Hi,
it's easy. I think the idea is picking up some electromagnetic anomaly.
The idea behind ? What I think...
The short turn + toroid coil act like an IB transformer, the e.g. flux that chain with the loop generates a very small eddy current flowing in the closed aluminium turn. Normally the coil on toroid is about insensitive to the background noise cause the magnetic flux at toroid is inside the toroid... but when the eddy current flows inside it the toroid coil will get some coupled magnetic flux cause of flux lines that concentrate at inner side of it, thus offering some (limited) coupling with the signal at antenna loop.
It's like in some directive rf meters... just at right direction you can spot the emission cause of the geometric properties of loop coil and lose coupling of receiver transformer (right Esteban ?).
The second one, open loop is similar, but... instead of using magnetic coupling between the eddy current (and its magnetic field) and the receiver toroid core, you have the aluminium loop acting as a voltage generator (only ac will work) connected to the primary of a transformer (for impedance separation)... now... if a voltage will develop at aluminium turn you'll get a scaled value of that at output of transformer depending on turns ratio there...with hi impedance separation between loop and load.
Also this approach is common in old rf locating stuff... and meters.
The first approach I think is more easy cause you don't need perfect match of impedance required to get it work... the second is probably sensitive to the transformer construction more than one could think.
Now the problem is (one way or another): does it work ???
Could this approach pay as LRL ???
Kind regards,
Max