Re: Electrical Properties of Brass

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Mark Broker by way of Terry Fritz <teslalist-at-qwest-dot-net>" <mbroker-at-thegeekgroup-dot-org>

Terry, all,

The skin effect only applies for conductive materials.  Aluminum oxide is a 
moderately good insulator, and would behave in much the same way as the PVC 
or vinyl insulation on hookup wire.  Also, AFAIR, the depth of oxidizing is 
also much thinner than .010".  I have heard .0001", but take that with a 
grain of salt.  The aluminum oxide prevents further oxidation lower than a 
few atom layers near the surface.

However, it is still difficult to make a good electrical connection with 
the aluminum, since oxidation occurs in minutes.  Connections have to be 
made air tight to prevent the oxidation process, which probably already 
started before the connection was made air tight (work in an argon-rich 
atmosphere, perhaps?).

So I, too, would agree with Terry's recomendation against using it.

Once you've optimized your TC, you could coat a copper primary with a very 
thin layer of polyurethane.  Then just scrape away the PU at the tap point. 
This doesn't make future modifications/tweaks too easy, though.

Happy Easter

Mark Broker
Chief Engineer, The Geek Group



On Sat, 19 Apr 2003 09:20:53 -0600, Tesla list <tesla-at-pupman-dot-com> wrote:

>Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
>
>Hi,
>
>"I" would never use aluminum.  Aluminum forms thick (skin depth thick) 
>resistive oxide layers.  These are terrible for RF currents.
>
>The resistivity for pure Al is 2.7e-8 so at first it looks like a good 
>choice.  But a 10 mil layer of fluffy white AlO2 and it goes bad fast...
>Since pure aluminum oxidizes in minutes and can reach 5 mil in a day 
>easily, the RF resistivity can be just about anything but is generally 
>bad.  Other metals form oxide layers too but they are far thinner than 
>those of aluminum.
>
>Many people do use aluminum, but I would avoid it.
>
>Cheers,
>
>Terry
>
>At 05:03 AM 4/19/2003 -0700, you wrote:
>>Terry,
>>
>>That's really interesting! Can you do a similar
>>analysis of aluminum roof flashing? I've often thought
>>about cutting it into 1" ribbon and using it for
>>primary coil stock.
>>
>>Regards,
>>
>>--- Tesla list <tesla-at-pupman-dot-com> wrote:
>> > Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
>> >
>> > Hi Jeremy,
>> >
>> > Resistance = Resistivity X Length / Area
>> >
>> > The numbers I have here are all in meters.
>> >
>> > Copper = 1.7 x 10E-8  Ohm-Meters
>> > Brass = 6.39E-8 Ohm-Meter
>> >
>> >
>>http://www.matweb-dot-com/search/SpecificMaterial.asp?bassnum=MBRASE
>> >
>> > So brass is about 4X more resistive than copper at
>> > DC.
>> >
>> > Skin Depth = 1 / SQRT(pi x f x Resistivity x
>> > Permeability)
>> >
>> > Permeability = pi x 4e-7  H/m  X  Ur
>> >
>> > Ur copper = 1
>> > Ur brass = 1.05
>> >
>> >
>>http://www.npl.co.uk/electromagnetic/dclf/magnetics/lowpermeabilitylc.html
>> >
>> > So the skin depth is about 1/2 that of copper since
>> > the resistivity is 4X.
>> >
>> > The skin depth of copper is 2.602 / SQRT(F) inches.
>> > At 154kHz that is
>> > 0.00663 inches.  So brass is roughly 0.0033 inches.
>> > Your 16mil brass is
>> > perfect.  At 2 inches wide, it is about the
>> > equivalent of 0.159 inch copper
>> > tubing.
>> >
>> > So I will say your brass strips will be about 50%
>> > more lossy than 1/4 inch
>> > copper tubing.  Lots of "if, ands, ors, and buts"
>> > too all this... but the
>> > brass will work "ok" for a 15/120 NST system.  You
>> > may loose about 2 to 4
>> > inches as compared to 1/2 inch copper tubing or
>> > something but probably not
>> > a big deal.
>> >
>> > Cheers,
>> >
>> >          Terry
>> >
>> >
>> >