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Author Topic: Aluminum tubing, copper coils: corrosion worries?  (Read 20622 times)
JAHAM2BE
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« on: April 24, 2013, 07:37:35 AM »

I'm building a short vertical dipole (2m total length) to be operated on 7 MHz and higher frequencies, initially with 5 watts output power. The dipole radiators are made of aluminum tubing. The loading coils will be made of 2mm-diameter copper wire. I plan to fasten the copper coils to the aluminum tubing with stainless steel hose clamps.

Will there be any problems over time, such as corrosion, with the dissimilar metals being clamped together? If so, what should I do to minimize the problems?

Also, if I run higher power (up to 50 watts) in the future, will that accelerate any problems caused by the dissimilar metals?
« Last Edit: April 24, 2013, 08:05:41 AM by JAHAM2BE » Logged

W8JX
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« Reply #1 on: April 24, 2013, 08:08:13 AM »

Biggest factor in dissimilar metal is environment not power. I would suggest Stainless Steel terminal lugs on coil and SS hardware and it should be stable. Direct cooper to aluminum will not play well in moisture.
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WB6BYU
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« Reply #2 on: April 24, 2013, 08:33:52 AM »

You can reduce corrosion by adding some dielectric grease to the joint:  it squeezes
out of the actual contacts, but helps prevent moisture and corrosive atmosphere
from getting in the gaps.  I use one called Ox-Gard, which is sold for use with
aluminum electrical wiring.  Another brand is NoAlOx.  (Both are also useful for
lubricating sliding joints in aluminum tubing, making it possible to take the antenna
apart years later.)  These are sold in the electrical supply stores.

Otherwise, you can use a stainless steel bolt to join the two and put a stainless
washer between them so there is no direct contact between the copper and
aluminum. 

The best method in practice is to do both.

You can connect them directly together for experimenting, but I'd recommend
taking further steps to protect the connection for more permanent use.
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JAHAM2BE
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« Reply #3 on: April 24, 2013, 08:47:51 AM »

In another post here on eham, it was stated that stainless steel should be kept out of the conductive path for low-loss connections:

http://www.eham.net/ehamforum/smf/index.php/topic,83113.msg597452.html#msg597452

Although not as bad as a small loop, a short dipole still has a low radiation resistance and thus requires some degree of attention to loss minimization. In this context, is it acceptable to have the antenna  current flowing through the stainless steel, or should some other metal be used to bridge the Cu-Al interface?
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WB6BYU
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« Reply #4 on: April 24, 2013, 09:50:27 AM »

You might just have to experiment and see.  You can use a stainless bolt/screw
to clamp the wire to the aluminum and see if putting a stainless washer between
them makes any difference.

Or you can use soft aluminum tubing for your coils.
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K3VAT
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« Reply #5 on: April 24, 2013, 01:18:03 PM »


Or you can use soft aluminum tubing for your coils.

I've successfully used this type product for winding a loading coil on a 4" diameter form.  You can search for similar size and lengths of soft aluminum wire or tubing.  Small tubing (0.25") may provide both the workability you want while providing lower resistance (compared to #12 Al wire), plus no worries on dissimilar metal connections.  Some solid copper wire, like those 25' rolls of 'grounding wire' that are sold by Lowes and HomeDepot (anywhere from #4 up to #10) are often hard to work with, especially when trying to wind a uniform coil while maintaining coil spacing.

http://www.etsy.com/listing/115278048/6-ga-aluminum-wire-5-ft-coil-soft-pure?ref=similar_items_sash

GL, 73, Rich, K3VAT
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W8JX
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« Reply #6 on: April 24, 2013, 01:59:17 PM »


Or you can use soft aluminum tubing for your coils.

I've successfully used this type product for winding a loading coil on a 4" diameter form.  You can search for similar size and lengths of soft aluminum wire or tubing.  Small tubing (0.25") may provide both the workability you want while providing lower resistance (compared to #12 Al wire), plus no worries on dissimilar metal connections.  Some solid copper wire, like those 25' rolls of 'grounding wire' that are sold by Lowes and HomeDepot (anywhere from #4 up to #10) are often hard to work with, especially when trying to wind a uniform coil while maintaining coil spacing.

http://www.etsy.com/listing/115278048/6-ga-aluminum-wire-5-ft-coil-soft-pure?ref=similar_items_sash

GL, 73, Rich, K3VAT

Aluminum tubing can work except you need to get bend right first time because it work hardens and will crack if you try to rebend it much.
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KB4QAA
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« Reply #7 on: April 24, 2013, 03:12:24 PM »

ordinary tinned ring terminals can be crimped/soldered to the copper coils and then bolted to the aluminum tubing and treated with conductive grease such as GB Oxigard, or Penetrox or similar.

The tinned terminal as an intermediate has low enough index to prevent corrosion.
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W8JX
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« Reply #8 on: April 24, 2013, 03:19:04 PM »

The tinned terminal as an intermediate has low enough index to prevent corrosion.

Not same as galvanizing. Due to lead content it can increase corrosion potential in moisture. 
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