RE: Aluminum Wire (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 28 Sep 2007 00:56:17 -0500
From: sparks@xxxxxxxxxxxxxx
To: Tesla list <tesla@xxxxxxxxxx>
Subject: RE: Aluminum Wire  (fwd)

> Why would aluminum be considered "bad" for RF?
> My antennas work great...and they're made out of aluminum!

So does mine!  See http://w5jgv.com/2007_160m_dipole

But!  Having said that, the problem is that Tesla coils have some pretty
enormous current flowing through the coils - especially the primary -
when they are running.  Since the aluminum wire has a higher resistance,
the losses will be greater, and that equates with smaller sparks unless
you use larger wire for your coil.  Remember I^2R losses.  On the
primary, that's not going to be too large of a problem; you generally
can go with large wire there, but the secondary is another matter.
Doubling the wire size can quickly make the secondary grow too big to
work properly.

The skin depth of aluminum is slightly greater than copper.  At 100 KHz,
copper exhibits a skin depth value of about 0.0085 inches, while
aluminum has a calculated skin depth of about 0.012 inches. At that
depth, the current flowing in the wire is about 37% of the total
current.  Transformer wire is commonly sized for 4 to 6 skin depths in
thickness, minimum.  Since the skin depth is larger in  aluminum than in
copper, you will need even larger diameter wire than will be required
due to the difference in DC resistively alone.

I did a measurement today just to compare wire types and sizes. Using
some aluminum electric fence wire, the data measured with my Wheatstone
bridge, show that to equal the DC electrical resistance of # 12 copper
wire, you have to use # 9 aluminum wire.  That's quite a large size
increase.  Note that aluminum electric fence wire has more resistance
than the aluminum commonly used for electrical cables.  However, it is
very strong, and works quite well for wire antennas.  For antenna work,
the skin depth is so small that even the thinnest wire or tube commonly
used for antennas works just as well as if you had used copper.

73,

Ralph   W5JGV - WD2XSH/7


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