At one time the FCC regulated the output of unlicensed radio transmitters, which includes metal detectors, to an output of 100 milliwatts. That's no longer the case. Now there are no restrictions below 9 khz and above that it's based on e-field strength at a distance, in effect also no restrictions when applied to metal detectors, but that doesn't mean the detector mfg's can build detectors that go a lot deeper than what is achieved with current detector technology. One mfg says it is possible to put the cheapest metal detector in a controlled laboratory situation and tune it to detect a coin in air at almost 40 inches, but if you take it outside and try to use it on the ground it will be completely useless and won't detect anything. That's because the huge amount of energy required to detect a coin at 40 inches will result in the detector simply detecting the ground. That's the problem. Doubling the the gain doesn't give double the depth, but it does double the signal the coil recieves from the ground. To double the depth of detection requires a transmit current (or receive gain) increase of 2 to the power 6, that's 64 times as much is currently used and would render a detector using current technology completely useless.
C-Scope, who makes detectors in England, states, <b><i>"It is in the fundamentals of electromagnetics where the laws of physics establish limits which cannot be exceeded. That explains why you cannot get more depth out of a metal detector. Metal detector R&D engineers all understand these laws very well and they all have to develop their detectors within the same constraints. Any detector manufacturer who tells you that they have a new development which gives dramatic increase in depth has to be treated with a lot of suspicion."</i></b>
One detector designer has said, on more than one occasion, that it's possible to add some depth by using different coil designs, and some look to advanced pulse type units as the next step in achieving more depth. In any case, it'll be interesting to see what develops in the years ahead.
