K7ITM wrote:
Huh? You wrote, "if tight winding results in a lower Q/other
effects, why space the windings for air-core, crystal radio coils,
period?" Do you not want a higher Q? Generally, people try for the
highest unloaded Q they can get, under some set of constraints.

Sorry, I made a typo. Rather, why do people tight-wrap coils, period?
Just ease of construction?

Close spacing lowers the Q mainly because the current in the wire is no
longer radially symmetrical, if you look at a cross-section of the
(round) wire. That raises the RF resistance of the wire. For decent
(low-loss) form material, it's mainly the RF resistance of the wire
that determines the loss and therefore the Q. Generally, highest Q for
a given diameter and length is obtained by spacing the wire about two
wire diameters, center to center, at least for high frequency work. If
you want to use Litz wire, there's an optimum stranding...more, finer
strands are not necessarily better as you get to either lower or higher
frequencies. You should be able to find info on that, if you do some
searching.
SNIP
Cheers,
Tom

Thanks for the information! Is there an advantage to wrapping cylinder
coils as opposed to spider or torroid, other than ease of construction?
I'm debating which one I'd likely get the best result with... I'll
probably make both to try it out, but I'd like to know which one would
'probably' work best.

Thanks again,
Dave

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

dave.harper wrote:


K7ITM wrote:

Huh? You wrote, "if tight winding results in a lower Q/other
effects, why space the windings for air-core, crystal radio coils,
period?" Do you not want a higher Q? Generally, people try for the
highest unloaded Q they can get, under some set of constraints.


Sorry, I made a typo. Rather, why do people tight-wrap coils, period?
Just ease of construction?


Close spacing lowers the Q mainly because the current in the wire is no
longer radially symmetrical, if you look at a cross-section of the
(round) wire. That raises the RF resistance of the wire. For decent
(low-loss) form material, it's mainly the RF resistance of the wire
that determines the loss and therefore the Q. Generally, highest Q for
a given diameter and length is obtained by spacing the wire about two
wire diameters, center to center, at least for high frequency work. If
you want to use Litz wire, there's an optimum stranding...more, finer
strands are not necessarily better as you get to either lower or higher
frequencies. You should be able to find info on that, if you do some
searching.

SNIP

Cheers,
Tom


Thanks for the information! Is there an advantage to wrapping cylinder
coils as opposed to spider or torroid, other than ease of construction?
I'm debating which one I'd likely get the best result with... I'll
probably make both to try it out, but I'd like to know which one would
'probably' work best.

Thanks again,
Dave


Me again...I should have read the later threads before my earlier reply.

Tom is correct about the Q using litz. Some guys have tried 48 ga litz
and said it nosedived in performance from the more-common 46 ga litz.
I've seen that explained with a critique of skin depth in that the rf
resistance of 48 is considerably higher at those freqs. Strand count
seems to still be in the 'more-is-better' range at BCB. 660-strand is
commonly used in DX sets...although I haven't graduated to that level of
expenditure myself

As to which to try....in a single-tuned set you won't notice the
difference. If you have a strong local BCB station the toroid will do a
very effective job of decreasing direct pickup by the coil. You might
still want a trap inline, though. Guess which type of coil makes the
best trap in this scenario!

My own tests gave a slight nod to the spider web coil. Not enough to be
noticeable in reception but enough for "spec-talk". The spider-web is
also less prone to proximity effects and even direct pickup because it
is 'directional'.

My own dx set which is admittedly tailored for my particular environment
uses a toroid on the first tuned stage, a toroid inline trap, then a
spiderweb on the detector stage with a loose coupled trap made with a
loopstick ferrite. A little of each, huh? I can receive stations
within 80-100 kc of the 5kw local that is 1/4 mile away on 1370.

My best recommendation would be to seriously consider a double-tuned
set. Its a whole different world than a single-tuned one.

-Bill