I apologize if this post is off the subject of computational
electromagnetics. However, there has been some interesting discussion
lately about materials of construction and the practical aspects of wire
antennas. I hope the regular readers do not mind.
On Mon, 12 Feb 1996, Richard Looke wrote:
> We all know that at certain frequencies, stranded wire is a much better
> conductor than solid for a given wire gage due to the so-called 'skin
> effect'.
I assume you are talking about multistrand insulated wire, the so-called
Litz wire. In this situation the insulation is critical. In fact, the
wire is normally stranded in such a way as to weave the conductors in
such a fashion that all conductors are brought to the surface
periodically down the length of the wire.
> With that in mind, what happens to the superior properties of the stranded
> wire if you tin it (i.e. dip it into a bath of molten tin/lead) for the full
> length of the wire? Will it behave more like solid? Completely like a solid?
> Or perhaps electrons traveling through this wire would prefer the skin of the
> copper, but find additional path through the skin of the solder, thus making
> the tinned stranded copper an even better conductor.
Tinned wire will behave more like the solid wire. However, the
resistivity of lead and tin is considerably higher than aluminum or copper.
This has a significant effect on the skin depth. Note the following
table for information about the pure materials.
Material Resistivity, ohm-cm skin depth, 1 MHz
Aluminum 2.62e-6 0.081 mm
Copper 1.72e-6 0.066 mm
Tin 11.4e-6 0.170 mm
Lead 21.9e-6 0.235 mm
> What sorts of activities occur in the valence regions between the
> copper-tin/lead interface?
I am not sure what is meant here. The tinning is generally used to
minimize corrosion. Copper oxides do form rectifying junctions.
> I've heard arguments about this in the past. Especially when dealing with
> ground busses. You definately want to use stranded if you care at all about
> RF. But when you solder up a connection, say midpoint on the bus wire, should
> you be concerned about letting the solder 'wick' out past the joint?
[If you really want to hear arguments with lots of hype and virtually no
fact, talk to audiophiles about speaker wire and interconnects!] :-)
Braid conductors are often used for ground busses at RF because of the
reduced inductance of the flat braid over round wire. Flat copper strip
is slightly more effective but not as flexible and convenient. Since the
braid is normally made up of non-insulated conductors, which over time
will oxidize and corrode on the surface, I would expect the soldered
connection to have a minimal effect on the overall RF resistance.
On a practical note, I would like to add something to the comments Buzz Shaw
(hbshaw_at_top.monad.net) wrote:
> Coating elements does extend the life of copperweld. The best coating I
> have found to date is high quality car wax. When applied every 3 - 4
> months the copper stays bright and trouble spots are easily found.
> Polymer coatings not only effect the electrical performance but also crack
> and let in the bad stuff and none I have ever tried has the UV resistance
> demonstrated by simple car wax.
Silicone car wax is the material of choice here. If you need to use
paint on your antenna, acryllic paints offer the best ultraviolet
resistance of most commercial paints. Ordinary Krylon (tm) spray paint
holds up extremely well outdoors and it is a low loss dielectric too. I
usually spray all bolts and other connections with clear Krylon before
erecting an antenna. I do not live near the ocean so I cannot predict
how well it will hold up there, but I have been quite pleased with its
performance in east Tennessee.
73, Barry L. Ornitz WA4VZQ
Eastman Chemical Company Research
ornitz_at_eastman.com
Received on Tue Feb 13 1996 - 17:31:00 EST
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