I was thinking you was thinking more power would be better. Two box detectors really don't us that much power and your batteries should last hours and hours.
From C-Scope's Plain Truth series of articles:
"It is in the fundamentals of electro-magnetics where the laws of physics establish limits which cannot be exceeded. Metal detector R&D engineers all understand these laws very well and they all have to develop their detectors within the same constraints. To demonstrate the point further.......it is possible to put the cheapest metal detector in a controlled laboratory situation and you can tune it to detect a coin in air at one meter! However, if you take that machine outside and try to use it on the ground it is absolutely useless and won't detect a thing. This is because the huge amount of energy in the search coil is simply detecting the ground.
Doubling the gain of the detector doesn't give you twice the depth but it does give twice the ground signal! (The magnetic field from the transmit coil to the target diminishes as a cube law. This magnetic field induces circulating eddy currents in the target and these eddy currents produce an opposing magnetic field, which also diminishes as a cube law. It is these which are detected by the receive coil. - So we are talking of a 6th power law of signal against distance (3rd power out and 3rd power back) . So to double the depth of detection requires a transmit current (or receive gain) increase of 2 to the power 6 (iex64, which equals 64 times as much). This also explains why you cannot get more depth out of a metal detector. 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.
Dave Johnson, from an interview with him and John Gardiner:
"Getting extra depth out of a VLF, multi-frequency, or PI machine is very difficult, because these machines follow an inverse 6th power law relationship between signal voltage and depth. If everything else is maintained equal, doubling the depth requires 64 times as much signal. If this is done by increasing transmitter power, doubling depth requires 4,096 times as much battery drain. That's the basic reason why depth increases come so slowly in this industry. The biggest impediment to getting usable depth in the ground, is interference from magnetic and electrically conductive minerals in the ground, which can produce signals hundreds of times as strong as that of the metal target you're trying to detect and hopefully identify.".
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