[Elecraft] OT antenna question -- solved, soon to be fixed

[Lewis Phelps]

I meant to reply to this query earlier,  but got sidetracked.  What I was going to suggest was that you look at the interfaces between stainless steel and aluminum.  They are potentially problematic, because SS and aluminum have very different galvanic potentials, and “in the presence of an electrolyte” create a battery with a significant corrosion-causing voltage.  This will cause serious galvanic corrosion of the aluminum.

More specifically, the aluminum alloys most often used for antennas have a galvanic potential of -0.8 volt to -1.0 volt.  Depending on the stainless steel alloy employed, it can have a galvanic potential of about -0.05 volt to -0.25 volt, which means a voltage differential of anywhere from 0.6 volt to 1.0 volt. If there is moisture in the air (which means most places in the world), it will create an electrolyte, and corrosion will commence. 

I have always been amazed that the standard practice in antenna building, both commercial and home brew, has been to use aluminum tubing and stainless steel fittings. That is, in my mind,  a recipe for disaster.

How to eliminate galvanic corrosion?  

1. Use aluminum bolts, nuts and fittings with aluminum tubing.
2. Use cadmium-plated or zinc-coated fittings with aluminum tubing.
3. Use stainless steel fittings with stainless steel tubing.
4. Use brass or copper fittings with brass or copper tubing
5. Use soldered copper joints.
6. Protect interfaces between dissimilar metals so that no moisture can ever get into the metal joint. 

There are other approaches that provide low galvanic potential difference, but the above are tested and useful.

Lew N6LEW


> On Jan 14, 2016, at 9:39 AM, Vic Rosenthal 4X6GP/K2VCO <k2vco.vic at gmail.com> wrote:
> 
> I would like to thank the numerous hams who responded to my plea for help.
> 
> The problem that was causing my SWR to rise with high power was not in my shack. The tuner, balun, coax and open wire line were all 100%. The trouble was at the connection of the feedline to the antenna.
> 
> The connection was made where stainless steel bolts passed through the tubing. The bolts heated up rapidly when running over 500w. At first I thought the connection was poor, but improving it did not help. From the way it was heating (away from the point of contact of the lugs to the antenna tubing) I think that it was due to the ferrous material (stainless steel) of the bolts heating due to eddy currents in the strong RF field.
> 
> I have experienced this phenomenon in amplifiers, but never an antenna!
> 
> I will change the method of connecting the line on Sunday and I hope it will solve the problem.
> -- 
> 73,
> Vic, 4X6GP/K2VCO
> Rehovot, Israel
> http://www.qsl.net/k2vco/
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Lew Phelps N6LEW
Pasadena, CA DM04wd
Elecraft K3-10 / KXV144 / XV432
Yaesu FT-7800 
Lew at N6LEW.US
www.n6lew.us

Generalized Law of Entropy: Sooner or later, everything that has been put together will fall apart.