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From: "Barry L. Ornitz" <>
Subject: Re: Cad done?
Date: 21 Dec 1998

Tim Shearer asked how CADWelding was done.  Louis Bybee physically
described the process but got the chemistry wrong.  The reaction looks like
ignited gunpowder, but is entirely different.

CADWeld is basically copper thermite - a mixture of copper oxide and
aluminum powder.  Once ignited, the two react to reduce the copper oxide to
metallic copper and oxidize the aluminum to its oxide.  The process is
quite exothermic (liberates lots of heat) and the metallic copper that is
produced is white hot.  This flows to the bottom of the mold and slightly
alloys with the metal being connected.  The bond is somewhat like brazing,
but since it is done at a much higher temperature, more alloying of the
base metal occurs.  It is an exceptionally effective bond for grounding and
power connections.  [Iron thermite, made with iron oxide and aluminum is
often used to weld railroad rails.  It takes a huge acetylene torch or a
big arc welder to produce this much heat.]

I am surprised that a CADWeld joint could be a significant source of
intermodulation distortion if the joint is prepared properly.  There are
some intermetallic species produced such as aluminum, iron, carbon and
silicon alloys - especially if the joint is not cleaned before "firing".
These could certainly have nonlinear voltage/current curves.  I would think
that these could be minimized by carefully controlling the CADWeld process,

But I am even more surprised that mechanical bolted stainless steel
hardware is suggested as the replacement.  From a chemical corrosion
standpoint, connecting copper and steel together mechanically - even with
stainless in between, is asking for corrosion.  In fact, stainless steel
can be both anodic and cathodic to ITSELF.  This means that a single piece
of stainless can act as a galvanic battery and promote its own corrosion.

                Dr. Barry L. Ornitz     WA4VZQ

From: "Barry L. Ornitz" <>
Subject: Galvanic Corrosion - was Re: Dissimilar metals in ant question
Date: 24 Dec 1998
Keywords: corrosion, galvanic series, dissimilar metals

Reg Edwards, in message <01be2fc1$e1922ec0$e85fac3e@default>
opened his mouth and inserted his foot again.  I am quoting his entire
reply to point out specific errors.

>I think severe corrosion occurs only when a DC current flows, or is
>allowed to flow, across the junction of the dissimilar metals.

True.  But the current is produced BY the dissimilar metals.  And the
corrosion does not occur at the ohmic contact either.

>If I have understood you correctly,  you are concerned only with the
>top part of an antenna which is very well insulated from ground.  So
>no DC current can flow through the junction and it won't matter in
>the slightest what metals are involved.

Totally wrong.  The two metals in good electrical contact with each other
form an ohmic contact.  But where the metals are near each other, in the
presence of a conducting electrolyte, DC current will be generated and the
aluminum, being the anodic metal of the couple, will selectively corrode.
Connection to ground is totally irrevalent.  The corrosion occurs where the
aluminum contacts the electrolyte.

>But to be absolutely certain, provide further protection by lacquering
>the junction to keep rain-water away.  Specially the acid rain stuff.

Good advice, but again irrevalent.  Rain water is always conductive enough
to promote galvanic corrosion; acid raid is hardly required.  And more than
the junction must be protected.

Since Reg always asks for references, let me quote from the following
website which specifically discusses the electromotive series and galvanic
corrosion in a Materials Science course:

    "For galvanic corrosion to occur, five requirements must be met:
     - One metal must serve as the anode.
     - One metal must serve as the cathode.
     - There is an ionic path connecting the two electrodes.
     - There is an electrical path connecting the two electrodes.
     - There is a difference in electrochemical potential between the two
electrodes. "

In this situation, the aluminum is the anode and the copper is the cathode.
The ionic path occurs whenever dust and moisture cover the two materials;
it does not have to be liquid water.  The electrical path is obvious as the
two metals are clamped together.  The electrochemical potential between
copper and aluminum is approximately 2 volts.  So galvanic corrosion WILL

Keeping the ENTIRE antenna away from moisture with paint or lacquer _might_
work (not just the junction), but it is next to impossible in general
practice.  Wind flexing the elements, sunlight degrading the paint, oxygen
attacking the paint, and dust and pollution in the air all will contribute
to eventual cracks forming in the moisture barrier, followed soon after by
galvanic corrosion which will further damage the paint.

The best situation is to keep the antenna from having ANY dissimilar
materials in it.  Where this is not possible, select materials which are
close together on the galvanic series of metals.  I have included a
condensed list below.  Note how far apart copper and aluminum are.  The
closer two metals are to each other on this list, the lower the
electromotive potential is between them - and the electromotive potential
difference is what drives the corrosion.  The rate at which corrosion
occurs, however, is determined by the current flow between the two
materials.  Thus corrosion will occur relatively slowly where the two
materials are coated with a thin layer of slightly conductive dust, but it
will occur quickly when the same two materials are immersed in salt water.
Ohms law still applies though, so the lower the potential, the less current
that will flow.

ANODIC (Least Noble) Material
    Magnesium alloys
    Aluminum 25
    Aluminum 17ST
    Steel or iron
    Cast iron
    Chromium-iron (active)
    18-8 Chromium-nickel-iron (active)
    18-8-3 Chromium-nickel-molybdenum-iron (active)
    Lead-tin solders
    Nickel (active)
    Inconel (active)
    Hastelloy C (active)
    Copper-nickel alloys
    Silver Solder
    Nickel (passive)
    Inconel (passive)
    Chromium-iron (passive)
    18-8 Chromium-nickel iron (passive)
    18-8-3 Chromium-nickel-molybdenum-iron (passive)
    Hastelloy C (passive)
    Carbon and graphite
CATHODIC (Most Noble) metal

In an earlier post, I mentioned that the stainless alloys can galvanically
corrode by themselves without the presence of a dissimilar metal.  This
characteristic is a property of almost all nickel alloys not just the 18-8
(400 series) and 18-8-3 (300 series) of stainless alloys.  It is indeed
unfortunate that these alloys were given the name "stainless" since they
can often corrode under relatively mild conditions.

           Dr. Barry L. Ornitz     WA4VZQ

From: "Barry L. Ornitz" <>
Subject: Re: Galvanic Corrosion - was Re: Dissimilar metals in ant question
Date: 25 Dec 1998

Reg Edwards wrote in message <01be3057$945a81c0$01ffabc3@default>...
>I'm not a chemist.

I am not a chemist either, but except for some of the finer points of alloy
corrosion, the chemistry is quite simple and is discussed in many web
sites.  I gave a reference to one that was very simple to understand; did
you read it?

> All I know from experience is that if two
>non-exotic dissimilar metals are bolted together, are kept dry, and
>there is no DC circuit, the construction will last for years.

Copper and aluminum are certainly not exotic metals.  In fact, in the
galvanic series I listed, all of the metals and alloys are quite common.
If they are bolted together, there _IS_ a DC circuit.  Keeping something
dry means different things to different people.  As I posted, even small
cracks in the paint or other coatings will allow corrosion to proceed.  As
the materials corrode, the water resistant coating is damaged and things
get worse.  A thin layer of moist dust is all it takes for corrosion to

>At least as long as the period till one gets sick and fed up with that
>particular band antenna anyway.

You must have a short attention span.  Copper and aluminum can corrode very
rapidly; it is one of the worse metal combinations commonly seen.

>Of course, I would not even dream of joining lead, tin or cadmium to
>gold where migration and poisoning can occur.

Pray tell how poisoning occurs, or what you even mean by poisoning.  Yes,
these metals diffuse into each other fairly rapidly (at least compared to
most other metals).  But the time scale is still exceptionally long -
especially at room temperature.  To give you an example, I looked up some
diffusion coefficients and found that for aluminum into silicon, for
example, the diffusion coefficient was 1.1E-7 m^2/s in the 800 DegC region.
Since the diffusion coefficient follows an Arrhenius relationship with
temperature, at room temperature this would work out to something
approximately 80 orders of magnitude lower.

>Neither would I encourage use of dissimilar metals in such structures
>as suspension and girder bridges, 1000-foot TV station masts,
>skyscrapers, Jumbo-jets, spacecraft,  where human life expectancy ,
>failure probabilities, statistical factors of safety, etc, affect
>engineering economics.
>But 2-foot long top rods of mobile antennas  -  -  -  -  -  -  -  -
>-  -  I ask you ?
>Reg,   G4FGQ        Click below.

The antenna in question was a fixed collinear base station antenna.  I
expect its owner wants it to work a long time.  Any corrosion in the joint
will not only reduce the ohmic contact between the top of the antenna and
its base, it will also be a nonlinear region where harmonics and
intermodulation distortion will be produced.

        Dr. Barry L. Ornitz     WA4VZQ

From: "Barry L. Ornitz" <>
Subject: Reg Waffles again - was Re: Galvanic Corrosion
Date: 25 Dec 1998

Reg Edwards wrote in message <01be30aa$1b2a8ea0$7e3eac3e@default>
responding to my statement:
>>  Any corrosion in the joint
>> will not only reduce the ohmic contact between the top of the
>> antenna and
>> its base, it will also be a nonlinear region where harmonics and
>> intermodulation distortion will be produced.
>Barry,  I am certain your statements such as above are correct.  And
>I hasten to assure you my remarks below are not inended to be
>antagonistic in any way.


This doesn't cut it.  You are trying the same stunt you have pulled so many
times on this group.  Why not admit you are in over your head, and that you
gave incorrect information?

>But unless you can give quantitative numerical estimates of the ohmic
>contact resistance expected to be developed between aluminium and
>copper, due to dissimilar metals emfs, when firmly clamped and kept
>dry for a period of 12 months, and the level (in dB ?) below radiated
>power of intermodulation distortion, etc,  to be expected for a given
>transmitter power at Robert's operating frequency, your information,
>although of academic interest,  is not of much practical value.

Again, Reg falls back on this approach.  He needs "specific" information.
Well, I actually have some information on this hidden away in one of a few
dozen packed boxes.   I see no need to find this just for Reg's interest.
{I think the article I have is from a patent application so I might not
have to look for it after all.  If I find it, I post the relevent details.}

To begin with, Reg did not specify what he meant by dry.  We are talking
about a real antenna here, not something kept under nitrogen in a
dessicator.  Suffice it to say that copper oxide was one of the earliest
semiconductors to be used in rectifiers.  Also Reg, you obviously did not
read the reference I listed in my first article.  If you did, you would
realize that the increase in ohmic resistance is due to the corrosion
components in the joint and has nothing to do with the difference in
galvanic potential between the metals.  But it is this difference in
galvanic potential that causes the corrosion, however.

>I suspect the intermodulation products, if present at all, would be
>so far below the transmit power level as to be undetectable in normal
>radio communication traffic, receive or transmit.  And nothing the
>licensing authorities need be alarmed about.  There will always be
>the 1-in-a-thousand case when someone might notice something not
>working quite as it should for a day or two.  Easily cured by
>tightening the hose-clip which is probably stainless steel anyway.

I guess Reg has never seen TVI caused by harmonic generation from a copper
antenna wire touching a galvanized steel or an aluminum gutter.  This is a
very real problem and it has been documented in every ham and professional
radio magazine that I know of.

>Robert's best plan would be to obtain aluminium tube or rod.
>Diameter need not be exact but within +/- 25 percent of the original.
>The principal  beneficial effect would be to end this academic

Exactly what I suggested in the first place.  And if you really want to see benefit, stop trying to argue about things you
know nothing about.

        Barry L. Ornitz     WA4VZQ

From: "Barry L. Ornitz" <>
Subject: Reg Edwards and Passive Intermodulation Distortion
Date: 25 Dec 1998
Keywords: passive intermodulation distortion, dissimilar metals; corrosion

In an earlier post Reg Edwards asked for specific references to
intermodulation distortion caused by dissimilar metals.  The list below
should be sufficient (but probably not for Reg).  I was unable to find the
particular patent I was searching for.  If it turns up, I will post an

Summitek Instruments manufactures testing equipment to measure such
distortion.  Quoting from their literature, the following is useful
information about the "real world" and protecting joints and antennas from

"... And then Mother Nature takes over...

   The natural elements are the enemy of those portions of the
   network that are exposed to them.

    - Wind-induced vibration
    - daily temperature variations
    - moisture in its various forms
    - thermal loading by the sun
    - air borne dirt

  Each of these works to break down and erode the quality of
  the components in the network, and ultimately the
  communication channel. The weakening of joints, separation
  of junctions, invasion of moisture, oxidation of materials,
  and contamination by dirt and dust all give rise to passive
  intermodulation and desensitization of the receiver.
  The result is a cell site performing below its design

I hope this post will end the discussion,

        Dr. Barry L. Ornitz     WA4VZQ

References on Passive Intermodulation Distortion and Metals

Arazm, F. and Benson, F. A.: 'Non-linearities in Metal
Contacts at Microwave Frequencies'. IEEE Trans., 1980, 22,
(3), pp. 142-149

Betts, J. A. and Ebenezer, D. R.: 'Generation of
Intermodulation Interference Due to Nonlinear Effects in the
Near- Field Regions of Multiple-Transmission Communication
Systems'. Proc. AGARD Conf. on Aerospace Telecommunication
Systems. May 1972, pp. 25.1-25.12

Bond, C. D., et al.: 'Intermodulation Generated by Electron
Tunneling Through Aluminum-oxide Films'. Proc. IEEE.
December 1979. 67, pp. 1643-1652

Lee, J. C.: 'Intermodulation Measurement and Analysis of
Some Conducting Materials Commonly Used in Aerospace'. IEEE
Int. Conf. on Communications, Vol. 2, June 1980, pp. 25.6.1-

Lee, J. C.: 'Intermodulation Measurement in the UHF Band and
an Analysis of Some Basic Conducting Materials'. TN 1979-70,
Lincoln Lab., MIT, USA, November 1977

Lui, P. L., et al.: 'Measurement of Intermodulation Products
Generated by Structural Components'. Electron. Lett. 1988.
24. (16), pp. 1005-1007

Lui, P.L., et al.: 'A Survey of Non-linear Junction (Rusty-
Bolt Effects) Detection Techniques'. IERE Int. Conf. on Land
Mobile Radio. UK, December 1987

Lui, P.L: A Study of Intermodulation Interference due to
Non-linearities in Metallic Structures. Ph.D. Thesis,
University of Kent, UK, February 1990

Martin, R. H.: 'Intermodulation Product Generation Studies
on Materials, Connectors and Structures'. IERE Int. Conf. on
Electromagnetic Compatibility. April 1978, pp. 243-2508

'Passive Intermodulation Products in Antennas and Related
Structures'. IEE Colloquium Digest No. 1989/94, June 1989

Watson, A. W. D.: 'Improvements in the Suppression of
External Non-linearities ( 'Rusty Bolt' Effects) which
Affect Naval Radio Systems'. IEEE Int. Symp. on
Electromagnetic Compatibility, 1983, pp. 157-160

From: "Barry L. Ornitz" <>
Subject: Re: Reg Edwards and Passive Intermodulation Distortion
Date: 25 Dec 1998

J. Fred Riley wrote in message
>Well, actually, the intermodulation distortion is caused by a
>non-linear device formed by metal oxide, most probably in the form of
>a diode.

The nonlinearity does not necessarily have to be an oxide, although it
often is.  And it does not have to be a rectifier.  Any nonlinearity in the
voltage/current relationship will generate harmonics and intemodulation.
Copper sulfide, a common problem with copper in an area where coal is used,
is also a very good rectifier, and aluminum oxide in thin layers can
produce a nonlinear conduction through quantum tunneling.  There has been
considerable work in recent years to eliminate nickel and iron from cable
connectors too; the problem here is the nonlinear magnetic characteristics.

Your anecdote was an excellent one.  There are so many possible sources for
intermodulation that it is indeed very difficult to find the worst one.  I
have seen fences around MW broadcast sites that were welded wire rather
than conventional chain-link fences, just to avoid such probelms.  Your
detective work was excellent in realizing that the second harmonic might
not come from the transmitter.

        73,  Barry   WA4VZQ

>One of the tougher cases I ever had to solve  was the radiation of
>second harmonic signal from an AM station in Paris, Illinois.  Turns
>out the diode was formed where barbed wire was wrapped around a nail.
>And the resonant frequency of the "insulated" wire was about a half
>wavelength at the second harmonic.  Because the fence was near the MW
>tower, it was difficult to separate the source of the fundamental and
>the second harmonic radiation.
>Fred, W8OY

From: "Barry L. Ornitz" <>
Subject: Ground Wire to Tower, was Re: Galvanic Corrosion
Date: 26 Dec 1998

J1aguiar wrote in message <>...
>Re: Galvanic Corrosion. When using a galvanized tower, grounding often
>becomes a problem with copper conductors. What is the proper way to
>connect the bottom of a galvanized tower to a copper clad ground rod.
>Aluminum wire ?

Galvanized iron is iron that has been coated with zinc for corrosion
protection.  Zinc being anodic to the iron will corrode first, protecting
the iron underneath.  Aluminum can be attached to zinc without excessive
corrosion, however since the zinc layer is thin, you should design the
connection to work directly with the steel.  Since iron is anodic to copper
by a considerable voltage, the normal practice is to use one or more metals
in between to lower the individual galvanic potentials.  Stainless steel of
the 300 series (particularly 308 or 316) is the material often used (*).
The tower would be bolted, either itself or with a strap, with a stainless
bolt using a stainless washer with a star lockwasher to "bite" through the
galvanizing and surface oxidation on the stainless.  The copper grounding
conductor would then be attached to the bolt again using star lock washers
and flat washers.  The result would be a steel to stainless to copper
contact.  But understand such a connection will still corrode - just more
slowly than a copper to steel direct connection.  The key is that that
there should be enough compressive force that the star lock washers
continue to bite into the steel and copper.  You might even consider the
use of Belleville washers to maintain the compressive force.

But remember, this connection still needs to be maintained.  It should be
checked often, and retightened as needed.  Every year or so, the connection
should be taken apart.  All the surfaces should be polished free of any
corrosion before reassembly.  Keep the joint as dry as possible since a wet
joint will hasten the corrosion, but remember the joint will corrode
anyway.  The use of one of the zinc granule containing greases that are
commonly used with aluminum may be used but the "teeth" in the lock washers
are the real key.

I have no experience using CADWeld on the small towers likely to be used by
amateurs.  You would preferably remove the galvanizing at the location of
the joint before firing the CADWeld.  The question I would have here is
whether the CADWeld joint gets hot enough to weaken the structural
properties of a thinwall tower leg.  Perhaps Gary Coffman or someone else
could address this.

>And the other question, is it proper to ground the tower though the steel
>rebar condstruction in the concrete base, and if a ground rod was placed
>inside the concrete during the time of installation. Is it better to keep
>the ground out of the concrete.?

If it is possible to avoid running the ground through the concrete, I
believe it is best to do so.  Every large antenna installation that I have
seen did it this way - especially those taking frequent lightning hits.
The worry is that enough current might flow through the conductor that it
would heat and expand causing the concrete to crack.  Most rebar is
installed for structural reasons, not for electrical ones, and the internal
connections are often not very good electrically.  I would personally not
want to rely on rebar for the tower lightning protection.  Again I would
defer to Gary Coffman on this subject since he deals with similar issues

(*) Since the lockwashers "bite" through the passivated stainless surface,
the stainless is considered "active" in the galvanic series I posted

        73,  Barry L. Ornitz     WA4VZQ

From: "Barry L. Ornitz" <>
Subject: Re: Bimetallics and Ground Rods
Date: 12 May 1999

Reg Edwards wrote in message <01be9ce0$decae9c0$9eecabc3@default>...
>How come alloys of aluminium and copper, used as structural
>materials and for such things as ladders, are extremely
>corrosion resistant ?  I've had a ladder stored outside for
>35 years in acid rain and it still takes my weight.

Because, as you say, they are alloys.  The copper atoms are imbedded within
the aluminum matrix.  The corrosion will occur locally at the surface where
a copper particle is exposed.  The aluminum dissolves away leaving the
surface copper atom to fall out (actually it only takes a microscopic
amount of surface corrosion to produce an essentially all aluminum surface.
Aluminum alloys containing copper only have small amounts of copper which
is added mainly to provide a harder alloy.

Better cut down on the kidney pies if the ladder stays out much longer!

        73,  Barry L. Ornitz     WA4VZQ

From: "Barry L. Ornitz" <>
Subject: Re: Alum to Copper Grounding Questions
Date: 14 Jun 1999

Pete Rimmel N8PR wrote in message <>...
>To get a good connection betweeen the copper and aluminum, go to an
>electrical contracting outlet and buy some 'Noalox' It is a zinc paste
>which you smear on both wires and then clamp them together...  It will
>give you "No-Aluminum-Oxidation" in the connection and keep it clean and
>low resistance.

Noalox offers no galvanic protection between copper and aluminum surfaces.
It is merely a thin paste of powdered zinc in a light grease, and it is
designed to be used between aluminum connections.  The zinc particles will
pierce the aluminum oxide layer and allow a better aluminum to aluminum
connection.  Zinc is used because of its hardness compared to copper, and
the fact that it is close to aluminum in the galvanic series.

Zinc is slightly anodic to aluminum, and it offers no protection in this
application.  Aluminum is quite anodic with respect to copper.  What is
needed is a material between aluminum and copper in the galvanic series.
Typically used is stainless steel.  Polish the stainless surface with a
fine abrasive before making the connection.  This assure an active surface
for corrosion protection.

        73,  Barry L. Ornitz     WA4VZQ

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