On to my 2nd Coil for my Minelab SD

[mario]

After my first some what successful attempt at creating a spiral coil (I still need to experiment with shielding), I decided to do another coil. My first coil is really sensitive, but I just didn't have much luck in the depth department. I need a general purpose coil so I am now trying to build a bigger coil.

I know there are advantages to having a spiral shaped coil, such as speed (my first coil is really fast -even on the small objects!), but I decided to attempt a mono loop. Hopefully the higher capacitance won't make this coil too slow.

I first did my calculations as usual, using Gary's' Inductance calculator. I used Litz 13/30 (19 AWG) this time. Last time, using thinner litz got me a slightly high resistance (.898 Ohm Vs Target .5 Ohm). Although my coil worked, I will never really know if the slightly higher resistance has anything to do with my coil's failure to detect medium sized objects at depths greater than the 7-8 inch threshold.

The variables:

Inside Diameter: 11.5 Inches
Target Inductance: .3mH
Wire Gauge: 19 AWG
Coil Length: .25 Inches

The calculator's estimated Results:

Inductance: 0.3252 mH
Resistance: 0.5864 Ohms
Turns: 24

After finishing my winding, my final values didn't vary that much.

Obtained results (after tightening the coil with craft tape):

Inductance: .312 mH
Resistance: .520
Turns : 25

My guess is that the discrepancy lies in the fact that the litz wire isn't actual 19AWG, but a close approximate.

I used some craft gift tape (made out of non conductive material -I hope) to tighten my windings.


and voila!


I want to shape this coil into a perfect elliptical shape. Any one have any suggestions?

[mario]

Ok, I'm having 2nd thoughts about this coil. I'm now considering salvaging the wire and making a dual field coil.

So many options, so little wire...

[porkluvr]

Do you know the voltage rating of your litz wire? I don't detect a "serving" layer from your photo.

It's one thing to wind a planar coil where the opposite ends (and voltage extremes) have some distance between them,

but... now you're going to scramble the windings together send a high voltage pulse through them. Keep your fingers crossed.

[mario]

I have no Idea. 200 Volts is the maximum output of the Minelab PIs right? The guy who shipped me the wire said the bundle was comprised of stranded magnet wire, with each strand individually shielded, which I verified.

I have 225 feet left, and I worked hard to create that tight bundle with the wrapping tape. I put in close to 3 hours already in creating it. I'll probably use the remaining wire to create my dual field coil. I haven't made a decision just yet...in any case, a dual field coil like the one Rov made back in June is a much more productive design than a mono loop. This coil will be my work horse so I'm looking for a good design and just don't see any real advantage of continuing with a single field mono.

[mario]

coil gets the green light

After doing some preliminary testing, I decided to complete the coil. I shielded my coil with scotch 24 shielding tape and finished the job by wrapping a tight layer of black electrical tape. I enclosed the coil in a dual field coil housing I had ordered.

I was really impressed with the coil's ability to find some medium sized targets at decent depths. I could find a US .10 dime at 10 inches!

I waited a few days for weather conditions to improve and headed toward desert areas by black mountains, near boulder city area. I had only been out hunting for less than 2 hours when I found the following:


Here's a different view



Yes, meteorites!

Both signals were strong. Because I had found most of my lead targets at surface level, I had expected something special after realizing these targets were buried a little deeper. Except for hot rocks, the ground was pristine.

No gold this time, but with two meteorites, It wasn't a complete waste.

Alas, I did lose a cell from my custom Li-Po battery pack and won't be prospecting for the next few days. With 2 cells, I can only get 6.3 Volts out of my regulator. I ordered a replacement cell and should be getting it next week.

Good coil, next time lets hope for some yellow!

[ghaj]

After doing some preliminary testing, I decided to complete the coil. I shielded my coil with scotch 24 shielding tape and finished the job by wrapping a tight layer of black electrical tape. I enclosed the coil in a dual field coil housing I had ordered.

I was really impressed with the coil's ability to find some medium sized targets at decent depths. I could find a US .10 dime at 10 inches!

I waited a few days for weather conditions to improve and headed toward desert areas by black mountains, near boulder city area. I had only been out hunting for less than 2 hours when I found the following:


Here's a different view



Yes, meteorites!

Both signals were strong. Because I had found most of my lead targets at surface level, I had expected something special after realizing these targets were buried a little deeper. Except for hot rocks, the ground was pristine.

No gold this time, but with two meteorites, It wasn't a complete waste.

Alas, I did lose a cell from my custom Li-Po battery pack and won't be prospecting for the next few days. With 2 cells, I can only get 6.3 Volts out of my regulator. I ordered a replacement cell and should be getting it next week.

Good coil, next time lets hope for some yellow!

Hi mario,
what will happen if I make a mono loop for SD2000 out of normal wire (one-strand cable) instead of litz wire? will it work? what is the benefit of litz wire in this application?

[mario]

Hi mario,
what will happen if I make a mono loop for SD2000 out of normal wire (one-strand cable) instead of litz wire? will it work? what is the benefit of litz wire in this application?

Hi,

You say "normal wire (one-strand cable)."

Is this magnet wire, coaxial? can you be more specific?

The reason why people recommend tin-plated copper or litz over other wire types has to do with wire properties. For instance, the eddy currents on regular wire types (like silver plated copper) settle quicker than the more preferable types. However, a lot of members have used silver plated, multi-stranded wire successfully.

Some thing else to consider is the wire shielding: Teflon has a more preferable dielectric constant than PVC.

Ideally we would like tin-plated litz. Unfortunately, I have yet to find a manufacturer who makes this type of wire.

One last thing I will mention about your future coil relates to shielding.

I have not been very lucky with scotch 24 shielding tape. I find it strange that some people have been able to use coaxial cable with their units because the shielding is very thick on coaxial cable. When I first used scotch 24 tape on my coil I must have lost depth by 2-3 inches easily. People will not believe me what I currently use for shielding: a non metallic fabric I found at a local thread shop. Experiment with different shielding material because shielding is very important.