Thanks for the information, everyone. I'll read up on a few of the
references. I have found a few web pages that compare the sensitivity of
the shielded vs. unshielded loops, and the shielded loop always ends up with
slightly less sensitivity.

Could anyone answer the following question? I'm trying to make things
simpler here than what happens what a real antenna: Let me take an ideal
circular conductor, say, a meter in circumference. Next I'll stick it
inside a perfectly conducting loop of pipe (the shield) that has an
infinitesimally small slit in it to prevent creating a shorted turn.
Finally I'll create a magnetic field at a very low frequency (say, 1Hz -- so
that the loop is clearly electrically small) and insert the loop into it
such that coupling is maximized.

Question: Will the current on the inner conductor be identical to the case
where there is no shield? Is there any current on the shield? Does
anything change if I ground the shield?

Hmm... I'm thinking I should move this over to the electromagnetics
newsgroup!

---Joel