Need LRL Schematic

[J_Player]

I have this small hand held stud sensor that will detect studs and other objects up to 3 inch distance. I would like to modify it to locate gold from a long distance. 50 feet would be nice, but I can live with 20 feet or maybe even 15.
Any modification must be kept within the plastic case of this stud sensor. I cannot mount large antennas and coils. The reason is because I must disguise the stud sensor to appear as a cell phone or other electronic thing that someone might carry in their pocket without drawing attention.

So I need a small long range locating circuit design that will fit inside. The existing electronics works by sensing capacitance changes due to coupling of nearby objects. But none of the existing electronics need to be used. Surface mount parts are ok if necessary, as long as all the parts fit inside the plastic case, including the battery. This may sound like a hard problem to solve, but here is your chance to finally get the recognition you deserve.

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Please post your circuits here so we can decide which is best to convert my stud sensor to a pocket LRL.
(see below for stud sensor details).

Best wishes,
J_P