-ex- wrote:
Comments interspersed, and staying with the BCB range scenario...


dave.harper wrote:

-ex- wrote:


In practice the coil Q is determined primarily by the form dielectric,
wire size, wire spacing, diameter/length ratio/neary coupling effects,
etc. R is far enough down the list that its generally not even
considered.


Ah, so the voltage drop across the coil (due to the small internal
resistance) and the close proximity of the wires give it some
capacitance? Does this affect performance or just screw add unwanted
capacitance?

The internal capacitance of the turns isn't enough to radically change
the basic LC resonance. Instead it tends to result more like dielectric
leakage

Wouldn't adding space between wires cause some eddy currents and lower
the L of the coil?

Again, not significantly in the BCB example. Take for instance, a 4"
diameter coil wound with #18 wire, however many turns it takes. Lets
say 60. Winding the coil close-spaced as opposed to about
one-wire-diameter spacing will require a few less turns (maybe 10%) to
get the same L. But the close spacing WILL result in lower Q once you
re-establish the same inductance. There can be more than one reason for
this...is it the winding spacing or the length/diameter ratio or more
dielectric loss that causes this? (Its certainly not the R). Its
impossible to say because you can't have one without the other!

Smaller coils, say toilet-paper tube size, don't exhibit this effect -
or at least not to the same degree. But there's a whole different
geometry there and its not optimum.

Nobody really knows exactly what goes on here other than trial-and-error
experiments to see how they behave.

Thanks again for the reply.

From what I've read here and elsewhere, I'm debating either making a
spider coil or a tight wound 4" cylinder ( about 1" length). Is there
any significant advantage to either?

I can see with a powered ferrite core how the spacing would make less
of a difference... but if tight winding results in a lower Q/other
effects, why space the windings for air-core, crystal radio coils,
period?

Thanks!
Dave