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**Roughwater** · 07-06-2009, 03:40 AM

Large coil

Hola Slick,

I too plan to build a 1 M coil or larger. I have built several smaller one's up to 18 inches that have worked well and will do a 1 M next. I have some ideas but got all my guidance from a guy who goes by the handle "Reg."

I have been using scotch 24 tape for my shielding but Reg says carbon paint for the correct resistance (low 25 ohms) could work but most carbon paint I've found is .8-1K ohm and silver paint looks to be too low?

I used teflon coated wire 24-26Ga wrapped with archery serving string (it's waxed) and makes nice half hitches and wrap as tight as possible. Once I complete wrapping with the half hitches I spread silicone II over the coil. I don't think I will pot the coil when I make a 1 M coil as I think it would be best if it remains flexable. However I plan to also wrap it with a foam wrap which will help protect the wrappings. Probably best to read Regs suggestions and my info at: http://www.geotech1.com/forums/showthread.php?t=15459

Also at: http://tdi.invisionplus.net/?mforum=tdi&showtopic=381

Hope this helps,
Terry

**Slick** · 07-06-2009, 04:29 AM

Terry,

Thanks for the reply! What are you using your big coil for?

Based on some reading I have done in the last hour or so, and your reply, it seems "coil winding" isn't exactly the precision operation I thought it was (good news, LoL

).

I had read that the interwire capacitance was a problem, so suspected there needed to be SOME gap between each successive winding.

However, it seems that the actual layout of the windings I have seen are pretty haphazard, i.e. most coils seem to be simply in a "bundle", or coil of wrapped wires, gathered in a bundle, either insulated or coated (magnet wire).

I had thought there was a more precise wire-to-wire alignment/placement needed, so was prepared to do a micrometer type layout of wire with a precise gap between each wire, all in a plane, not a bundle.

Is there a benefit to just wrapping it all up in a bundle vs. some other method? Am I just trying to make something simple overly complicated??

As far as shielding, with a single coil for a PI design, do you even want to shield??

All experiences/suggestions appreciated!

Thanks,

Slick

**Reg** · 07-06-2009, 06:09 AM

Hi Slick,

I have not build Gary's PI so you might want to check with him to see what he feels would work best. What I have seen is he seems to like to build his coils flat single layer types. Again, I have not tried that design so I can't tell you what to expect if one were to compare the different coil designs.

Now, you can scan this site for a lot of ways to build coils that include simple winding techniques, to basket weave, single layer, etc, etc.

The best way I have found is to simply try differet ideas to see what fits your requirements the best. Only you can decide that.

Now, Terry gave some basics that will work and is simple to do.

You asked a question about shielding and whether is it necessary? That depends upon what you will shoot for as your minimum delay. The shorter the delay the greater need for a shield. Since shielding really isn't that much of a rocket science, one can try both ways.

Now, if no shieldng is used, then you will probably have to operate at a longer delay setting to avoid excessive false and erratic signals. This is fine for the iron and stony iron meteorites, but may not be the best for the stony types. Since the stony make up about 95% of the meteorites that hit the earth, that is a lot of meteorites to ignore.

Reg

**Roughwater** · 07-06-2009, 11:10 AM

Large coil

Hey Slick,

I'm glad Reg answered your questions as he is one of the real experts here. What I do know is that the coils I built using this simple design seem to work great! I suppose there could be an advantage to making a coil using parallel conductors in the form of a ribbon but it seems it would be challanging to build and would complicate using it with a simple housing like PVC.

For a large coil I was just planning to use PVC pipe as the housing and maybe either tape or strap the coil to the pipe frame as the meteorite men seem to have done. If more protection of the coil would be in order I could maybe slit the pipe and couplings so that most of the winding is inside the PVC ?

As to what I would use my large coil for, probably the same thing you plan to use your for, hunting meteorites but seems it could also find use for locating large deep nuggets, veins or a cache.

Terry

**Slick** · 07-06-2009, 06:10 PM

Reg,

Thanks for the reply!

This is my first whack at a diy detector circuit and coil. So went with Garys kit to be able to start with a known qty, already tested. Those coil wire parameters came from him. He was unable to wind a coil that big, and shipping it would be a risky adventure as well!

I am enthused on your comments about the coil. I do like to experiment, and those materials are relatively inexpensive to putter with!

Electronic materials are often a bit of a challenge to come by here (Mexico) as well, so 'experimenting' goes hand-in-hand with 'diy', LoL

I am aiming for depth and size over 'quality', and given that I will mostly be in virgin territory (desert area, dry lake beds), I will hopefully eliminate the need for overly tight discrimination. But, I do understand the 'stony vs. iron' issue, so will have to find a balance there.

I suspect I will have some more detailed questions about shielding after I get things powered up and get some sample signals.

Thanks again for the pointers!

Slick

Originally posted by Reg View Post

Hi Slick,

I have not build Gary's PI so you might want to check with him to see what he feels would work best. What I have seen is he seems to like to build his coils flat single layer types. Again, I have not tried that design so I can't tell you what to expect if one were to compare the different coil designs.

Now, you can scan this site for a lot of ways to build coils that include simple winding techniques, to basket weave, single layer, etc, etc.

The best way I have found is to simply try differet ideas to see what fits your requirements the best. Only you can decide that.

Now, Terry gave some basics that will work and is simple to do.

You asked a question about shielding and whether is it necessary? That depends upon what you will shoot for as your minimum delay. The shorter the delay the greater need for a shield. Since shielding really isn't that much of a rocket science, one can try both ways.

Now, if no shieldng is used, then you will probably have to operate at a longer delay setting to avoid excessive false and erratic signals. This is fine for the iron and stony iron meteorites, but may not be the best for the stony types. Since the stony make up about 95% of the meteorites that hit the earth, that is a lot of meteorites to ignore.

Reg

**Slick** · 07-06-2009, 06:35 PM

Terry,

If I stay with the 'ribbon' method, what I had in mind was splitting the pvc pipes in half, then splitting some wooden dowels (or other cheaper wood stock) in half and gluing into the bottom half of the split pvc pipe assembly with the flat surface of the wood 'up'.

That would provide a flat surface on which to lay/mount the coil. That would probably work for your bundle design or a ribbon design. I can see a lot of variations on this theme, so experimenting would be in order!

For the initial prototype, I plan on leaving the pvc pipe 'open' so I can experiment easier with shielding, but for protection, the final design would probably be enclosed by somehow securing the top half of the split pvc pipe to the bottom half.

The 'Meteorite Men' didn't appear to have split their pvc pipe, so can only guess that they use some method of connectors/joining on their coil wiring at the corners?

I didn't see any external wiring (could have just missed it), but their comments and actions in turning off the cameras every time they had to repair it made me think the wiring was inside, not outside! Plus, not much protection for the wiring if outside of the pvc pipe. And the way they are using it seems to require more than normal ruggedness, LoL.

Slick

Originally posted by Roughwater View Post

Hey Slick,

I'm glad Reg answered your questions as he is one of the real experts here. What I do know is that the coils I built using this simple design seem to work great! I suppose there could be an advantage to making a coil using parallel conductors in the form of a ribbon but it seems it would be challanging to build and would complicate using it with a simple housing like PVC.

For a large coil I was just planning to use PVC pipe as the housing and maybe either tape or strap the coil to the pipe frame as the meteorite men seem to have done. If more protection of the coil would be in order I could maybe slit the pipe and couplings so that most of the winding is inside the PVC ?

As to what I would use my large coil for, probably the same thing you plan to use your for, hunting meteorites but seems it could also find use for locating large deep nuggets, veins or a cache.

Terry

**Roughwater** · 07-06-2009, 11:55 PM

Meteorite men coil

Hey Slick,

I copied the Meteorite Men show to my Dish Network DVR. I was just reviewing the show in order to better see how the meteorite men built their large coils. Early into the show they open up a corner of the PVC on their 1 Meter coil to expose the actual coil winding. It looks like a fairly thick round insulated cable but it appears to be laid loose inside the PVC and not potted. They may not have even glued all the pvc together in order to allow it to pack smaller?

They also (very quickly) show the meteorite men putting their PVC sleigh together and the coil is not inside any of the PVC tubing. They actually show them taping and straping the coil to the outside of the PVC sleigh.
I'm sure this would also allow compact packing of their coil. They seem to do it smart and simple.

Terry

**thedigger** · 07-07-2009, 10:52 AM

In regards to shielding for large coil,I built several years back 3ft mono for a minelab 2200.the shielding was very tricky,found that I had to apply it by trial and error in the goldfields.the final outcome was that I used a high resistance wire .1mm at about 120 ohms per yard spaced at about 12mm apart.
I had one wire running around the coil {with a break in the middle}and the wires winding around at 12mm apart,I then cut the bottom wire on the bottom wright around so the windings were like a fish bone pattern,I hope this explains what I did.
The end result was that we got the 3ft coil to balance perfect,it had very good sensitivity.

**Roughwater** · 07-08-2009, 02:47 AM

Large coil shielding

Digger,

Appreciate the input and I believe your type of shielding would work fine for a whites TDI. However for the 1M that I'm building I'll stick with the scotch 24 tape as it worked great on the 7" and 18" coil I built so I am hoping it will do the same on the 1 M coil. If it doesn't work well your shielding suggestion may become plan "B".

Slick,

I got a start on my 1 Meter coil. The board I used wasn't 3ft wide so for now, it's actually 28X44". I'll post a couple pics of the start of it on this website under coils for a TDI. I ended up with 10 turns as 9 turns read about 290 uh on my meter, 10 turns read about 370uh.

Terry

**authere** · 07-08-2009, 05:10 AM

Shielding

Hi The Digger,
Well done with the shielding of your 36' coil,with your .1mm wire was it magnet wire or S.S. or what?, as i am in the throws of shielding several coils and they are being a pain in the a#s. I'm using a method developed by Stefan whereby several strands of .14- to .2mm magnet wire are laid side by side and put on tape which is then wrapped, spirally to make a shield. I'm making(trying to) coils for minelab machines and the tolerences are tight, i get the tolerences spot on ,as the ones i make have a Q of 3.8 to 3.9 to which i maintain so the coils should work, but putting the shield on and i have all sorts of problems. The main problem i am having is the coil when stationary gives good depths in air (testing) but as soon as i ground balance and tune the detector and start swinging they sound off, stationary they are quiet.
My thoughts are that the shielding is wrong, but have not as yet found a solution to the problem,but looking at your design i would say there is too much shielding. My 6" coil i have 40 strands x .14mm magnet wire and the 28" coil has 20 strandsx .14mm m/w, can anyone direct me to the right solution, i have put a lot of work into these mother su## ers and i have run out of ideas.
Regards Ron

**Roughwater** · 07-08-2009, 10:51 AM

Shielding for ML

Digger,

Not sure I can help you a lot as I am a novice at this so I hope Reg might step in and better answer your question. I suspect tolerances aren't quite as tight for the TDI as as for the ML PI detectors as power levels are lower and the delay is adjustable.

All I can tell you is what works well for the TDI: After I wrap a series of half hitchs the length of the coil I spread Silicone II over it and immediatly wrap 1 layer of slightly overlapping scotch 24 tape which is not really tape but a thin wire mesh. I honestly thought it would be too much shielding but the coils respond very well and provide great depth on the TDI. Attached is a pic of the tape. It has to be cut however is it comes in the form of a flattened tube. I just cut it down the middle lengthwise and spiral wrap it leaving a small gap and ground one end.

Attached Files

**thedigger** · 07-08-2009, 11:10 AM

Hi Ron
The larger the coil the less shielding is needed.I used high resistance wire the type used for elements,I think it is made from nickel and something else which escapes me at the moment,about 130 ohms per meter.
You could purchase that from any company that sells resistance wire,one good point it is very strong.
For smaller coils I use a silver plated mesh that I got from http://www.lessemf.com/fabric.html
Do not seem to have any problems with smaller coils,but back to the larger coil,as I said in previous post you will need to do it by trial and error in the goldfields,but I have given you a starting point about the lay out.

**thedigger** · 07-08-2009, 11:23 AM

Hi Ron & Roughwater
Another point is that I try to keep at least 3mm gap between the coil and the shield,when using a shell case I fix the coil on some 3mm spacer on the shield.
I hope this helps

**thedigger** · 07-08-2009, 11:27 AM

Hi Ron & Roughwater
Another point is that I try to keep at least 3mm gap between the coil and the shield,when using a shell case I fix the coil on some 3mm spacer on the shield. (balsa wood)
I hope this helps

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