The "shield" is actually the antenna, and the gap in the "shield" is
the feedpoint of that antenna. You will do well to make the "shield"
out of a good conductor, and to get the benefits of rejecting
vertically-polarized electric fields generated nearby, you should make
the antenna very symmetrical. See the discussion in King, Mimno and
Wing's "Transmission Lines, Antennas and Waveguides." I think I have
a .pdf file of the relevant section somewhere. I particularly like
that one for its qualitative explanation, clearly presented. I've
seen other decent explanations in places like Johnson and Jasik's
antenna book. The explanations in such texts that I've seen all
agree.

A key advantage of the "shield" is that it simplifies the task of
making the antenna symmetrical, though I've seen a lot of old ARRL
pubs that completely miss that point. If you realize that that's what
you're trying to accomplish in the "shielded" construction, you'll
find you can do quite well with a multi-turn "unshielded" loop, too.

Cheers,
Tom (one with a last name)

"Joel Kolstad" wrote in message ...
When you build a loop antenna, it's common to wrap it in, e.g., aluminum
foil that's grounded so as to prevent electric field pick-up (I'm thinking
of HF loops here, 30MHz). A slit is made in the wrapping so that a shorted
turn isn't created, thereby nulling out the magnetic field that the loop is
trying to detect in the first place.

Something I don't understand, though... normally, if you were thinking of
using aluminum for EMI shielding purposes, the skin depth of aluminum at
10MHz is all of ~1mil. Hence, a regular sheet of aluminum foil would
significantly attenuate both the magnetic and electric fields on its 'far'
side. Why doesn't this apply in the case of a shielded loop antenna? It
seems to me that the ~95+% 'coverage' of the shield (everything minus the
slit to prevent the shorted turn) would be what dictates the overall
shielding effectiveness, not the presence of the slit itself.

Looking for insight,
---Joel Kolstad