[Elecraft] toroid turn spacing-how it matters(?)

[Rick]

I have to recall my old school days here.  I wish I had an RCL bridge to
verify what I am about to say. Oh well, here goes:

Ideally the spacing between the turns would not affect the inductance of a
toroid type inductor.  The core captures all of the magnetic field from each
wire turn regardless of the location of the wire with respect to the body of
the toroid. The wire simply has to pass through the center of the toroid for
this to be true. One way to envision this is to imagine the total magnetic
field about the wire. As the wire is moved away from one interior edge of
the toroid body, it must approach another.  No matter where you place the
wire inside the toroid the magnetic coupling is the same! (It's how those
clamp-on current probes work, too.)

Since people do see an affect when they move the windings around, I suspect
the affect is due to the presence of inter-winding capacitance.  Less
capacitive coupling between turns would result when they are spaced apart.
The affect would certainly be more noticeable at the higher frequencies
where any small finite capacitance would have a greater affect.

Rick
KC0OV
K2 1360