I forgot to mention a few things on my previous post. They bubbled
up later as I was doing yard work!
My basket weave coil is about 4.1" diameter and 1.2" in length. So
it is "loop-like". I think that a longer and narrower solenoidal
coil will work more poorly as an antenna. Thought I should mention
this in case anyone throws something together to fool around with.
I was thinking of the spider web type coil because the turns count
could be easily manipulated for alignment purposes. I think it would
be easier to unwind a portion of a turn off the broad flat spokes of
a plastic form than from a solenoidal type loop. I wonder if the
wire unwound could be left intact (if short) for an iteration or two
until the "ballpark" is found. Then wire cutting could happen. Off
the coil its inductance would be fairly low at 600 kHz.
I think it was John who furnished the nice one-pager on the SRF stock
ferrite bar coil complete with measurements of inductance (+/- 580
uH). Does anyone know how much it is possible to vary the inductance
by sliding the smaller winding across the customary range that is
used in alignment? That would give an indication of how much an air
core loop would have to be varied during alignment.
Interesting situation on the MW vs LW windings used with the same
variable. I know that more turns and less C create a higher voltage
at the loop terminals. This is handy with crystal set loops because
the higher performance diodes will work more efficiently with higher
voltage. But I think this should not make any difference with a
magnetically coupled arrangement like you describe. In fact, if the
wire sizes for both windings are the same and the windings are the
same size I would think that the shorter winding would be superior
because the total copper losses would be lower in the shorter length
of wire. And, the capacitor would be used at a higher value to
resonate the pair at the same frequency. Unless the capacitor is a
high quality one (ceramic insulators, no corrosion at joints) it will
have a higher Q at higher capacity settings. This idea is (once
again!) from a Ben Tongue paper where he documented that a small
receiver type phenolic insulated variable had lower losses at higher
settings of C.
I suspect that having two loops on one frame might be the answer to
this situation. I believe that unused windings and tapped windings
are usually avoided in constructing high performance loops. Guess it
depends on convenience and what is desired for efficiency.
How big is your box loop? My current crystal set is a 3' x 3' box
loop of #14 bare solid wire. In the last listening contest in
January I logged about 90 stations with it in a week or so. I don't
use a spotter radio -- I like to do it all with the crystal set.
Just nuts I guess. Ought to fit in well here!
near Syracuse, NY