A couple coil experiments What do you guys think?

[Chet]

digit sent me three coils to compare with a couple of mine. SRF was measured be holding a open test coil above a operating PI coil and measuring the resonance with a oscilloscope, probe held near test coil. Inductance was calculated_ soldered a .01uf capacitor across the coil and measured resonance the same as SRF. Inductance=1/4pi^2/.00000001/resonance^2. Not a lot of difference except at 50mm distance. Don't know why his winding would be higher. Had 2mm foam spacer, graphite painted on side away from coil top and bottom for a shield.

Hi Green


Great charts as usual. I think the higher amplitude measurements at 50 mm distance for coils 1, 2 and 3 are because of the smaller inter diameters. The 20 mm target is facing a larger percentage of the magnetic lines of force within the smaller concentrated areas of these three coils.


As the target distance increases to 300 mm coils 4 and 5 show an improvement over coils 1, 2 and 3. I believe this is because the average mean diameter of coils 4 and 5 is larger and provides increased depth as expected as coil diameter increases. Of course this only works as long as the target is large enough to be detected. Which in this case the 20 mm target is large enough.


Since Digit is a nugget hunter another test might be conducted with a smaller target such as a 6 mm or 10 mm from the bottom of a coke can. I think the bottom of the can may be closer to the average thickness of small nuggets.


Coils 1, 2 and 3 are somewhat shelf-shielding by the nature of how they are wound. I have 200 mm wound the same way. For me the self-shielding hasn’t worked as well as conductive paint. The self-shielding coil seems to need more separation from the coil windings to the outside coil housing surface to be effective. This can be provided by foam or material wrapped around the coil.


Since you already have some graphite painted shields another test would be to compare the self-shielding coils with graphite shielded coils. This could be accomplished by conducting your coil self-resonance frequency test with the coil in the air versus against or near a bag of clay or soil. The amount of frequency shift could be calculated into how much ground capacitance effect was encountered by each coil.


I am surprised at how well the vertical wound coil #5 performed in the self-resonant frequency test. Are the windings spaced apart from each other?


Thank you again for all of the testing that you share with us.
Have a great day,
Chet

[green]

Hi Green


Great charts as usual. I think the higher amplitude measurements at 50 mm distance for coils 1, 2 and 3 are because of the smaller inter diameters. The 20 mm target is facing a larger percentage of the magnetic lines of force within the smaller concentrated areas of these three coils.


As the target distance increases to 300 mm coils 4 and 5 show an improvement over coils 1, 2 and 3. I believe this is because the average mean diameter of coils 4 and 5 is larger and provides increased depth as expected as coil diameter increases. Of course this only works as long as the target is large enough to be detected. Which in this case the 20 mm target is large enough.


Since Digit is a nugget hunter another test might be conducted with a smaller target such as a 6 mm or 10 mm from the bottom of a coke can. I think the bottom of the can may be closer to the average thickness of small nuggets.


Coils 1, 2 and 3 are somewhat shelf-shielding by the nature of how they are wound. I have 200 mm wound the same way. For me the self-shielding hasn’t worked as well as conductive paint. The self-shielding coil seems to need more separation from the coil windings to the outside coil housing surface to be effective. This can be provided by foam or material wrapped around the coil.


Since you already have some graphite painted shields another test would be to compare the self-shielding coils with graphite shielded coils. This could be accomplished by conducting your coil self-resonance frequency test with the coil in the air versus against or near a bag of clay or soil. The amount of frequency shift could be calculated into how much ground capacitance effect was encountered by each coil.


I am surprised at how well the vertical wound coil #5 performed in the self-resonant frequency test. Are the windings spaced apart from each other?


Thank you again for all of the testing that you share with us.
Have a great day,
Chet

Hi Chet
Think your right about the diameter. Charted the #2 and #4 coil with the 6x6mm can bottom. I tried the resonance with coil in air and near a plastic bag with the California clay. Looked like there was a small change but scope resolution wasn't good enough to say how much. The #4 coil changed about 40 counts when I brought my hand near the coil without the shield and zero counts or close to it with the graphite shield above and below the coil. #2 coil changed about 40 counts wired one way and maybe 30 counts with the wires reversed. The shield above and below helped a lot but didn't get the change to zero. Probable due to coil thickness and inner diameter of the shield rings was just slightly smaller than the coil I.D. #5 coil, there was some small spacing not on purpose. Tried tapping the wires down spaced as close as I could. SRF dropped, 3.73MHz to 3.67MHz. Added two turns, inductance increased, 256uH to 310uH, SRF 3.05MHz.

[Chet]

Hi Chet
Think your right about the diameter. Charted the #2 and #4 coil with the 6x6mm can bottom. I tried the resonance with coil in air and near a plastic bag with the California clay. Looked like there was a small change but scope resolution wasn't good enough to say how much. The #4 coil changed about 40 counts when I brought my hand near the coil without the shield and zero counts or close to it with the graphite shield above and below the coil. #2 coil changed about 40 counts wired one way and maybe 30 counts with the wires reversed. The shield above and below helped a lot but didn't get the change to zero. Probable due to coil thickness and inner diameter of the shield rings was just slightly smaller than the coil I.D. #5 coil, there was some small spacing not on purpose. Tried tapping the wires down spaced as close as I could. SRF dropped, 3.73MHz to 3.67MHz. Added two turns, inductance increased, 256uH to 310uH, SRF 3.05MHz.

Hi Green


The 6x6 mm appears to be a good test for this size coil. The self-resonant frequency / shield tests seem to reinforce my experience with the two different ways of shielding.


The #5 coil is working so well that I might try to wind a similar one in the future. Adding uniform spacing between the wires would reduce the self-capacitance even more. The increased height of the vertical winding should not be much of a problem.


Thank you again especially for such a fast turnaround on the testing and results.
Have a great day,
Chet

[godigit1]

Hi Green,
Great job on the graphs. Thank you for putting my coils to the test.
The yellow coil #2 was the one that teated the best on my barracuda. Giving me almost two extra inches. I now think That Dan may have hit it in his posts about critical damping. I set my damping to another 300 uh coil and was rolling with it for all my 300 uh area coils. That one may have had better attributes for the barracuda.
When testing on my TDI #2 was still the best of my coils and barely beat by a 3dss 8" coil with 300 uh made from the stranded 26 awg. and only because I could get the threshold and gain little higher with the self shielding coil.
All that said they seem to hold up pretty well depth wise against the larger coils. As 6666 has said maybe too complicated for any good use in comparison.

Also Thank you Chet for your insight and knowledge your evaluation is spot on. Ive been trying to reduce capacitance with these coils but think that a properly spaced vertical stack is probably the way to go and then using the paper and carbon shielding. I already have a way to do it with a tooth pick size spacing between all wraps and layers. I will keep trying these formless coils in that manner see what I can come up with.

P.S
My measurements on the coke can were redone with pressure on the calipers and I now get .0035 on the peligrino can and .0045 on the coke can Still a .001 difference with the coke can. Must be the area I cut from.
Thanks again everyone!
Jim

[green]

[The #5 coil is working so well that I might try to wind a similar one in the future. Adding uniform spacing between the wires would reduce the self-capacitance even more. The increased height of the vertical winding should not be much of a problem.]

Wondering if the spacing is needed. The #5 coil, vertical winding, 200mm diameter, 25 turns, 310uH measured 3.05MHz SRF. Calculated capacitance 8.8pf, less than a foot of lead wire. I have been thinking circuit resonance(no damping resistor) 1MHz or above would be good enough. 84pf total. A couple of other threads are detecting short TC targets with coils that have to have a lot lower circuit resonance than 1MHz. I'm not understanding it.

[Chet]

I think additional transmit gain from adding 5 or more turns and additional receive gain from adding 5 or more turns would be worth the extra spacing to reduce the capacitance. As long as we keep the overall self-resonant frequency near 1 MHz I think the coil will be fast enough.


Have a good day,
Chet

[green]

I think additional transmit gain from adding 5 or more turns and additional receive gain from adding 5 or more turns would be worth the extra spacing to reduce the capacitance. As long as we keep the overall self-resonant frequency near 1 MHz I think the coil will be fast enough.


Have a good day,
Chet

I've been using AWG28 enamel coated wire. Maybe Teflon coated would give enough spacing.

[Chet]

I've been using AWG28 enamel coated wire. Maybe Teflon coated would give enough spacing.

Teflon coating may work if the same gauge or smaller wire is used. Larger copper surface area will increase the capacitance. If we estimate (wild guess) 0.5pf per turn; then 50 turns would be approximately 25pf. If we can reduce the cable capacitance and keep the MOSFET capacitance low this should make a good gain in signal strength.
Have a good day,
Chet

[baum7154]

Hi all, interesting results in the coil tests. I've been away for a week and have been catching up on the coil results. Do I understand that the #5 coil is Graphite Shielded inside and outside in sandwich fashion? If so does it show any influence from approaching the ground from the unshielded gap in the sandwich?

As far as adding additional turns it would be good to know how many turns would get the coil to about 320uh. I think this is a good value for sensitivity and speed. Perhaps 3 turns would do it. My 328uh Racetrack coil has 28 turns of 24 awg 600V PTFE stranded wire. It will operate at 6.98us sample delay but my detector wont sample any sooner so I don't know where it actually stops working.

The difference between 250V and 600V PTFE gives significant improvement to the coil speed.

Regards,

Dan

[green]

Hi all, interesting results in the coil tests. I've been away for a week and have been catching up on the coil results. Do I understand that the #5 coil is Graphite Shielded inside and outside in sandwich fashion? If so does it show any influence from approaching the ground from the unshielded gap in the sandwich?

As far as adding additional turns it would be good to know how many turns would get the coil to about 320uh. I think this is a good value for sensitivity and speed. Perhaps 3 turns would do it. My 328uh Racetrack coil has 28 turns of 24 awg 600V PTFE stranded wire. It will operate at 6.98us sample delay but my detector wont sample any sooner so I don't know where it actually stops working.

The difference between 250V and 600V PTFE gives significant improvement to the coil speed.

Regards,

Dan

http://www.geotech1.com/forums/showt...255#post214255

Hi Dan, Link above describes coil test. The shield is just above and below the coils. Trying to see if the way the coil is wound effects signal strength. Or is it just diameter if inductance and Tx signal are the same for all tests. Need to wind a 200mm mean diameter spider web(flat basket) and a 200mm vertical single layer with full shielding and check resonance and amplitude vs distance. For me it's finding a way to use the spool of awg28 magnet wire I have and learn something about coils.

[baum7154]

Thanks Green,
Some how I thought you had a vertical wound coil with vertical shield rings inside and outside of the coil ring.

Dan