OT antenna question -- solved, soon to be fixed

[Vic Rosenthal]

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/
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

[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 <[hidden email]> 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/
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]
>

Lew Phelps N6LEW
Pasadena, CA DM04wd
Elecraft K3-10 / KXV144 / XV432
Yaesu FT-7800
[hidden email]
www.n6lew.us

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





______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

[Vic Rosenthal]

I asked my XYL to press the key and watch the SWR meter, and to let go
when it hit 3:1. Then she ran out and waved to me on the roof, at which
point I went up the ladder and felt the bolts. Very high tech.

If I'm right that it's the bolt material, just changing to brass bolts
should work.

73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/

On 14 Jan 2016 20:14, [hidden email] wrote:

> Nice work Vic!  Let us know how you determined the stainless screws to
> be heating?  Do you have an IR sensing gun with a tight pattern?
> Also be curious to hear how you plan to change the connection method and
> if successful?  All I can think of is a stainless hose clamp.
>
> 73, Mike  AC5P
>
>
> On Thursday, January 14, 2016 11:40 AM, Vic Rosenthal 4X6GP/K2VCO
> <[hidden email]> 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/

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

[Clay Autery]

Interesting.  My SWAG was the obvious transmission line/feed point
impedance mismatch, but I would have never thought about the fasteners
being the issue.
I'll be reading up on eddy currents now and how they apply to impedance.
Is it possible that it's not actually the ferrous materials and the eddy
currents generated, but the mating of dissimilar metal conductors
separated by a thin layer of aluminum or other oxide setting up some
level of capacitance, and/or some additional inductance added by the
stainless fastener and feed-line attachment (a 1-turn inductor?)?

All that put together in some freak fashion would lead me to believe a
small L-C network was created that may have caused a transmission line
to antenna impedance mismatch uncorrectable at the station... indeed,
under-reported in the SWR readings.

Just a huge guess and just curious.  Adding this to my "file drawer" of
troubleshooting ideas, but I'd like to understand the science.

Of course, it could be simpler...  simply multiple interfaces of
sufficiently incompatible metals that it sets up galvanic corrosion in
multiple layers.

> ...The trouble was at the connection of the feedline to the antenna.
> (...) 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 will change the method of connecting the line on Sunday and I hope
> it will solve the problem.

Assuming the root cause is galvanic corrosion (and potential
introduction of capacitance and/or inductance), I would surmise the
solution to be setting up a connection where no two incompatible metals
are touching.
Something like a bi-metal connector kinda like the one on this page
(only correctly sized):  http://www.utilex.com/bi-metallic_connector.htm

If I'm way, way off base, please educate me.  I'm on this list to learn.

73,

______________________
Clay Autery
(testing 02/20/16)

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]