I think that the principle of operation of all lrls start from the first (Alonzo PD) that was a metal detector (an induction balance) modified with the addition of a ferrite and an amplifier stage. My first lrl was a PI in which the signal from an antenna was mixed with the signal of the PI oscillator. My last LRL in practice follows the same principle, except that the internal oscillator does'n drive a coil and serves only to provide a signal to the mixer. So I think it is a useless job to build an MD and then modify it to use it as an lrl.
Esteban said that even an FM receiver (not tuned to any station) could work as lrl and therefore no MD is needed, All you need is an oscillator that is mixed with the signal from the FM receiver. I am convinced that almost all lrls are passive receivers and therefore there is no reason to construct transmitting coils and moreover there is not a particular frequency that for example allows to reveal only gold. Obviously you are free to experiment, in this field you need a lot of practice since the theory is very little.
|