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Dual D On A Different Pi


Reg

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I was curious just how a dual D coil would work on a TDI and this is what I found but first, I need to say my Dual D differs a little from ML's or Minelab's design a little. The reason is simple, I wanted to use what I already had made and just modify the mounting.

My main TX coil looks basically the same is those in the posts of the dual D coil, but my RX coil is a little different. Instead, my two D coils are part of one large winding shaped into two D's, with a half twist in the leads passing through the center of the TX winding A simple half twist puts one winding out of phase with the other RX winding, so cancellation is the same.

Ok, here are the results. First, the coil is quite sensitive, very quiet, and requires no ground balance signal at all. At least, it doesn't at my house. The detector is so quiet with or without the GB on, it is clear that I could increase the gain and, thus, the depth when using this coil.

The coil housing I used is a 12" by 9" so it is smaller than the factory 12". However, the sensitivity is close to the same I am also betting I can increase the windings in the receive with little to no major noise effect,and possibly a little gain. I also have an idea to increase the gain another way.

Right now, I see a depth increase on the TDI since I can run with the GB off just fine even over really bad ground, but I feel I can do much better with a few other tricks.

So, it is possible even the old detectors have life left.

Reg

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Hi Reg,

You mention increasing receive windings for more sensitivity. What is the relationship between windings and sensitivity and the trade-offs involved? I ask because Minelab claims over 6000 feet of wire is used in the GPZ 14 coil which seems like an insane amount of copper. I am sure getting depth or sensitivity is a lot more complicated than just spinning more wire so was hoping you could help enlighten me?

I wish there were more oddball coils available. Stuff like the Big Foot coil for White's detectors I would really like to lay hands on for other detectors. And wasn't it Garrett that had that stacked concentric design? I think Jimmy Sierra also produced a version, supposedly very resistant to EMI and good at working near metal items like fences.

Your work with this folded D design is also very interesting. Thanks for the post.

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Hi Steve,

I am not the one who can give you the truly technical answers to your questions, since I work more on trial and error than trying to calculate things out.  Most people work using my technique anyway.

Now, as for ML's claim of 6000 feet of wire, well Litz wire is made up of multiple strands and they may be using a type that has 45 strands , so each foot of wire will be 45 feet of strands.  If they are using more strands per foot, then the number of feet adds up quickly.  Now, lets say they are aiming for 450 uh or so tx, then that will be maybe well over 100 feet of primary wire, or 4500 feet just for the TX. 

Personally I feel they are using more strands in their litz than I mentioned. 

Now, I have no idea just what they are doing for the receive.  A typical PI DD coil will have maybe 450 uh winding, but a VLF can have 4 to 8 mh.  I have no idea just what the sq wave pulse will use. 

As I mentioned in a different post, the coil action is similar to an air core transformer, so one needs to know more calculations than the average person would understand to calculate that out.  So, wind one with extreme number of receive windings (actually a pair, one for each side) and see how it works.  Since the coils are mounted so they oppose each other, the result is going to respond as a total near zero.

I suspect the greater number of windings in each receive winding will increase the importance of a better balance, but I haven't tried much of anything yet.  What I do know is my "balance" technique in trying to make each side identical is a joke and is embarrassing to look at.  I put the coil together so crudely and quickly, I wasn't even sure it would work, but it did surprisingly well. 

Now, there are lots of different coil designs out there.  Yes, Garrett used a stacked coaxial design in some of their coils back in the late 70's and 80's.  Jack Gifford used the coax in some of his designs also in some of his pre Tesoro days.

One that most people do not know about is Eric Foster used a Garrett stacked coax housing and built a coil for his PPD1, which was a discriminating PI. by sampling during the on time, he could easily tell iron junk from non ferrous objects. Coaxial coils worked extremely well close to fences, but they didn't ground balance that well.  At least, the ones I used didn't. 

Yes, there was a discriminating PI built back in the 80's.  I understand it worked quite well on a saltwater beach, but sucked on dry land, since there was no ground balancing built in this detector. 

Getting back to my design of the dual D, well it was built the way I built it because it was fast and very convenient.  I didn't have to wind anything.  I just used what I had already made.

Reg

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Wish it were sharper but in this closeup of the GPZ 14 internals the twin receive coils appear to be wound with a fine single strand and the single transmit coil looks braided. Bunch of copper no matter how you look at it, explaining why they weigh three pounds. Makes me wonder on the practical limits on how small they can make this type of coil.

Hey, you coil works - who cares what it looks like!

post-1-0-03543400-1427695141_thumb.jpg

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Hi reg like your idea of the DD COIL your playing with im going to keep my old TDI because there is so much you can do with this detector. Like to hear some in ground tests sometime.

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