In a post below there was a reference to a 100 milliwatt restriction on the transmit power of metal detectors. At one time that was true, in fact all unlicensed radio transmitters, which includes metal detectors, were restricted to 100 milliwatts output. The text books when I was taking an electronic course in the early 1960's qouted the FCC regulations and made it very plain that to exceed 100 milliwatts would be a violation. But after a discussion a few years ago with Carl Moreland, who has the Geotec forum, both of us went through the FCC regulations and found no reference to a 100 miliwatt restriction. I contacted the FCC and asked where I would find the info and they directed me to Section 15 of the Regulations. According to Section 15 there is no restriction on power output for transmit frequencies below 9 khz. From 9 khz to 90 khz it's it's based on e-field strength at a distance, basically 2400 divided by the frequency.
There's a lot more involved in making a detector go deeper than just the transmit power. The text below is from C-Scope's Plain Truth articles on their website and explains the effect of increasing transmit power, or gain, and why detectors have been restricted to pretty much the same depth levels for years, and also why we keep getting new models with more bells and whistles but no improvement in performance. The links are to other articles on their website that are also interesting reading.
<i><b>"It is in the fundamentals of electromagnetics 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 furher.......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 metre! 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<p>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 (that's 64 times as much!). Resonating the receive coil gives free gain and a filtering , but requires more stringent temperature drift measures ". <p>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. You CANNOT get around the laws of physics!"</b></i><br><center><font face="Verdana" color="#000099"><img src="http://www.cscope.co.uk/metdet/newsiteimages/TitlePlainTruth.jpg"> <br><b>OPERATING FREQUENCY<br><b>SENSITIVITY <br><b>DISCRIMINATION <br><b>GROUND EFFECT</b></center><br>
There's a lot more involved in making a detector go deeper than just the transmit power. The text below is from C-Scope's Plain Truth articles on their website and explains the effect of increasing transmit power, or gain, and why detectors have been restricted to pretty much the same depth levels for years, and also why we keep getting new models with more bells and whistles but no improvement in performance. The links are to other articles on their website that are also interesting reading.
<i><b>"It is in the fundamentals of electromagnetics 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 furher.......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 metre! 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<p>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 (that's 64 times as much!). Resonating the receive coil gives free gain and a filtering , but requires more stringent temperature drift measures ". <p>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. You CANNOT get around the laws of physics!"</b></i><br><center><font face="Verdana" color="#000099"><img src="http://www.cscope.co.uk/metdet/newsiteimages/TitlePlainTruth.jpg"> <br><b>OPERATING FREQUENCY<br><b>SENSITIVITY <br><b>DISCRIMINATION <br><b>GROUND EFFECT</b></center><br>
. Every new model Charlie Garrett has released since the Garrett BFO I had in the early 1970's has been advertised as having 20% more depth than the previous model. That BFO would get a quarter about 5 inches deep in my mild ground, if his depth claims were accurate his current top model should get one at least 20 inches deep