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RE: RF conductor materials (fwd)



---------- Forwarded message ----------
Date: Mon, 4 May 1998 07:16:47 -0400
From: "Thornton, Russ #CSR2000" <ThorntoR-at-rc.pafb.af.mil>
To: 'Tesla discussion Group' <tesla-at-pupman-dot-com>
Subject: RE: RF conductor materials (fwd)

I have a couple of comments:
>
>Hi Mike,
>        I was so super busy at the time you originally sent this that I
>didn't give a good response.  Sorry about that. :-(
>
>Aluminum and copper are not used in high-power high-Q RF circuits due to the
>problem of oxidation.  Bare aluminum and copper almost instantly begin
>oxidize in air.  Over time this layer can become very thick.  Aluminum oxide
>is an excellent insulator and is used as such in the common form of alumina.
>Copper oxide easily forms with similar results.  Since RF currents travel on
>the outside surface of conductors, this can have a very significant effect
>over time as the oxide layer builds up.  Aluminum house wiring used years
>ago keeps fire departments busy today :-(( 

I have to disagree with this statement in one way and ask a question in
another.
If the oxidation becomes an insulator, then why isn't it considered just
that an isulator?  Every flexable RF cable I deal with has an insulating
coating.  Why doesn't the RF electrical path just take the top most
conducting layer of the cylinder albeit at a smaller or reduced radius?

As far as the house wiring using aluminum, your statement is a little
misleading. The fires were not started because of oxidation causing
resistance along the lenth of the wire runs.  Instead it was because of
oxidation and maelability of aluminum at the connections, e.g. switches,
receptacles, breakers, etc.  I grew up in the electrical contracting
business and believe it or not aluminum is still used in house wiring.
There are restrictions however.  It is no longer used for circuit runs
but is very adequate for service from the meter to the the breaker
panel.  Also the utility company uses the heck out of aluminum.

Just my 2 centavos(in honor of cinco de mayo)
>
>There are four common solutions used in the industry:  
>
>Coated copper - like magnet wire.  The coating protects against oxidation.
>In high power RF circuits the coating may not be reliable however.   
>
>Brass - does not oxidize badly and is a good cheap alternative.  Over time
>it will give much better performance than copper.  Used extensively in the
>industry.
>
>Silver coated copper - At first this sounds like it would have the same
>problem as aluminum and copper.  So why is it used as a coating on so many
>high power RF components?  Nope!  Not because it is a better conductor.
>That really has nothing to do with it.  Silver oxidizes to silver sulfide a
>conductor (a neat trick! :-)).  Not a good conductor, but much better than
>brass if you need the low losses.  At my company we pump 100 amps of
>continuous RF through conductors of silver coated copper with good results.
>We cheat a little by pumping cooling water through the conductors to keep
>them cool too :-))
>Anything else will melt down very quickly except...
>
>Gold coated copper - Excellent conductor of heat and RF and immune to
>oxidation.  Often used on RF power transistors and other "high value" places
>where maximum performance is needed.  Cost is the killer for most normal
>applications.  
>
>        In our relatively low frequency uses, copper is generally very good
>(although my recent testing raises many questions about what frequencies we
>really need to design for).  Litz wire is good at our frequencies but
>expensive.  (We get custom Litz cables over 1 inch thick for 300KHz at
>100KW, the cost is stunning!)  Aluminum should be avoided since the oxides
>are so aggressive and thick.  Of course, everybody uses it all the
>time.....just because :-))
>
>        Terry
>
>If it looks simple, look closer!
>
>
>
>
>
>At 10:38 PM 4/15/98 -0500, you wrote:
>>
>>----------
>>From:  Hollmike [SMTP:Hollmike-at-aol-dot-com]
>>Sent:  Wednesday, April 15, 1998 7:29 PM
>>To:  tesla-at-pupman-dot-com
>>Subject:  Re: 40MHz Spark Gap Behavior
>>
>>Terry, 
>>    I was just reading the specs on your cap and have one question:  How
>>come
>>you chose brass to use as the "bus" for the doorknobs.  Aluminum is less
>>resistive than brass, and copper is far superior to either.   It may not be
>>that significant, but it could lower the Q of your primary system.  
>>Just Curious,
>>Mike Hollingsworth
>>
>>
>>
>
>Russ Thornton
>CSR 2040, 
>Building 989, Rm.  A1-N20
>Phone: (407) 494-6430 
>Email: thorntor-at-rc.pafb.af.mil
>
>
>