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So What Is Inside That DD Coil?

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If you have been following my Garrett ATX rebuild thread at http://www.detectorprospector.com/forum/topic/462-garrett-atx-strip-down-rebuild/ you will have seen that I sacrificed a Minelab Commander 12' x 15" DD coil for the project.


So may as well get an education on what Minelab has in there! First the coil, then with top cut off. The inner portion is completely wrapped in conductive paper which creates the electrostatic shielding.




Tear that all off, and you see a foam oval. Flip it over, and incised into the bottom are the coil windings, with the distinctive back to back Ds. A bit of detector mythology has it that the DD refers to the initials of a guy named Donald Dykstra that supposedly had something to do with DD coil design, but I think that has been discredited. http://compass-metal-detector-forum.548136.n2.nabble.com/Compass-and-quot-THEIR-quot-own-DD-coil-information-td7598726.html




I believe, and JP can correct me if I am wrong, that Commander coils always used Litz wiring http://www.globalspec.com/learnmore/electrical_electronic_components/wires_cables/litz_wire They never really touted it nearly as much as Nugget Finder later did, and now Coiltek with their recent coils offering Litz wiring. I think Commander coils in general have not gotten the attention they deserve, as people seem to assume aftermarket is always better. Something I am a bit doubtful of myself. The main problem with Commander coils really has always been that they are heavy by comparison to some aftermarket alternatives. That, and because Minelab has in general forbidden bundling Minelab product with Minelab coils dealers often promote other coils.


Anyway, now you have had a look under the hood!

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Hi Steve, your correct, in fact Minelab have always used Iitz wire in all their coils dating right back to the SD 2000.

I get a bit of a giggle sometimes when I read advertising about large nugget finds being found with X brand coil, at some stage people have to recognize that the box attached to the coil might have had something to do with the discovery too!!

The Commander coils are built like proverbial brick Outhouses (stand alone outside toilets for those who don't follow the Aussie vernacular:-) ), but they are VERY good coils, built by people who make VERY good metal detectors.

Very interesting thread Steve thanks for putting this up.


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Them Minelab Coils are tough as nails! As you know with a metal detecting Wife, I have to have two of everything. I have 10 Minelab coils and two NuggetFinder Coils. My opinion, is Commander Coils hands down are the best and I don't treat mine easy. That big 12x15 DD coil is one of my Favs for patch hunting. Seems to skip right over small lead birdshot and I've found nuggets from sun bakers to well over 12" deep. Steve, hope you get that going on your ATX!


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Hey Rick, no chance of getting that coil to work on the ATX. I am just stealing the top to make a lid and mounting bracket for the ATX coil. Sacrificed in the name of detector prototyping.


I agree the coils are very robust. Actually, I think they are waterproof, Minelab just does not want to say so. The inside of the rod mounting ears are filled with epoxy:




The hole drilled through the ears to mount the rod with a bolt passes through the epoxy filled area. Impossible for water to get in that way, unless the epoxy separated from the housing somehow.




The coil halves are sealed so well that after some effort trying to chisel my way around the seam I just resorted to cutting the top out with a Dremel tool. No water getting in that way. That only leaves the jam nut where the cable enters the coil. The top part is a rubber compression fitting that for sure will block water from entering.


The jam nut housing is fastened from the inside with a huge nut, and it was so tight I gave up trying to get it loose and just cut it off. I saw no sign of sealant at the base of the area where the black meets the white in the photo below. Not saying it was not there, but it was not obvious. This would appear to be the only place water MIGHT enter the coil. Personally, I would have no issue myself after seeing this to subjecting a Commander coil to a little light immersion. But just to be safe, a little silicone around the base of that jam nut - that baby is going to be 100% waterproof. No, I am not offering a warranty with that statement!




Finally Fred, the windings are sunk into epoxy more to hold them into place than to make them waterproof per se, as there are thicker and thinner spots. In some places the wiring is more or less exposed on the bare windings. The other half is wrapped in some sort of fabric - fiberglass cloth? Anyway, pretty well sealed up.


The foam is dense but if compressed does not bounce back. Putting a very heavy weight on top of the coil or hitting the side really hard could permanently compress the foam, but it would take a real effort. Unlike some foam I have seen used in coils this stuff should not break down and crumble. A lot of coils made have used a urethane foam type of stuff that as the coil gets knocked around would crumble around the edges internally, allowing things to move around, and then the coils false a lot.


Good job Minelab.



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Thanks, Steve....I ride my detector/coils pretty easy but it is good to know what the inside is made of. I hate false signals from bumping a bush and most of my coiltek coils did that....


I wonder if a few small holes and some low-expansion foam would solve most falsing problems on coils that arenot already fully embedded???



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