Getting ready to build the MPP

[Silver Dollar]

I used a piece of wire 66 feet long that I had wound a mono coil with it
came out pretty close to the optimum value of 300 uh...

[6666]

Have a look in this thread on how to make the form and how to wind it (look around #111);
http://www.geotech1.com/forums/showt...-PI-coil/page5

Check out post #111 you should be able to work out how to wind the coil from the pictures I posted, thanks SD.

[KRinAZ]

Have a look in this thread on how to make the form and how to wind it (look around #111);
http://www.geotech1.com/forums/showt...-PI-coil/page5

Check out post #111 you should be able to work out how to wind the coil from the pictures I posted, thanks SD.

Yes, thx much 6666!, found those a bit earlier - good that you posted those - that russian translated lacing image is pretty much the missing key.

It's still vague to me how to wire it but I'm guessing it will be clearer when I actually sit down and lace it up. Picked up Lexan today - .093" - and a 1/8" bit for my Dremel - I have an idea on an easy way to do some nice clean Lexan machining with just the Dremel and a simple jig - basically using it as a tiny router. It will cut the slots exactly the 1/8" width and terminate each slot with a round end as if I cut down to a drilled hole - but a cleaner straighter cut. Will shoot photos of course. Ultimately, if I get this basket weave thing (what's the correct term for these coils?) working, I'm going to make a full lace up set of directions and photos - kind of like the Boy Scouts knot making tutorials where they show every twist and turn making complicated knots look easy...

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[KRinAZ]

I used a piece of wire 66 feet long that I had wound a mono coil with it
came out pretty close to the optimum value of 300 uh...

OK, cool, gives me a clue on wire length I need - do you remember what wire gauge & insulation it had?

.

[KRinAZ]

[QUOTE=Silver Dollar;197631]
That's the easy part, I still haven't figured out how to package it. I saw someone
who packed foam tubes around it then taped and glassed it to male a big donut
type housing. I cannot find the thread just now...

Found it - #30 on page 2 of the Chance PI coil thread: http://www.geotech1.com/forums/showt...-PI-coil/page2[/QUOTE] .

[Chet]

Hi KRinAZ, Attached is a photo of my coil. It is in an old 8” coil housing that a friend gave me. The coil form was box knifed out of foam filled art board which was lying around. It wasn't strong enough for the wire used so the windings have some BO-BOs from broken or bent sections. Even with the mistakes the principal goal was maintained by having good crossover of the wires (no parallel wires that would cause more coil capacitance). It is wound with 18 gauge Litz wire which was also available in one of my boxes of stuff.

Qiaozhi has a coil calculator at the top of the Coil Forum. Because of the zig zag pattern this coil will require some additional wire and turns to achieve the required inductance. It is easier to remove extra turns than to add if the inductance is wrong.

Air is the best dielectric for a fast coil. Sections of foam filer (tubes) used in the Chance PI Postings is a good way to stabilize the windings from bumps against rocks and bushes. Avoid using very much solid material such as epoxy or goop directly within or against the windings.

The coil works well on the bench. It will be tested outside after through experimenting and devising some kind of Ground Balance circuit. Click on the photo to expand or shrink.

[Silver Dollar]

I used awg 24 Teflon coated wire. I just hope the hot melt doesn't slow it down too much!

I just fired up my Hammer Head yesterday so will be able to test it soon.

To wrap mine I'd need to cut the Lexan. In working with mine I found it can crack if you
are not careful. I might just put it in a full case like the Minelab one above.

[Silver Dollar]

It's still vague to me how to wire it but I'm guessing it will be clearer when I actually sit down and lace it up.

I was a bit lost at first too until I figured out the key is after 1 complete loop you go forward one slot and start looping on the other side.

[KRinAZ]

Hi KRinAZ, Attached is a photo of my coil. It is in an old 8” coil housing that a friend gave me. The coil form was box knifed out of foam filled art board which was lying around. It wasn't strong enough for the wire used so the windings have some BO-BOs from broken or bent sections. Even with the mistakes the principal goal was maintained by having good crossover of the wires (no parallel wires that would cause more coil capacitance). It is wound with 18 gauge Litz wire which was also available in one of my boxes of stuff.

Qiaozhi has a coil calculator at the top of the Coil Forum. Because of the zig zag pattern this coil will require some additional wire and turns to achieve the required inductance. It is easier to remove extra turns than to add if the inductance is wrong.

Air is the best dielectric for a fast coil. Sections of foam filer (tubes) used in the Chance PI Postings is a good way to stabilize the windings from bumps against rocks and bushes. Avoid using very much solid material such as epoxy or goop directly within or against the windings.

The coil works well on the bench. It will be tested outside after through experimenting and devising some kind of Ground Balance circuit. Click on the photo to expand or shrink.
Looks good Chet, boy I'd be happy acquiring some of those shells! Thx for those tips - I will heed them. So...I just emailed Don at DBCoilShells with the coil specs (will be 8"OD, appx 1" wide, appx 1/4" thick) to see if he has/can build shells...

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[KRinAZ]

I used awg 24 Teflon coated wire. I just hope the hot melt doesn't slow it down too much!

I just fired up my Hammer Head yesterday so will be able to test it soon.

To wrap mine I'd need to cut the Lexan. In working with mine I found it can crack if you
are not careful. I might just put it in a full case like the Minelab one above.
Yesterday I also picked up some new sharp 35 tooth thin metal jigsaw blades, will use those to cut the lexan into the ring shape (with a nice slow feed rate), then use the dremel to cut the slots (also nice slow feed rate) and then will clean up and soften the edges of the cuts with 320 grit sandpaper strips.
I should have my magnetics separator (for prospecting) project finished today, if I have time left I will cut the Lexan, otherwise should have it cut and pic'd tomorrow.

Best wishes on the Hammerhead!

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[KRinAZ]

I was a bit lost at first too until I figured out the key is after 1 complete loop you go forward one slot and start looping on the other side.

Thx for that, I'll watch for that step

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[KRinAZ]

I can get some Litz wire - either unserved 7/36 (28AWG equiv) or served 12/36 (25AWG equiv), about 8000 feet - I know nothing about Litz wire - never heard of it until joining this forum - would either of these work for my basket weave coil, is one better than the other for this coil (I'm thinking the 7/36 would be better as it's smaller equiv gauge), and served/unserved (what is that!?) does it matter which one for the coil build? Would about $55 be a good price? To me it seems like a lot of wire for the $ but I've of course never shopped Litz wire. It's not chinese I'm told...

[Davor]

Litz improves coils by means of eliminating eddy currents in the cross section of a wire. E.g. you can detect a mains cable that is made of thick bulk wire, but you can't detect a cable of a same thickness made of braided wires. Same goes with a coil, and the copper decays quite slowly. Tau of copper is ~140us. This signal presents itself as a constant offset on top of all other signals, but because it also produces noise, it is a good idea to reduce it as much as possible. Hence Litz.
The other method is using braided wires, as they too reduce cross-section eddy currents.

[KRinAZ]

Litz improves coils by means of eliminating eddy currents in the cross section of a wire. E.g. you can detect a mains cable that is made of thick bulk wire, but you can't detect a cable of a same thickness made of braided wires. Same goes with a coil, and the copper decays quite slowly. Tau of copper is ~140us. This signal presents itself as a constant offset on top of all other signals, but because it also produces noise, it is a good idea to reduce it as much as possible. Hence Litz.
The other method is using braided wires, as they too reduce cross-section eddy currents.
OK thanks much Davor - do you know if the 7/36 unserved Litz would be a good choice for the basket weave coil and if 8K feet for about $55 is a good price?

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[Chet]

Hi KRinAZ
Litz wire for a metal detector fast coil should be made up of many individual small (44-48 AWG) Polyurethane-Nylon coated wires. Served means that the entire Litz bundle of small wires is wrapped with a nylon textile or yarn for added strength and protection.


Litz wire is designed to minimize the power losses exhibited in solid conductors due to "skin effect". Skin Effect is the tendency of radio frequency current to be concentrated at the surface of the conductor. Litz wire reduces this effect by increasing the amount of surface area without significantly increasing the size of the conductor.


Litz wire composed of many strands of finer wires is best for fast coils. Polyurethane-Nylon coating is the best for soldering because doesn’t require pre-stripping before soldering. It’s hard to strip enamel coating off of 330 individual 44-48 AWG wires for soldering.


Frequency use vs. AWG size of the individual wire strands used within the Litz bundle.
All Values in KHz; a fast coil should be in the 800-3200 kHz range.
1 To 10.0 - 30 AWG
10 To 50.0 - 33 AWG
50 To 100.0 - 36 AWG
100 To 200.0 - 38 AWG
200 To 400.0 - 40 AWG
400 To 800.0 - 42 AWG
800 To 1600 - 44 AWG ------ Fast coil
1600 To 3200 - 46 AWG ----- Fast coil
3200 To 5000 - 48 AWG ----- Hard to find


There are several listings on eBay for Litz wire. The larger (mm) diameter listed in advertisements is normally the overall wire plus serving/insulation measurement. Litz wire made up of 330 strands of #46 AWG (330/46) or (250/46) would be approximately equivalent to an overall 22 AWG solid wire size. Litz 175/46 is approximately equivalent to 24 AWG. These sizes would be good for a fast coil.

Smaller wire will reduce coil capacitance a small amount and be a little faster. I haven't seen any literature on comparisons of coils wound with different size Litz wire. With this modified basket weave there probably is not much difference since there is no parallel wires as wound in commercially built Litz metal detector coils.

Smaller overall wire size will increase the resistance of the coil which results in increasing the time constant of the coil (current charge time) for each transmit pulse. This in turn will limit the pulse repetition frequency (transmit pulses per second). There is a lot of leeway and trade-offs but its good to build in some margins for experimenting with different charge currents, pulsewidths and pulses per second rates.