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