Most of the good ideas are not covered by patent its just a case of taking them up. The British Rimertron company (now Viking) produced the first detector with all the works in the headphones (Rimertron Crown), Richard Dodds a true windowing signal booster (small items at depth boosted , large near surface stuff muted), Fieldmaster added a double D widescan coil to their motion machines when Whites etc said only concentric coils could be used. The Bulgarian external pre amp for the Sovereign is now starting to be copied.
On the other hand multifrequency metal detecting was developed by the U.S. firm Westinghouse first but U.S. companies didn't apply it to a hand held machine. U.S. loss, Australian gain.
On the other hand multifrequency metal detecting was developed by the U.S. firm Westinghouse first but U.S. companies didn't apply it to a hand held machine. U.S. loss, Australian gain.