Single layer Spiral coil

[chemelec]

I would like to know if any one could direct me to a site showing depth results of different coil sizes for a minelab SD,GP, GPX. I would like to see how well my coil stacks up against other coils.

This coil is relatively fast, but I noticed that after 8 inches, it looses sensitivity dramatically. A dime or bullet won't register at greater than 8 inches - Which is really strange considering that I can find small metal targets easily at the less than 8 inch threshold. I'm able to get a BB (1/6 gram) sized lead object at about 2-3 inches... an iron BB comes in at 5 inches.

Is it normal for PI to loose dramatic sensitivity in this non linear fashion?

Also, Air tests on a soda can come in at 2 feet.

A Lot depends on the Type of Metal. I Believe your US Dimes are mainly composed of Zinc.

[mario]

depth comparisons

I would like to know if any one could direct me to a site showing depth results of different coil sizes for a minelab SD,GP, GPX. I would like to see how well my coil stacks up against other coils.

This coil is relatively fast, but I noticed that after 8 inches, it looses sensitivity dramatically. A dime or bullet won't register at greater than 8 inches - Which is really strange considering that I can find small metal targets easily at the less than 8 inch threshold. I'm able to get a BB (1/6 gram) sized lead object at about 2-3 inches... an iron BB comes in at 5 inches.

Is it normal for PI to loose dramatic sensitivity in this non linear fashion?

Also, Air tests on a soda can come in at 2 feet.

[mario]

I tried it. No luck

[mikebg]

Unfortunately, the detector was able to detect the shielding and all I got was a persistent hum. I'm going to try next conductive paint if I find any.

Mario, try to cut alu foil as shown below

[mario]

Thank you!

After watching th[FONT=verdana,geneva,lucida,'lucida grande',arial,helvetica,sans-serif]is[/FONT] video I see my mistake. I tried a quick fix by enclosing the coil in some aluminum kitchen foil shielding and grounding the shield to the detector.



Unfortunately, the detector was able to detect the shielding and all I got was a persistent hum. I'm going to try next conductive paint if I find any.

[mikebg]

Could it be due to the coil having a larger number of windings and therefore more capacitance than the machine could handle? I measured capacitance at 18.98 uF.

By the way, some more coil specifications in case you don't have a LCR meter, and would like to construct the coil:

Windings: 39 turns
Outer Diameter: 7 and 1/2 inch (exact)
Wire type: Litz 330/46 with nylon shielding
Inductance: 301 uH
Resistance: .898 OHM
Capacitance: 18.98 uF (measured at connector)


Last issue that I'm having is the coil's sensitivity to hot rocks. I dug faint signals thinking they were small lead shards. Once I dug faint signals, I noticed they were hot rocks. It frustrated me since I particularly got rid of my old vlf metal detector because I got tired of digging the hot rocks. Is the spiral coil design not suitable for hot rocks?!?!? I was told PI was better at avoiding them.

Mario, the equivalent capacitance of the search head can not be measured by C-meter.
It is calculated after measurement of L and the resonant frequency. Even with a long microphone cable, the real value of calculated capacitance is several thousand times less than yours 19uF.
If you find some hot ricks placed in shallow, some of them are meteorites. Search WEB for details because some meteorites are very expensive. You can get good money for them if offer to the jewelers, collectioners, museums and researching institutions.

Minelab uses graphite paper for coil shielding. You can see this in video at:

http://www.youtube.com/user/woodypoopoo

See also:

http://detectormods.com

[mario]

I played with the coil today. It's really sensitive. I was able to find a lead bb at 3 inches. It's also deep. I'm able to find bronze targets 1/2cmx1cmx2mm at 8 inches. It's also a very quiet coil. If I keep the coil at a consistent level off the ground (which can be difficult since ground isn't naturally leveled), I notice a very stable threshold every time, which surprised me. Ground balancing helped out a lot with stabilizing this coil.

The only problem I'm having is the aforementioned issue with lowering the coil. As it approaches the ground, the machine sounds like it hits a large target every time. it's really annoying, and I know I could improve the coil by shielding it. I just don't want to ruin the coil by using the wrong shielding material. I really need your advice on this issue guys. My SD 2100 v2 should not be that different from the SD 2000 in terms of components.

Could it be due to the coil having a larger number of windings and therefore more capacitance than the machine could handle? I measured capacitance at 18.98 uF.

I also read some where that the coil needs to be grounded. What does that mean? How would you go about grounding a coil housing? where would you ground the wire? right now, the two sole wires from the microphone cable I'm using are connected to the RX and TX terminals. I striped the copper ground wire mesh that was inside the cable to stabilize the coil.

By the way, some more coil specifications in case you don't have a LCR meter, and would like to construct the coil:

Windings: 39 turns
Outer Diameter: 7 and 1/2 inch (exact)
Wire type: Litz 330/46 with nylon shielding
Inductance: 301 uH
Resistance: .898 OHM
Capacitance: 18.98 uF (measured at connector)

I was able to attain a better inner circle by winding the coil to 7 1/2 inches (The eight .10 US dimes were placed at 2 inches from the center), and then removing 5 turns from the inner circle. It allowed me to remain very close to the required 300 uH inductance. However, resistance is close to .9 OHM and is not effecting performance as I expected (Could the coil be even more sensitive if its resistance was at .5 OHM?). The coil remains quiet sensitive.

Last issue that I'm having is the coil's sensitivity to hot rocks. I dug faint signals thinking they were small lead shards. Once I dug faint signals, I noticed they were hot rocks. It frustrated me since I particularly got rid of my old vlf metal detector because I got tired of digging the hot rocks. Is the spiral coil design not suitable for hot rocks?!?!? I was told PI was better at avoiding them.

[mario]

After testing the unit I realized my coil needed shielding. As I lower the coil, its very noisy. I need to shield the windings with scotch 24 tape. However, since the coil is spiral in design, scotch tape will not be the most appropriate material. Does anybody have suggestions? could I use epoxy as shielding?

I noticed Gary's coil had a thin epoxy layer. Can this layer lower the noise effect of lowering the coil?


Also, I have another question. I measured the resistance (at connector) to be 1.6 OHM. I know Minelab specifications call for .5 OHM. Is the higher Resistance bad for sensitivity?

Thanks for any suggestions

[mario]

NEW UPDATE!

Ok gentlemen, I have been working on this project for a while now. I decided to take some of your advice and asked Gary to make me a coil with the requirements needed for my Minelab. He did a really great job.


As you can see from the picture above, Gary's rig is simply amazing. It was a perfect circle. Everything was on track until I received my coil housing from Hay's electronics. Unfortunately I made the assumption that the coil housing was a precise 8 inch coil housing, so I asked Gary to make a coil 2mm short of 8 inches. I therefore provided him the wrong outer diameter, which really sucked because I was not able to fit the coil inside the coil housing. When I finally got my coil housing, It turned out the coil housing was more like 4mm short of 8 inches, rendering my coil housing useless. I had two options, make a coil housing out of Resin OR make another coil meeting the coil housing specifications.

I decided to make a coil.



As you can see from the image above, my first coil was not that particularly tight. I realized that I had to come up with a set up to tighten the windings, which brings me to my next photo


I used a special U shaped vice to keep the two tile slabs together. Unfortunately, since I did not use a strong tile material such as granite, the slab broke into two pieces as I tighten the vice. I cut the tile slab with a tile cutter. For spacing I used 8 dime coins. They accommodated the wire thickness perfectly.

And here's a picture of my final coil. I used litz wire 330/46. I was able to get 301 uH, with a .9 Ohm Resistance. Unfortunately, the resistance was a little higher than I would have liked but couldn't do anything about it. I suppose I needed thicker litz...



Here's a picture of my final coil inside the housing:

[chemelec]

Gary, I designed a so complex form for winding flat spiral, that it's even hard to describe the structure and procedures in use.

The main metallic disc here is denoted as position 1. He did not warp because the metal is thick and there are not any slots, only small openings for leads of wire. Position 7 is the opposite disc with less diameter because after winding, sectors 2 should be tighten with four clamps against warping. The eight sectors pos. 2 and disc 7 may not be made of metal.
Reel is formed when on disc 1 is placed disc 4 for distance because is thick as wire. Then through an opening in 1 is inserted wire lead and tighten the eight sectors pos. 2 each with a screw pos. 3 with hidden head. Then with four screws pos. 6 is tighten the opposite disc pos 7. After winding are used 4 small clamps to prevent warp of four diametrically opposite sectors, for example as indicated by numbers 1, 3, 5 and 7. Remove the four screws pos. 6 and the disc 7. Then remove the screws that hold the sectors with numbers 2, 4, 6 and 8 (which are not tightened with clamps). This opens four diametrically opposed sectors of coil to enable covering wire with very thin sheet glue. After glue hardens, removed sectors are inserted and tighten with clamps, and removed sectors 1, 3, 5 and 7.

Looks like a Lot More Complicated and Slower of a procedure to wind a coil, than on my New Jig.
Sorry No Picture at present time. Have Company here from England!

[mikebg]

Gary, I designed a so complex form for winding flat spiral, that it's even hard to describe the structure and procedures in use.

The main metallic disc here is denoted as position 1. He did not warp because the metal is thick and there are not any slots, only small openings for leads of wire. Position 7 is the opposite disc with less diameter because after winding, sectors 2 should be tighten with four clamps against warping. The eight sectors pos. 2 and disc 7 may not be made of metal.
Reel is formed when on disc 1 is placed disc 4 for distance because is thick as wire. Then through an opening in 1 is inserted wire lead and tighten the eight sectors pos. 2 each with a screw pos. 3 with hidden head. Then with four screws pos. 6 is tighten the opposite disc pos 7. After winding are used 4 small clamps to prevent warp of four diametrically opposite sectors, for example as indicated by numbers 1, 3, 5 and 7. Remove the four screws pos. 6 and the disc 7. Then remove the screws that hold the sectors with numbers 2, 4, 6 and 8 (which are not tightened with clamps). This opens four diametrically opposed sectors of coil to enable covering wire with very thin sheet glue. After glue hardens, removed sectors are inserted and tighten with clamps, and removed sectors 1, 3, 5 and 7.

[chemelec]

Improvement of Gary's form. Eight more openings are made to glue more surface at winding large coils. They are smaller as seen in the attached image.

That Aluminum Plate WAS one of my Earlier Jigs.
But it WARPS too Much, Especially as you wind larger coils.
I would think that your Extra Grooves will also cause MORE Warpage.

My Newer Jig I now use is made of 2 Plates of 3/4 Inch Lexan, 14 1/2 inches in Dia. and it will wind a coil Up to 13 inch Dia.
And it uses Special Designed Center Piece to offset warping, So No need for Extra Grooves.

I use My Mill Machine to hold this Jig, and thats the Biggest diameter it can hold.

But this is OK, As Shipping Coils larger than 13 Inches becomes a Problem anyway.

[mikebg]

Gary's winding form

Improvement of Gary's form. Eight more openings are made to glue more surface at winding large coils. They are smaller as seen in the attached image.

[IBGold]

Hi all,
The silver plated wire is no good in Minelab PI machines as the wire is seen as a solid wire and will support eddy currents I made this mistake when I first started making coils and bought a large amount of 16 AWG and 20 AWG silver Teflon stranded wire and it was no good so if someone wants some it is sitting here wasting space only trouble is I am in S.Australia, the enameled silver wire would be great but 17 AWG would be optimum let me know if you find some.
Yes the 660/46 is close to 18 AWG but for correct inductance and usable resistance OK for coils to about 14" max.
Regards, Ian.

[chemelec]

Just a Few Comments:

Due to Insulation Thickness, Making a Coil with PVC Coated wire will become Quite Large and Heavy to get the required inductance.

Same goes for Teflon coated wire, but Teflon coated wire will be a faster coil than the PVC. For Better Bonding it is Easier if the Teflon is Acid Etched beforehand.
Litz wire will also make a Fast Coil, but again it requires more turns due to insulation thickness.

I Find that Soild 18 AWG adds a considerable amount of Interwire Capacitance, as compared to 20 or 22 AWG.

A Typical Coil that I wound with 22 AWG Enamelled wire is 6" ID and 8" OD.

The 22 AWG was Calculated as .025" and spacing of .002" to allow for a tiny gap and insulation thickness.
Calculated inductance was 408.223 uH.
Actual Measured inductance is 408.8 uH.
Pretty Darn Close to Calculated value.
(DC Resistance is 1.07 Ohms, and Free Air Resonance is 1.88 Mhz)

Kinda Wish I had some 22 AWG Enameled "Silver coated" wire to see how well it would work out, as a comparison.
Should be LOWER Resistance!