No answers - nice - I'll tell you what this circuit does:
It recognises the "antenna function" of the searched metal object.
I found this out by thinking about the somekind special movements you have to do with the PD.
The style how the ferrite-antennas are arranged within the PD are typcial for "minimum bearing".
Forget about the unscientific "ghost stuff" on this page but read about the EMF-meters and how they work:
http://www.ghostgadgets.com/_knowled...explained.html
and here:
http://www.ghostgadgets.com/_knowledge/emfmeters.html
If you are really interrested and not just like making fun out of everything - like little kids do with sex - you will find out that there exists already very sophisticated EMF & magnetic field detection devices and that you have incalculate all 3 dimensions and the "antenna function" of your own body if you want to get real authentic test results!
I hardly doubt they will detect ghost, poltergeists or demons but at least if they are sensitive and professional enough they will detect metal objects that disturb the sourrounding field.
btw. for shure there exists some resonance-frequency for special materials or element! Microwave ovens work this way (resonating water molecules). And even the electrostriction is some sort of resonance. How good this works with detecting gold etc. is a different question.
I have no time writing any more because finally after 1 week and alot hours of research and reading (not here...) I want to start with the ferrite antenna tests!
The ferrite antenna receives the magnetical pulses or in our "passive pistol case"
it detects the vanishing of the "magnetical noise" if a metal object is "near".
Now the only question is if the "natural" VLF-noise is enuff or if I some kind of add. long wave transmitter is needed.
First I have to find out which of my ferrite-staffs has the best permeability and if a shielding (perhaps some can) or reflector improves the directiveness (probable only some magnetical one).
btw. it guess the ferrite-circuit is used for vague "middle to long range" detection and the bfo for pinpointing / long wave transmitter...