Jim in Idaho Posted May 15, 2020 Share Posted May 15, 2020 Nice work, Steve! I think the resistors in that circuit are to prevent the coils "ringing" when the pulse turns off. Note both coils are wired in series, so the pulse fires them together. A folded mono is the same. You just take a larger diameter coil, and squeeze it together so it makes a figure-eight with one end half the size of the other. Make a half-twist at the juncture. Then you fold the small portion over on the large portion, and you have a folded mono. Electrically, they're pretty much the same. I don't know where they put the damping resistor on the FM. Jim 2 Link to comment Share on other sites More sharing options...
Glenn in CO Posted May 15, 2020 Share Posted May 15, 2020 19 hours ago, Steve Herschbach said: I decided to go to the source - the Dual Field patent. "A new dual field search coil for pulse induction metal detectors has multiple coplanar wire coils of different diameters connected in series with the output of a pulse source, overcoming the loss of target size resolution associated with a single coil search loop. Small objects are sensed by an internal small coil and larger objects are sensed by the larger outside coil, and the overall depth of target sensing remains similar to that of a single coil construction. The smaller coil is isolated from the pulse source by the inductance of the larger coil and will ring at a frequency determined by its own parameters. To prevent the smaller coil from ringing, a second damping resistor is connected across the terminals of the smaller coil at the junction of the large and small coils making up the modified search loop." So my question is if I'm reading this information right, does the smaller coil achieve the same depth as larger outside coil? If that is true would the 7.5" Dual Field coil have the same depth as the 12" Dual Field coil? I'm basing these questions on coin size targets. Link to comment Share on other sites More sharing options...
Steve Herschbach Posted May 15, 2020 Share Posted May 15, 2020 2 hours ago, Glenn in CO said: So my question is if I'm reading this information right, does the smaller coil achieve the same depth as larger outside coil? If that is true would the 7.5" Dual Field coil have the same depth as the 12" Dual Field coil? I'm basing these questions on coin size targets. No, I’m not sure how you are reading that into it, especially if you read the full source article. The max depth on coin size targets is determined by the outer coil, a coin being a large target. The inner coil enhances sensitivity on stuff like ear rings and small gold nuggets. 2 Link to comment Share on other sites More sharing options...
Glenn in CO Posted May 15, 2020 Share Posted May 15, 2020 19 hours ago, Steve Herschbach said: Small objects are sensed by an internal small coil and larger objects are sensed by the larger outside coil, and the overall depth of target sensing remains similar to that of a single coil construction. Whoops! I guess I was reading that statement as the smaller coil would achieve the same depth as outside larger coil. Link to comment Share on other sites More sharing options...
Steve Herschbach Posted May 15, 2020 Share Posted May 15, 2020 That would be nice - you would not need the outer coil. 1 Link to comment Share on other sites More sharing options...
HardPack Posted December 13, 2022 Share Posted December 13, 2022 I assume the “folded mono coils” are also in series similar to the White TDI mono Dual Field but in a figure eight design folded back on itself with the smaller loop placed to the interior. The location of figure eight coil crossing would most likely be located at the heel of the coil. Both the waterproof White TDI 7.5 DF coil and an the after market waterproof 5x9 folded mono coil are good prospecting for tools for tight spaces and for those hard to reach partially submersed waterfall rock shelves that intersect the stream “V” channel. A better design would be a simple 5x9 waterproof mono coil with a sensitive front toe/tip. (That was for the Garrett Axiom engineers). 1 Link to comment Share on other sites More sharing options...
Jim in Idaho Posted December 13, 2022 Share Posted December 13, 2022 From what I see, that is correct. But, the interesting thing is you can get the two magnetic fields working together (adding), or contrary to each other, depending on how the "fold" was done, if I'm looking at this correctly. IMHO, the 5 x9 FM was nothing special as far as sensitivity goes. Jim Link to comment Share on other sites More sharing options...
HardPack Posted December 13, 2022 Share Posted December 13, 2022 I agree but the size & shape serves it purpose. 1 Link to comment Share on other sites More sharing options...
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