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Author Topic: connecting copper wire to aluminum tubes  (Read 5250 times)
WS4T
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Posts: 182




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« on: May 10, 2012, 09:02:14 AM »

I want to build a Battle Creek Special (see here: http://www.pi4cc.nl/link/mkII.htm).

I've built wire versions before using a fiberglass mast and it worked, but those were temporary because the traps and wire were too heavy for the mast. So now I want to build a more permanent version with aluminum tubes.

There's a number of places where I will need to connect a copper wire to an aluminum tube. From what I hear, it's very difficult to solder copper to aluminum, so I was thinking of using stainless steel bolts, washers and wire lugs to make the connections.

My question: Does that sound like a good approach to get a connection that will last a few years outdoors near the Baltic Sea?

If so, can anybody recommend a good source of stainless steel wire lugs, bolts and nuts? (something like this: http://www.alliedelec.com/search/productdetail.aspx?SKU=70084129)

Tnx + 73,
Gary, ES1WST (WS4T)
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W5FYI
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Posts: 1046




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« Reply #1 on: May 10, 2012, 09:18:51 AM »

Stainless is a good choice, and I'd shop for the hardware at a marine supply store (if it can hold up on a saltwater vessel, it should hold up as well on an antenna). However, I would recommend sealing the connections after you make them--to keep them mechanically tight and to minimize galvanic corrosion.

You can solder to aluminum, but it takes a bit more heat than a lightweight soldering iron can provide. What you need to do is scrape away the surface oxide in order to get the solder to stick to the aluminum--trouble is, oxide forms almost immediately! One way around this is to put some light machine oil on the place you want to solder, then scrape or sandpaper it so that the oil remains to block out air. Next apply a little flux to the area, then heat with a hot iron or torch until it's hot enough to melt the solder.

There's also something called "aluminum brazing rod," an aluminum alloy that melts at a lower temperature than most construction aluminum. You might be able to use it to "solder" copper to aluminum as well. GL
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K8AXW
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Posts: 3958




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« Reply #2 on: May 10, 2012, 09:43:42 AM »

I'd save myself a lot of grief and use the stainless steel bolts and nuts, and as mentioned, seal the connection.  Beam makers have been doing this for years.

What to seal it with is a whole new topic.  It an include special "paints," seal tape, duct seal (like a putty that never hardens) and silicone calking.  (Make sure it's non-corrosive.  A lot of caulking has acid in it)
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WB6BYU
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Posts: 13459




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« Reply #3 on: May 10, 2012, 10:57:04 AM »

I'd go with the stainless hardware - make sure there is a washer between the copper
and aluminum so they don't touch directly.

Of course, you could just use aluminum wire and eliminate the bimetalic joints, but you'd
still want a good solid connection with anti-corrosion compound.
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G3RZP
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Posts: 4834




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« Reply #4 on: May 10, 2012, 11:30:33 AM »

I've always got stainless steel hardware at Home Depot. At one time, you could even get left hand thread stainless steel hardware there - back 25 years ago!

Customs at Heathrow once asked why I was bringing 1/4-20 stainless steel hardware into the UK......but I got away with it.
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W6EM
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Posts: 879




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« Reply #5 on: May 10, 2012, 06:22:07 PM »

I'd go with the stainless hardware - make sure there is a washer between the copper
and aluminum so they don't touch directly.

Of course, you could just use aluminum wire and eliminate the bimetalic joints, but you'd
still want a good solid connection with anti-corrosion compound.

The surface of aluminum is basically coated with aluminum oxide, a non-conductor.  Before you apply your stainless washer at the interface, scour the aluminum surface with steel wool or sandpaper til its bright and shiny, then quickly coat the surface and the SS washer with electrical inhibitor grease (carbon-loaded, silicone grease).  No-AL-Ox is a popular brand at the hardware stores.  The inhibitor will help keep the surface from re-oxidizing under the SS washer.  SS is midway between the electronegativity of aluminum and copper.  Actually, come to think of it, better yet tin the copper wire with all-tin or "no-lead" solder if you can.  Tin is closer to SS than is copper.  Most UL-listed copper to aluminum electrical connectors are tin plated, just for that reason.

Good luck with it.

73

Lee

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K2OWK
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Posts: 1072




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« Reply #6 on: May 10, 2012, 09:09:22 PM »

Connecting aluminium to copper requires a bimetallic connector. This consists of a stainless steel tube with two sealed chambers where the copper is inserted in one side and the aluminium is inserted in the other. they are clamped in place by a pressure screw in each chamber. These can be purchased very inexpensively at places that that sell lightning rod equipment. These will last for a long time and keep the copper and the aluminum from reacting with each other.

To solder aluminum requires aluminum solder also known as swagging compound. The swagging compound will adhere to the aluminium, and then the normal solder will adhere to the swagging compound. This is the technique used to solder the copper tabs to the aluminium conductor in a foil wrapped capacitor.

Hope this helps.

73s

PS: Not sure how to spell swaging in this use.

K2OWK
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