In a solenoidal, helically wound coil, most of the magnetic flux is aligned
with the center of the coil and flows along the axis of the coil.
Relatively little flux is outside the coil because of cancellation effects.
For a single turn, current in different parts of that turn tend to partially
cancel flux in regions that are outside that turn.

To see this, draw a circle of wire and show a current at one point flowing
in one direction (out of the page). At the corresponding point
diametrically opposite, that same current flows in the opposite direction
(into the page). Inside the turn the two magnetic fluxes that encircle the
wire add in-phase, but outside the turn these same fluxes cancel in opposite
phase. This effect is more pronounced in a coil of small diameter.

Coils of that kind are pretty well self-shielding for regions beyond the
*side* of the coil.

But if a copper disc is placed near the *end* of the coil a large
circulating current is induced around the edge of the disk and a flux is
created that opposes the flux inside the coil (Lenz's law) and a significant
reduction of the inductance of the coil can occur. This effect has been used
to "tune" coils.

The completely enclosed metal shield helps to create a closed path for
leakage current that makes the shield much more effective.

The shield around the coil can also reduce capacitive coupling to adjacent
circuitry. This is often important.

Bill W0IYH

"Joel Kolstad" wrote in message
...
I've noticed that the various tunable inductors (and transformers) in a
metal
can are much better shielded than I might have initially guessed, based
upon
the premise that most all of the flux from the coil in concentrated inside
the
coil which is 'visible' through the hole in the can (so that the thing can
be
tuned!). In thinking about this, I've pretty much convinced myself that
the
shielding works as well as it does because electrically it still "looks"
pretty much contiguous at the frequencies you're typically operating the
coil
at (e.g., tens of MHz for a 1/4" hole), in a similar manner to how
perforated
enclosures make an effective shield so long as the holes are small enough
(...and how you can even calculate the attenuation based on "waveguide
below
cutoff" formulas).

Is that correct?

If so, it seems that if you tune the slug so far "out" that it protrudes
from
the top of the can, you're probably starting to seriously degrade the
shielding... right?

Thanks,
---Joel Kolstad