The 'slit' in the shield allows a voltage difference to exist across the
slit.
If there were no voltage difference, then you could short the slit with
no change in performance.

Thus the shield acts like a short dipole, with the ends bend up to almost
touch one another. There is a voltage gradient along this short shield
(dipole),
and current flow along the length of the shield (dipole).

The loop inside the shield thus is coupled to this short dipole. Multiple
turns of the loop 'add' (increase) the voltage since the dipole (shield) is
short compared to the wavelength.

Thus, the shield IS the antenna, and the loop is the means to couple the
energy out of the short dipole. Thus, it really isn't a magnetic antenna at
all - it's a dipole!

Tom, W8JI has a better, more detailed explanation at his website (a great
resource) at http://www.w8ji.com/magnetic_receiving_loops.htm




"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