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.


Does wire coating make a difference regarding the dielectric? Or is it
another capacitance-altering effect?

Yes it does. One of the tests on a good high Q coil is to set it up on
a Q-meter then touch a piece of your coil-form material (or
wire-insulation) to the coil and see how it behaves on the Q-meter. It
shouldn't move. Lossy core material/insulation will cause a visible
effect with this test. I'll reiterate in case someone jumps in and
reads this without reading the earlier parts of the thread...you won't
see this happen with a low-q coil but as you get higher in Q it becomes
more and more evident. In fact, with a big solenoid coil and Q500 you
pretty much have to tie the sample material onto the end of a stick to
do this test because of hand effects. Not to be confused with resonance
detuning effects.

There's no good rule of thumb for insulated wire other than a test like
this. There's quite a bit of insulation material in 660-strand litz and
thats darn good wire. No way to make a comparison because BARE litz
can't exist! I don't think I've ever heard a comparison made between
say bare 16-18 wire vs enamelled. I tend to think any difference would
approach the 'too difficult to evaluate' range.

-Bill