Before emit the BFO an only hertz, the amplification enlarge this "soft breeze". With this type of detector (I had one of 80's generation) I found a copper plate 120 X 60 cm (very big) at 50 meter distance and 1.5 meter depth with a coil of 11-12 cm diameter.
The old coil I saw was turn by turn, but other models has classic winding. The BFO is a transmitter since is an oscillator and at the same time is a receiver, so attention with the "obsolete".
Here exactly the middle and the end of 80's pistol I had: