An electrical signal cannot get inside the sheilded box...
But equally obvious a magnetic signal can...
No one seems to realize or want to admit that although electric properties and magnetic properties are linked, they have different properties.
And what we are trying to receive is an electro-magnetic
wave. It needs both
parts to function.
I spend a lot of time in a screen room at work. Sure, you can wave a magnet around
one side of it and detect the changing magnetic field, or wind a coil on each side of
the copper mesh and pass AC current though it. But my cell phone loses coverage
when I close the door, and you can't listen to the ball game on your AM radio inside
even though it uses a loop antenna.
If a "magnetic" loop really responded to magnetic fields, you would be able to. But
the term "magnetic loop" is a misnomer - it still responds to electromagnetic
fields like any other antenna. The ratio of the two fields may change somewhat,
but you still need both components.
Even though you could get a magnetic field inside the screen room, you can't get
radio signals. (Unless someone leaves a hole, or fails to filter an incoming wire.)