I s'pose Reg, the local expert on proximity effect, etc., should pop in
here and 'splain it all. Seems, though, like it's wrapped up in
practicalities. For low-frequency work you typically want a lot of
inductance, so you use fine wire so you can get a lot of turns in a
relatively small volume. The wire diameter is small enough that, at
low frequencies, the skin depth is large compared with the wire size.
I believe you will then find that the proximity effect won't have as
much influence on the Q as in the case where the skin depth is a small
fraction of the wire diameter. So for a 50Hz/60Hz power transformer,
you won't find the turns spaced apart any more than needed for
insulation.

You can do a Google search for conductor proximity effect and find a
bunch of references. The stuff at
http://www.national.com/nationaledge...c_article.html has some
nice pix to show the effect in a bit different environment than we're
talking about here.

The Q you actually obtain may depend on so many other things than just
the shape of the windings that it's not possible to tell you the "best"
geometry. But I can tell you that if you make a large coil of good
design, you should be able to get to a high enough unloaded Q that
doing better with a different geometry about the same size will give
you only small returns on the performance in the circuit. That is, if
you do manage to make a solenoid coil say 5 inches long and 5 inches
diameter, maybe getting the Q up near 500 if you're careful, then
operating it at a loaded Q of 100 (for a 10kHz bandwidth at 1MHz), the
loss in the coil compared with an INFINITE unloaded Q is so small as to
be nearly unnoticable. If my mental arithmetic is right, it would be
about a 1dB difference, just barely audible. And of course, you won't
get anything like that much improvement in Q with a different shape.
Plus--the standard solenoid shape is easy to construct! (There ARE
reasons for wanting higher unloaded Q, if you want to operate at a
higher loaded Q and if you want to build a multiple-resonator tuner,
but my impression is you are not there yet!)

Cheers,
Tom