spiral pi coil

[godigit1]

Reason I think zero spacing is best for spiral coil(flat spiral.jpg)capacitance is less than 10pf. My first attempt at a spiral coil wasn't good so tried a flat basket which worked. Tried a flat spiral again awhile back, success this time. Not sure which is better.

Picture of the 8inch figure eight coil I'm using, COIL Tx(oval) Rx 2(8inrd).jpg. Both Rx coils are wound the same way, put next to each other(one not inverted), outer wires connected. Inner coil wires used for Rx coil.

Not saying it's best way or even a good way to make a coil. Seems to give best detection distance of the things I've tried. Interested in what others are trying to see if there is a better way.

Figure8 Rx probably needs absolute valve circuit since each Rx coil gives opposite polarity signal?

I was having a hard time picturing it . thanks for the pictures.
It is like a big foot configuration. Nice job looks really clean.
When you say non inverted do you mean both rx coils flow the same direction not like a figure 8?
I did a dod a while back and it worked best with both coils in phase, though I think its supposed to be reversed to get the ground canceling benefit.
What kind of sensativity are you getting for this large a coil and where can you first sample?

[godigit1]

Thanks for the SRF. Calculates 74pf with 370uH. Lot more than 10pf I get with zero spacing. Have seen spacing suggested for a spiral coil in quite a few threads. Haven't seen an example where spacing caused a lower SRF(less capacitance). Maybe another reason why spacing would make a better spiral coil?

I read somewhere three times the wire diameter for spacing a flat spiral.
Reading up on transformers and Im not sure if i go this totally right or not.
more spacing makes for more capacitance. depending on coating dialectric value
My coil should have had a higher inductance i was thinking too much spacing was reducing the inductance of the coil.
thinking too much spacing and you loose field strength.
Tonight i will wind a bundle with the same amount of wire to see if I was just way off on the calculator or if the spacing had something to do with it.

[green]

I was having a hard time picturing it . thanks for the pictures.
It is like a big foot configuration. Nice job looks really clean.
When you say non inverted do you mean both rx coils flow the same direction not like a figure 8?
I did a dod a while back and it worked best with both coils in phase, though I think its supposed to be reversed to get the ground canceling benefit.
What kind of sensativity are you getting for this large a coil and where can you first sample?

The coils aren't flipped, connecting the outer winding on one to the outer winding of the other reverses the current in one of the coils. Center tap of coil is on outside of coil. Thought it might make a difference, don't know if it does.

I tried a DOD(looks similar to 7000), same as you, worked best connected in phase. 7000 isn't a PI, maybe operates different?

Can sample at 6us delay. Don't have a working detector at the moment, could detect a US nickel over 17 inches with 1A peak current.

[godigit1]

Your coil has very good depth on the nickle and is fast. I would think it would do pretty good for its size on small nuggets.
If I remember correctly you were using your Chance PI?
Thanks for the info i may try a coil like this as well.


I wound a bundle with the same diameter loop as the figure 8 spiral, works out to a 9 inch coil.
Calculated inductance was 480uh.
The bundle coil came out 540 uh at 7.3 ohms resistance.
The figure 8 came out 199uh at 7.2 ohms.
I was expecting some differance but thats a huge amount.
Im thinking way too much spacing for the 8 coil not enough for the bundle.
I still have to hook them up get srf to figure out capacitance.
Now to figure out proper spacing before building such a busy coil again.



Thanks again

[sinclairuser]

21ohms does seem high.
If I got the individual strand size correct .1mm is equivalent to 38 awg.
5 strands is equal to 31 awg finished wire size.
If I calculate for a 8 inch coil 60 feet of line 18.28 meters.
the resistance is .432794ohm/m for 5 strands 38 awg.
So Im getting 7.91 ohms or there about. you have three times what I calculate I must be way off on the wire specs or something.
Is this wire individually insulated like litz? maybe some strands did not solder well.

no mate, the strands are not individually insulated the strands are twisted together with a plastic insulation covering them, i'm going to re-wind this coil identically but with magnet wire this time, i will then post the uh, ohms and srf specs of that coil.
aly.

[green]

If I remember correctly you were using your Chance PI? Link to my detector https://www.geotech1.com/forums/show...571#post225571

[godigit1]

no mate, the strands are not individually insulated the strands are twisted together with a plastic insulation covering them, i'm going to re-wind this coil identically but with magnet wire this time, i will then post the uh, ohms and srf specs of that coil.
aly.

I know your over that coil wire but something had to be wrong. Sounds like your solder joints are all good. So my problem solving torturing my self mind tells me there are possible strand breaks within one of the wires.
Looked at it again and to get that resistance it is equal to one strand 34 awg =.855974ohm/m.
Just saying
I think you will have better luck with the magnet wire especially if you added a spacing between wraps.
I just did the srf and capacitance of the spaced eight and bundle and for the same amount of wire the capacitance differance is huge.
Coil 1
9 Id round "
25 wraps 30 awg
7.3 ohm
540 uh
588.2 khz Srf
135.67pf with 3 foot leads

#2
figure eight plane with a wrap that keeps the loop size the same diameter as a 9" loop
25 wraps 30 awg
7.2 ohms
199uh
1.786Mhz
35 .67pf with 3 foot leads

You can see the huge differance in the same wire though i think my eight has too much spacing for this guage wire by the low inductance.

teflon mega tape makes a nice wrap if you twist it, its 40 times thicker than regular plumbers tape Ive even used it to wrap ESC' and parts to quad copters for testing as well as coil wrap.

Best of luck with the next wind Ive had a few redoos my self lately .

[godigit1]

If I remember correctly you were using your Chance PI? Link to my detector https://www.geotech1.com/forums/show...571#post225571

Ok I did not remember correctly what else is new LoL.
Hope you get it working again?

[green]

I know your over that coil wire but something had to be wrong. Sounds like your solder joints are all good. So my problem solving torturing my self mind tells me there are possible strand breaks within one of the wires.
Looked at it again and to get that resistance it is equal to one strand 34 awg =.855974ohm/m.
Just saying
I think you will have better luck with the magnet wire especially if you added a spacing between wraps.
I just did the srf and capacitance of the spaced eight and bundle and for the same amount of wire the capacitance differance is huge.
Coil 1
9 Id round "
25 wraps 30 awg
7.3 ohm
540 uh
588.2 khz Srf
135.67pf with 3 foot leads

#2
figure eight plane with a wrap that keeps the loop size the same diameter as a 9" loop
25 wraps 30 awg
7.2 ohms
199uh
1.786Mhz
35 .67pf with 3 foot leads

You can see the huge differance in the same wire though i think my eight has too much spacing for this guage wire by the low inductance.

teflon mega tape makes a nice wrap if you twist it, its 40 times thicker than regular plumbers tape Ive even used it to wrap ESC' and parts to quad copters for testing as well as coil wrap.

Best of luck with the next wind Ive had a few redoos my self lately .

I think you will have better luck with the magnet wire especially if you added a spacing between wraps.

Why do you think adding space between the wraps would make it better? I think spacing reduces inductance and probably increases capacitance especially if enough turns are added for the same inductance?

Are your 3 foot leads twisted? I get about 1pf/inch when I use AWG24 twisted pair for lead. I wound a bundle coil awhile back with AWG 28magnet wire, inductance very close to calculated using the coil calculator. Calculated inductance winding the same coil with Teflon coated wire calulates about the same inductance I get winding the spiral coil using magnet wire with no spacing.

[godigit1]

"Why do you think adding space between the wraps would make it better? I think spacing reduces inductance and probably increases capacitance especially if enough turns are added for the same inductance?"

I may be totally wrong, what you say holds true in what I read in transformer windings.

But here is kinda where im coming from.
Many coil builders here say to use the thickest teflon 600v insulation on coils like the 3dss types. The 3d coil has very large spacing between layers and where the wires come together cross at close to 90 degrees all of this in a effort to reduce capacitance speed up the coil.
But thats not it it depends on coatind dialectric as well.
Magnet wire has a very thin coating, yes it has a good dialectric value but that actually helps its capacitive capability.
you have two coating thickness between the wires. that is a stable capacitance between wires.
Now you start increasing spacing capacitance starts rising to a point where you loose proximity effect or field strength I think.
Your right also as the spacing increases you loose field strength or inductance.
So here it is I think there is a point where the spacing reduces capacitance while reducing inductance as well but you can use a wire mass that say would carry 500uh inductance take it to 300uh while reducing capacitance. would this be of real benefit I guess I have to find out. Maybe its just better to use less wire.
My two recent coils I think are a good example I think of both extremes.
Your bundle came out close to calculated value. You probably built a real bundle coil, the one I built was I guess not really a bundle its wound on a spool and i wound it pretty tight. That may have increased capacitance and inductance
There is almost the exact same amount of wire between the two coils and there is over a 100pf more capacitance just using the magnet wire coating as spacing.
Ok so I have to go the other way now and take the spool down to 300 uh and see what the trade off is between inductance and capacitance. or add spacing every five wraps.
Ive said Im trying to learn and everytime I think I got something something contradicts this I think that comes down to the old saying there is always a trade off.
I understand the 500 uh coil is going to have a stronger field strength but the capacitance slows it down?
Plus I have yet to hook either of these coils up I dont really expect good performance out of 200uh I may add another coil to that coil. try and save it.

The leads were not twisted on these coils when I tested them should I retest that way?

Thanks for asking me why beacuse Im still trying to figure it all out.
Please feel free to continue to correct me its appreciated.
More people in this world need to ask "why?" and offer constructive critism and it needs to be taken constructively.

[green]

"Why do you think adding space between the wraps would make it better? I think spacing reduces inductance and probably increases capacitance especially if enough turns are added for the same inductance?"

I may be totally wrong, what you say holds true in what I read in transformer windings.

But here is kinda where im coming from.
Many coil builders here say to use the thickest teflon 600v insulation on coils like the 3dss types. The 3d coil has very large spacing between layers and where the wires come together cross at close to 90 degrees all of this in a effort to reduce capacitance speed up the coil.
But thats not it it depends on coatind dialectric as well.
Magnet wire has a very thin coating, yes it has a good dialectric value but that actually helps its capacitive capability.
you have two coating thickness between the wires. that is a stable capacitance between wires.
Now you start increasing spacing capacitance starts rising to a point where you loose proximity effect or field strength I think.
Your right also as the spacing increases you loose field strength or inductance.
So here it is I think there is a point where the spacing reduces capacitance while reducing inductance as well but you can use a wire mass that say would carry 500uh inductance take it to 300uh while reducing capacitance. would this be of real benefit I guess I have to find out. Maybe its just better to use less wire.
My two recent coils I think are a good example I think of both extremes.
Your bundle came out close to calculated value. You probably built a real bundle coil, the one I built was I guess not really a bundle its wound on a spool and i wound it pretty tight. That may have increased capacitance and inductance
There is almost the exact same amount of wire between the two coils and there is over a 100pf more capacitance just using the magnet wire coating as spacing.
Ok so I have to go the other way now and take the spool down to 300 uh and see what the trade off is between inductance and capacitance. or add spacing every five wraps.
Ive said Im trying to learn and everytime I think I got something something contradicts this I think that comes down to the old saying there is always a trade off.
I understand the 500 uh coil is going to have a stronger field strength but the capacitance slows it down?
Plus I have yet to hook either of these coils up I dont really expect good performance out of 200uh I may add another coil to that coil. try and save it.

The leads were not twisted on these coils when I tested them should I retest that way?

Thanks for asking me why beacuse Im still trying to figure it all out.
Please feel free to continue to correct me its appreciated.
More people in this world need to ask "why?" and offer constructive critism and it needs to be taken constructively.

First I'm just learning. I'm not saying you are wrong. Just saying we are not thinking the same. Probably means at least one of us wrong. One thing worse than being wrong is not knowing it. Helps if someone corrects or suggests something different.

The leads were not twisted on these coils when I tested them should I retest that way?

Just did a test with a shielded mono coil I have. 31inch twisted pair lead, 100nf capacitor across coil(SRF 29760kHz, 286uH) 31inch twisted pair lead, no capacitor(SRF 1.563MHz, 36pf) 31inch lead not twisted, no capacitor(2.1MHz, 20pf) no lead, no capacitor(2.56MHz, 14pf) The lead not twisted did add some capacitance, not as much as twisted. Probably should state if no lead, or with lead and what type of lead when giving SRF or coil capacitance.

[sinclairuser]

hi all, so i made another coil wound exactly the same as before only this time i used .5mm magnet wire,
8" flat spiral basket coil, 30 turns, 590uh, 4.3ohms, 3.5nf and srf of 1.1069hz.
that is as it stands i will probably remove some windings just to drop the uh a bit, there was definitely some thing amiss with the other one.
something i thought about was when using calculators for this type of coil, the uh always seemed low compared to what was expected for a given number of windings.
and i think there is actually more than 30, if you count them on one side its 30 but does the surface area of the 30 windings on the other side of the coil matter?.
is it 30 windings or more due to the windings on the other side of the coil form?.

[6666]

For interest
https://www.nuggetfinder.com.au/spiral-wound-coils

[green]

hi all, so i made another coil wound exactly the same as before only this time i used .5mm magnet wire,
8" flat spiral basket coil, 30 turns, 590uh, 4.3ohms, 3.5nf and srf of 1.1069hz.
that is as it stands i will probably remove some windings just to drop the uh a bit, there was definitely some thing amiss with the other one.
something i thought about was when using calculators for this type of coil, the uh always seemed low compared to what was expected for a given number of windings.
and i think there is actually more than 30, if you count them on one side its 30 but does the surface area of the 30 windings on the other side of the coil matter?.
is it 30 windings or more due to the windings on the other side of the coil form?.

Decimal point wrong place, frequency or capacitance?

[sinclairuser]

sorry green, 30 turns, 590uh, 3.5nf and srf of 110693.162 hz.
sorry about that, its not quite what a calculator says it should be(110754.17429501114), but in the ballpark.