Theories about coil-shielding

[mykaitch]

i got a couple of carbon cloth mats about 10 inches square from work, one fine one course, and on first inspection seemed possible but the mechanical connection is where it fell down, conductive glue, failed and direct connection was worse, but i discovered they are great for standing soldering kit on.

I have some 3M copper tape. The backing glue is designed to be conductive. You can even rub it down onto anti-static foam and get a solid connection.

[Waikiki_Sweep]

I have some 3M copper tape. The backing glue is designed to be conductive. You can even rub it down onto anti-static foam and get a solid connection.

I have purchased cooper tape on eBay. It is synthetic cloth covered by nickel than cooper.
Looks great but it has conductance only a few Ohms and I think it is to high conductive for a good shield.
Also I have checked that all my detectors can sense a piece of such tape. Signal is much smaller than signal from aluminum foil but it is here.
So I have worries that cooper tape will reduce sensitivity of metal detector like aluminum foil did.
Graphite shield works perfect so far.

[mykaitch]

I have purchased cooper tape on eBay. It is synthetic cloth covered by nickel than cooper.
Looks great but it has conductance only a few Ohms and I think it is to high conductive for a good shield.
Also I have checked that all my detectors can sense a piece of such tape. Signal is much smaller than signal from aluminum foil but it is here.
So I have worries that cooper tape will reduce sensitivity of metal detector like aluminum foil did.
Graphite shield works perfect so far.

Sorry, I meant as a means to connect to graphite etc. I did wrap my DD in foil AND copper though and it works great. It was just concentric that didn't work
I think the stuff on ebay is cheap junk. The 3M type is expensive and 100% copper - I even used it to make busbars for a high power amp PSU. AsI said,the backing is totally conductive.

[Waikiki_Sweep]

I did wrap my DD in foil AND copper

I am not sure that you need to put so much metal into metal detector coil because that cooper tape and aluminum foil acting like targets shifting phases in VLF and prolonging decays in PI detectors.

[Waikiki_Sweep]

As I can see PI detectors are more sencetive to proper shielding because of high voltage spikes at the moment of discharge of coil.
Such high voltage creates capacitive coupling to the ground and without shield you can get signals from wet or conductive environment even from some distance.
Also PI detectors are sensitive to the shield meterial. Foil slowing down the dacay and it is bad to detect fast decay targets like gold.

VLF probably can survive aluminum foil or other high conductive shields. I am not sure how much such shields reducing detecting range but there should be some drop in depth as well as phase shift probably.

[green]

[You say single layer on a hoop? Not sure what you mean but my DD is fine with foil]
A wood hoop 9.3 inch dia, .5 inch wide, 1/4 inch thick. One layer, 20 wraps awg 28 magnet wire around the outside. For playing it was easier than winding a spiral coil.

I recorded more data. Still 4 usec delay. First image is 1x1 and .5 x.5 inch aluminum foil. Second image is scope screen trace of amplifier out and coil fly back with different targets. Still getting some noise glitches from the circuit but the trend should be correct.

[Waikiki_Sweep]

[You say single layer on a hoop? Not sure what you mean but my DD is fine with foil]
A wood hoop 9.3 inch dia, .5 inch wide, 1/4 inch thick. One layer, 20 wraps awg 28 magnet wire around the outside. For playing it was easier than winding a spiral coil.

I recorded more data. Still 4 usec delay. First image is 1x1 and .5 x.5 inch aluminum foil. Second image is scope screen trace of amplifier out and coil fly back with different targets. Still getting some noise glitches from the circuit but the trend should be correct.

Interesting that 12x12mm (0.5 in) piece is detectable. Probably 8x8mm piece will be invisible for this detector.
How about unshielded coil? Is there any difference?
What wire you have used to make a coil? solid or stranded? What cable used to connect coil solid or stranded?

Also on second picture it looks like coil is ringing too much.

[green]

Interesting that 12x12mm (0.5 in) piece is detectable. Probably 8x8mm piece will be invisible for this detector.
How about unshielded coil? Is there any difference?
What wire you have used to make a coil? solid or stranded? What cable used to connect coil solid or stranded?

Also on second picture it looks like coil is ringing too much.

I'm not happy with the mono coil amplifier. Been playing with difference amplifier lately for IB coils, maybe DD or big foot type. Used the mono amplifier for shield not shielded comparison. Had a lot of oscillation with unshielded coil, didn't try but not sure if I could detect the .5 x.5 inch piece with the scope. Wire is awg 28 solid magnet wire. Using awg 28 stranded twisted pair to connect coil to circuit. Haven't figured out why the overshoot. Not sure it matters as long as it repeats, just looking for a change. I have a fluorescent bulb in the magnifier lamp on my bench. I think the shielded coil is a lot less noisy. Will try to record noise with and without shield when done playing with it shielded.

[Waikiki_Sweep]

I'm not happy with the mono coil amplifier. Been playing with difference amplifier lately for IB coils, maybe DD or big foot type. Used the mono amplifier for shield not shielded comparison. Had a lot of oscillation with unshielded coil, didn't try but not sure if I could detect the .5 x.5 inch piece with the scope. Wire is awg 28 solid magnet wire. Using awg 28 stranded twisted pair to connect coil to circuit. Haven't figured out why the overshoot. Not sure it matters as long as it repeats, just looking for a change. I have a fluorescent bulb in the magnifier lamp on my bench. I think the shielded coil is a lot less noisy. Will try to record noise with and without shield when done playing with it shielded.

If you worried for EMI noise you can build "8" shape coil for tests. It will cancel all EMI noises from any direction. Sometimes I have to find proper position for my ordinary monocoil on the bench to avoid to receive all radio and TV stations in the city, not only fluorescent bulb and cars ignition.
Try awg 28 stranded insulated wire for the coil. Magnet wire is good target also thin lacquer insulation on magnet wire perfectly builds capacitors in places where wires touching each other.
Many expensive PI MD using stranded insulated wire for the coil
For example Garrett Inginum LS:

[baum7154]

If you worried for EMI noise you can build "8" shape coil for tests. It will cancel all EMI noises from any direction. Sometimes I have to find proper position for my ordinary monocoil on the bench to avoid to receive all radio and TV stations in the city, not only fluorescent bulb and cars ignition.
Try awg 28 stranded insulated wire for the coil. Magnet wire is good target also thin lacquer insulation on magnet wire perfectly builds capacitors in places where wires touching each other.
Many expensive PI MD using stranded insulated wire for the coil
For example Garrett Inginum LS: [ATTACH]29083[/ATTACH]

I have had only mediocre performance in coils wound with enamel insulated solid conductor magnet wire…speed always suffers.

There is a significant difference in using either PTFE or TEFLON insulated stranded wire and it is even better to use the 600V rated insulated teflon…thicker insulation is better. I wound a coil a few years ago with 26 AWG stranded PTFE 250 volt insulation and it does not perform nearly as well as the 600 Volt stuff. I just received some 26AWG and 28 AWG stranded wire in 600V PTFE insulation. Will be using it to wind an elliptical spider coil. Not sure if the 28AWG will allow enough current for best detection. Will post elliptical coil performance in the coils forum.

Regards,

Dan

[Waikiki_Sweep]

I have had only mediocre performance in coils wound with enamel insulated solid conductor magnet wire…speed always suffers.

There is a significant difference in using either PTFE or TEFLON insulated stranded wire and it is even better to use the 600V rated insulated teflon…thicker insulation is better. I wound a coil a few years ago with 26 AWG stranded PTFE 250 volt insulation and it does not perform nearly as well as the 600 Volt stuff. I just received some 26AWG and 28 AWG stranded wire in 600V PTFE insulation. Will be using it to wind an elliptical spider coil. Not sure if the 28AWG will allow enough current for best detection. Will post elliptical coil performance in the coils forum.

Regards,

Dan

You are right about performance of insulated wires.

I am using AWG 24 in all my PI coils which is near 0.5 mm diameter and it conducts current much better than AWG 26 OR 28.

I told about AWG 28 stranded WIRE only because Green already using AWG 28 soild wire. Anyway stranded wire is better than solid.

But also I have one more observation - smaller diameter wire creates less inter-wire capacitance. May be some day we will go that direction by increasing power supply voltage with voltage converter to keep current the same.

Also you are right about PTFE. It has low dielectric constant. Teflon is one of the best dielectrics to create low capacitance coil.
Once I saw on sale 24 AWG PTFE wire with thicker insulation that has 1000V rate.
That 1000V is better for sure because coil will have even less inter-wire capacitance.

[mykaitch]

I am not sure that you need to put so much metal into metal detector coil because that cooper tape and aluminum foil acting like targets shifting phases in VLF and prolonging decays in PI detectors.

Yeah, should do. To be honest it was a FUBAR. The Alu yape ket snapping so I got mad and covered the lot in copper - still works great but if I do another one I now have some mylar backed Alu. Waiting for a Garret spider cover to arrive for my next concentric

[baum7154]

You are right about performance of insulated wires.

I am using AWG 24 in all my PI coils which is near 0.5 mm diameter and it conducts current much better than AWG 26 OR 28.

I told about AWG 28 stranded WIRE only because Green already using AWG 28 soild wire. Anyway stranded wire is better than solid.

But also I have one more observation - smaller diameter wire creates less inter-wire capacitance. May be some day we will go that direction by increasing power supply voltage with voltage converter to keep current the same.

Also you are right about PTFE. It has low dielectric constant. Teflon is one of the best dielectrics to create low capacitance coil.
Once I saw on sale 24 AWG PTFE wire with thicker insulation that has 1000V rate.
That 1000V is better for sure because coil will have even less inter-wire capacitance.

Yes, the reason I did order 26AWG and 28AWG is precisely because it gives much lower inter wire capacitance and I want to see how far I can push down the wire size without sacrificing detector performance. Also the silver plating on the smaller wire size will have less eddy current effect, though I have not seen a problem with the 24 AWG silver plated wire. Maybe it is an issue with larger wire sizes (18 to 22) but I have no need for those with my present builds.

Dan

[green]

Not suggesting aluminum foil is the way to go. A plot showing why I think it didn't change the 9 inch coil sensitivity. First image is a 2x2 inch piece of aluminum foil not cut and cut in a pattern shown in second image The cut foil has a TC of .4usec vs 3usec for the not cut.

[Waikiki_Sweep]

Not suggesting aluminum foil is the way to go. A plot showing why I think it didn't change the 9 inch coil sensitivity. First image is a 2x2 inch piece of aluminum foil not cut and cut in a pattern shown in second image The cut foil has a TC of .4usec vs 3usec for the not cut.

Cutting is a great idea!!!! Thank you!!! Should be used widely in such construction with different shield meterials.

I am not suggesting aluminum too also because of corrosion of aluminum and bad contact to cooper wires.