In article ,
wrote:

I found this very nice website with a section about Homebrew Butterfly
Capacitor with many photographs.

http://hps.infolink.com.br/py1ahd/gallery21.htm

Does anybody knows where in UK (preferrable in Dorset or London) can I
get aluminum foil, 0.5 centimeters thickness?

I think that figure, cited on the web site, must certainly be a
typographical error. There's no way that aluminum sheet which is 0.5
centimeters (5 millimeters) thick would make sense in this application.

I'd believe that it's supposed to be 0.5 mm (rather than 0.5 cm). I
can't offer a specific source, since I'm in the United States, but a
well-stocked hardware store might carry something of about that
thickness. When I look up aluminum sheeting at www.use-enco.com (a
U.S. mailorder supplier) the thinnest I see is 0.032", which is around
..8 mm I believe.

I do have one specific concern about the butterfly capacitor design
you're interested in. Capacitors used for resonating small magnetic
transmitting loop antennas need to have low resistive losses, due to
the high Q and the high currents in the loop. Butterfly capacitors
are often preferred to typical stator/rotor capacitors because they
have no "wiping" contacts (which have a relatively small contact area
and thus potentially a relatively high resistance). The most-favored
type use an all-welded or all-soldered construction, in order to
ensure a low-resistance path to all of the plates.

Unfortunately, the homebrew cap you're looking at uses threaded brass
rods, brass nuts, and aluminum plates. None of the connections are
soldered. The connections to the stator plates will be
interference/friction mating (i.e. brass nut, tightened onto aluminum
plate) and I'd be concerned that the aluminum will soon oxidize and
that the connections may rise in resistance, leading to increased
losses over time.

This issue could be largely eliminated if you were to use sheet brass,
rather than sheet aluminum, for the stator plates. You could then
flux and solder the rods, nuts, and plates together (before installing
the acrylic endplates, of course :-) and ensure a solid, reliable,
low-resistance current path.

I think you could continue to use aluminum for the rotor plates, if
you wish, since the current flow will be across the plate and there
will be little or no need for current flow between adjacent plates.

If you do use aluminum plates for the stators,, I'd consider adding a
thin smear of a conductive aluminum-compatible anti-oxidant grease
(Penetrox, NoAlOx, or a similar product) around the hole in each
stator plate, before tightening down the brass nuts. This would help
make a reliable contact, and exclude oxygen from the aluminum surface.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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