 20 amps max for 14ga aluminum single wire open air.
Now divide by 3.2 for the rating at 3.5MHz!

Steve G3TXQ
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WX7G
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« Reply #6 on: August 08, 2012, 11:53:11 AM »

The RF resistance of aluminum is 30% greater than copper.

At 3.5 MHz the resistance of #17 AWG aluminum wire is 35 ohms per 1000'. At 28 MHz it is 100 ohms per 1000'.
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W4VR
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« Reply #7 on: August 08, 2012, 12:29:09 PM »

It will work just fine...go for it...but if you can afford a larger gauge you would be better off!  Many blue moons ago I had a 300-foot longwire made of #8 gauge solid aluminum.  It did the job for me running 1 kilowatt.  I just bought some LMR 400 that has a solid center conductor made of copper clad aluminum.  In order to keep the cost of wire to a minimum I'm seeing more and more coax and antenna material made of aluminum.  I remember years ago when electricians were using aluminum wire in new home construction...they had problems with this down the road when the connections would oxidize and cause house fires.
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KL3HY
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« Reply #8 on: August 08, 2012, 01:08:14 PM »

This is great info--I appreciate the responses.

I did run across some 12.5 gauge aluminum wire, so that's an option too.  I'll probably stick with the 14 gauge to start with--the stuff is less than $50 per 1/4 mile spool, so it's pretty cheap.

Thanks,
Mike
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W0BTU
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« Reply #9 on: August 08, 2012, 04:22:00 PM »

I'm not a big fan of either aluminum (for more than one reason) or horizontal loops (which radiate straight up). But I cannot imagine that running the full legal limit will cause such antenna to melt and fall out of the sky. :-)
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K0ZN
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« Reply #10 on: August 08, 2012, 04:42:17 PM »

Either wire will work fine in the real world if you operate legally power wise.  That said, I think it would be "Penny wise and pound foolish"  to try to save a
couple of dollars on the wire and use 17.  "Man up to the bar" and buy at least 14 ga. or a little larger. (Personally, I would look hard at #12 or maybe even #10
if I was using Aluminum wire. )
 
If you are going to all the trouble to put up a fairly large antenna, do it right.  I promise you: the small wire will break at the WORST possible time !  There is a very accurate Murphy's law corollary that applies to ham radio: "An antenna will fail when it is either: very cold, loaded with ice, in a wind driven rain or you badly need for it to NOT break and don't have time to immediately fix it." The Inverse is that:  "An antenna will almost never fail on a beautiful, sunny, pleasant, 75 degree day."

Biggest problem with AL wire is mechanical issues, but it is not a deal killer by any means. Just DON'T connect copper direct to the AL wire.

73,  K0ZN
« Last Edit: August 08, 2012, 05:21:26 PM by K0ZN » Logged
K3VAT
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« Reply #11 on: August 09, 2012, 06:08:27 AM »

Either wire will work fine ... . Just DON'T connect copper direct to the AL wire.  73,  K0ZN

Have you heard of products specifically for bonding Al with Cu?  Products like alnox, pentrox, Cu-Al butter/paste.  These are chemical additives to aid in conductivity and lessening metal oxidation?  There are thousands of good performing stations where Cu and Al are used together!  Please see this description here: http://www.dxengineering.com/search/product-line/dx-engineering-penetrox-a-anti-oxidants?keyword=PENTROX
« Last Edit: August 09, 2012, 06:25:00 AM by K3VAT » Logged
K0ZN
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« Reply #12 on: August 09, 2012, 03:44:49 PM »


 K3VAT:   

Yes....I am fully aware of those products and various methods of AL-CU connection.  I have Pentrox in my shack.

I just wanted to keep the comments simple..... guess it was too simple...... I was just trying to point out the possible
problem with dissimilar metals and the galvanic tables, etc. etc.

73,  K0ZN
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KL3HY
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Posts: 120




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« Reply #13 on: August 09, 2012, 03:56:28 PM »

I appreciate the comments re: dissimilar metals--I'll probably just get a stainless bolt, nut and washer and attach them using that, with the washer separating them.
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W8IFI
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« Reply #14 on: August 10, 2012, 05:51:38 AM »

In a previous post a poster said a 80 meter horizontal loop radiates straight up. I suggest he read an article in QST of November 1985 titled "The loop skywire" by Dave Fischer.  The computer models say it radiates straight up but in actual practice that isn't so. The actual results differ. I've used that loop for years and found it works as well as other wire antennas for dx as well is local contacts. Dave has excellent credentials and knows that actual results do not reflect what the computer model tells you.
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