Help with Dual-field design

[Ism]

Hello everybody,

I am attempting a dual-field style coil and I have a couple questions that haven't been addressed in previous threads.

When I wound my 2 coils (5" and 7") and tested them, their individual values are 200uh. With the coils in series, placing one coil inside the other with both windings in phase (turns going the same direction), my combined inductance is a little over 510uh. If I have the coils out of phase, the inductance is a little over 320uh.

Which is the proper relationship for these coils so they work together?

Is there relationship between the diameter of the coils that enhances their field strength?

Should the center conductor of the coax be connected to the inside coil or outside?

And finally, I am using teflon wire as recommended but its 28awg. Are there any concerns with the smaller wire for TDI coils?

Thanks

Randy

[Reg]

Hi Randy,

Rather than answer your question about how to set up the coils, I recommend you simply try it and see which works best. You will know quickly once you try both directions. Then you can tell us. Just make sure you can easily swap wires or flip one of the windings and testing is easy.

To be honest, rather than try to measure and get things right, I test the coil to make sure the wiring is correct before I even consider finishing or sealing the coil. It is far easier to correct things then than try to open up a sealed coil.

Now, as for using the 28 awg wire, I am sure it will work fine. The coil may get a little warmer than one made with 26 awg, but that is ok. Actually, I have tried coils on my TDI I made using 30 awg wire that were made for a different PI and they worked ok also.

Now, what I have not done is make both coil windings the same inductance. I need to do that and compare that coil to a normal mono coil for depth.

If you have built a similar size mono that has only one winding, then maybe you can compare the two coils and let us know.

Reg

[Ism]

Reg,

Thank your for your reply. I have been spi-wrapping my coils and then attaching them to a plastic cardboard with ty-wraps. Its great for testing the coils without making anything permanent.
I jumped the gun a little with my first question. I saw that I was low on wire and was hoping to avoid the extra experimenting. However, I couldn't stand it so here's what I found.

When the coils are out of phase, they cancel each other out. So much so that the combination barely registers an object in front of it.

I then rewound the inner coil to end up with 380uh when it is in phase with the outer coil. This is definatly the combination to use.

I would be happy to share my findings in the future.

Randy

[Reg]

Hi Randy,

I am a little confused with your answer. Are you saying you couldn't get your original two windings to work at all or not right no matter what you did? I would think you should have been able to get the two coils you initially wound to work and not had to rewind a new inner one. You may have had to increase the delay a little but that would be all. Simply flipping the winding over or swapping the two ends of that winding should have changed the phasing and allowed it to work if the original phasing was incorrect. Once you got the original windings to work, then you could have wound a different inner winding and compare that combination with the original just to see if there was any significant difference between the two different winding combinations. That is what makes it fun. At least, it does for me.

Regardless, if you have a working coil, that is the objective anyway. So, congrats on advancing on the learning curve. It won't be long and I will be asking you questions.

Reg

[Ism]

Reg,

The two 200uh coils wired in series and placed one inside the other will produce the following results.

-in phase, aprox. 500uh
-out of phase, aprox. 300uh

I wanted one of each at 400uh for comparison testing. In this particular case there was no doubt that the out of phase version wouldn't serve our purposes

In the interim I have been doing some testing.

I now have 3 un-damped coils with the following properties;
Inner winding 5.25"
Outer winding 7.5"

Coil-A [Balanced] Inner=180uh, Outer=180uh -Total 395uh
Coil-B [Cen-high] Inner=220uh, Outer=120uh -Total=390uh
Coil-C [Cen-low] Inner=140uh, Outer=200uh -total 385uh

Tested on TDI with Gain-6/GB-3/Delay-10uh

Coil-B had slightly better results than Coil-A. Coil-C was less sensitive by better than an inch of depth. The results were impressive for raw coils in air. Coil-B was over 13.5" for a nickle and better than 7.5" for a silver dime with GEB on in high conductor mode.

One other thing I noticed. The closer to 400uh, the less interference I created when touching the controls of the TDI with my hand.

[Ism]

Updated findings

Adding the damping resistors may prove to be a challange with the imbalanced coil setup.

Coil-A [Balanced] Inner=180uh, Outer=180uh -Total 395uh
Coil-B [Cen-high] Inner=220uh, Outer=120uh -Total=390uh
Coil-C [Cen-low] Inner=140uh, Outer=200uh -total 385uh

Coil-A became very stable after adding a 1k damping resistor to each coil. Things became a little more difficult working with Coil-B. Finding the damping resistance of each coil would take more than I have the patience for right now. Especially since I am now very pleased with the performance of my balanced dual coil (Coil-A).

[Ism]

8" dual field style coil.

I finished a dual field style coil in a Hayes 8" spider housing. Inner and outer coils were 26ga teflon coated silver wire wound to 180uh. Total inductance 430uh after spi-wrap and scotch 24 shielding. Coax is RG58A/U. Not the best but what I had on hand and super flexible.

Testing results were average for coins and other large artifacts compared to other posts for coils of this size.

I don't have any gold nuggets to test with so I mashed a .177 cal lead pellet (3 grains) into an irregular nugget shape. I got 4 inches depth with my 8" coil compared to 1.5 with my 12" stock coil. I know this is apples and oranges, but it gives me hope that it may be a good nugget coil.

In addition, I just built an 18" dual field prototype w/18ga teflon silverplated wire, shielded and tested it. Can someone explain the following?

I tested the coil on my TDI (as per Reg's previously posted explainations) and the coil is actually around 12.5us.

When testing the coil in "all" with GEB off, The coil doesnt perform well at 10us (dime at about 6 inches) but if I switch to GEB on and switch to "high conductor", I can detect the dime at 15 inches.

I am not sure how this 18" compares to others but I am very pleased with its performance for deep objects. Thanks to many posts in the coil forum I've managed to cobble enough knowlege together to replace Spocks brain

Thanks

[Reg]

Hi Randy,

You might reduce the inner winding inductance some so your total inductance is closer to 300 to 350 uh total. Then the coil should work at 10 usec.

Now, as to your question concerning your 18" coil, I think your question is why do you get only 6" or so with the GB off but turn the GB on and then you can detect the same dime at 15" or so.

With the GB off, you are only checking with a single channel, the metal channel. If the coil isn't fast enough, the coil will act weak and have a muted signal plus a loss of depth. Turn the delay back and at some point you should notice a sudden increase in depth capability. This will be the minimum delay the coil will work properly. In your post, you indicate this is around 12.5 usec.

Now, when trying the coil at 10 usec and with the GB on, you are not only sampling at about 10 usec, but also at about 40 usec. The later sample is the one needed for ground balance. Since this channel is also amplified a lot, the signal is really very strong. Now, since the main channel signal is weak and muted, the stronger signal from the ground balance channel becomes the strong signal you hear. So, what you hear is mainly the later channel signal.

Now, if you were to turn the GB off and advance the delay to maximum of 25 usec, you should get a nice strong signal close to that you got at 10 usec with the GB on.

Let me know if this makes sense.

Reg

[Ism]

Absolutly Reg, thanks for the explaination. It makes all the sense in the world now. I will reduce the inductance as you indicated and report my findings.

[Ism]

18" dual coil

Hi Reg,
I lowered the inductance to 310uh on my 18" dual coil and it still fails to respond properly under 12us. I am completing the coil as is and labeling it 12us min.

I am curious if you or anybody else can tell me where I may have failed on this design. The following are the specs.

-2 coils 1-9" and 1-17" wound with 18ga teflon wire
-2 layers of spiral wrap over the windings
-1 layer of scotch 24 shielding tape
-2 1.5k damping resistors (1.2k was too much damping).
-pink shipping material to stablize the windings in the form.
-RG/58AU coax

The shielding tape is begun and terminated at the location where the winding wires enter the coil. There is a 1' gap at that location so the shield doesn't complete a loop.

I get a solid 26" on a mason jar of modern coins (air test) with the delay above 17us. And 12" on a merc dime at around 12-13us. So I am happy with that part of the performance. But for the life of me I cant think of where I went wrong. Does anything stand out as a potential speed killer?

[Reg]

Hi Ism,

I don't think you did anything wrong. It is extremely difficult to get any coil much over 12" diameter to work at 10 usec, let alone a coil that is 18" in diameter. So, I wouldn't worry about trying to get a little faster on the coil.

As for things to possibly try to make the coil a little faster, you could try to increase the resistance of the damping resistors in the coil housing, increase at least one to begin with, then try increasing the resistance of both to see if that will allow a little faster operation.

That is about all I can think of doing at this point.

If you decide to increase the resistance, let us know how it works out.

Reg

[Ism]

Thanks for your valuable help Reg

Actually I did increase the damping R and performance went up a little but speed didn't change.

I Thought about how constant the cutoff speed has remained. The only similarities from the first and second attempt that appear to influence capacitance are the coax, Teflon wire, and shielding gap. I don't believe its the coax but changing it to RG/59 would reduce total capacitance by around 50pf.

The shielding gap could maybe use a third layer of spiwrap. I thought the #18 wire insulation thickness might not add enough spacing between windings. (ratio of wire surface area/insulation thickness). I will work with #22 teflon wire next time. I also got RG/59AU for future coils. I might upgrade the coax to RG/59 before closing it up. I will share my findings as I progress.

I did take the coil out in the yard for a field test today and it performed nicely.

[Reg]

Hi Ism,

I prefer the w2964 Mogami coax. It has low capacitance (about 20pf per foot) but better yet has spiral wrap shielding so it isn't detected at 10 usec. If you can't find any, let me know, I have a little extra you can try. Just drop me an email.

Now, as I mentioned before, if you have the coil working at 12 usec, that is extremely fast for that size coil and probably about as good as it is going to get.

Reg