Radical Twist

Here's a puzzle for electronics enthusiasts. BFO metal detectors have a single active coil. IB metal detectors, too, have a single active coil. Yet IB represents a quantum leap in sensitivity over BFO. Why? Today I wired up a BFO metal detector with a radical twist. It picked up an old English penny at 170 mm -- by which I mean that the penny induced a shift of half a tone in a loudspeaker at that distance. My past BFO designs have generally picked up an old English penny at 90 mm -- and that seems about right for BFO. So this is a 90% boost. OBSERVATION: In fact, an incredible boost, when one takes the inverse square law into account. I am still thinking on what I might not understand. Is this real? Is there anything else, inside or outside my design, which is causing a 90% boost -- apart from what I intended? My experiments continue ...

CORRECTION: It is a minumum 100% boost, by which I mean increase in range.