Flat Speaker Wire Coil?

[FatBob]

I picked this up from a yard sale for a couple dollars. Has anyone ever tried making a PI coil from this stuff?

[ODM]

If coiled like the cable is in the package, it will create a coil with high parasitic capacitance. Coil windings need spacing between them and minimal shared surface area. Could work for a slower-coil PI detector of course.

[Davor]

I on the contrary believe this to have a potential for a decent spiral coil, and easily wound, just as it is wound on that reel. Point with spiral coils is that the capacitance adds up less so if the adjacent winding is the only thing that contributes it. You may observe it as each winding being a capacitor, and all those capacitors being connected in series.

[ODM]

Comparing coils with a cross section, a fraction of it is conductor and assume the rest to be insulator, a round conductor would have the lowest shared surface area and afford greatest spacing between windings. Capacitance is a function of surface area and inverse for distance, therefore highest d/a ratio would lead to lowest capacitance.

There are plenty of IF coils in RF engineering where zig-zag winding is used to increase self resonant frequency, where turns may freely rest beside turns that lag a whole lot. Is there a reason why flat winding coils would be a better alternative but rarely used, compared to round wire? When looking at HF coil wire types, why is flat litz braid rarely used?

Going to distributed model suggests operating at dimensions on ballpark with wavelength. However I have seen very few metal detector related publications refer to anything except the lumped model of a coil, I can't recall a single one that would present the search head as distributed model, but that could be sample bias on my behalf. Any examples I've missed?

[Davor]

No, you haven't.

Considering the length of a wire in, say, 300μH coil of under 30m, and λ/10 as length where transmission line behaviour kicks in, you have 1MHz as a frequency where it becomes notable. So for under 1MHz you may safely consider all parasitic effects lumped.

I'd expect a coil wound with flat cable like that to have high-ish inter winding capacitance, but sufficiently low total capacitance in a spiral configuration. This particular form may lend itself nicely for a differential coil. Perhaps not the fastest one around, but good in spiral configuration.

Otherwise, your reasoning is completely correct. Spiral coil is simply a special case.

[ODM]

What I'm interested in is the lack of examples in flat spiral coils. The only application I've seen is flyback ferrite cored transformers with a large ratio where copper and polyimide tape is used for high current windings, but that is not for SRF reasons.

This should be easily answered if FatBob has a scope or LCR meter, and some free time on their hands.

[green]

I picked this up from a yard sale for a couple dollars. Has anyone ever tried making a PI coil from this stuff?

[ATTACH]35618[/ATTACH]

[ATTACH]35619[/ATTACH]

I see 18ga, .5mm thick and solid speaker wire in the first picture. Can you describe the wire? Wire thickness with and without insulation.

[green]

chemelec has done a lot with spiral coils. I don't have the tools to make a jig so I have been winding spider web(flat basket) coils. Been doing some coil testing in the Litz coil thread. A vertical wind and vertical basket were close to the same so I'm going to guess a flat spiral and flat basket are close to the same also. chemlec gets high self resonance with the spiral coils. I would guess the flat 18ga solid would look like a target for a least the first 10usec. Don't know how much the flat wire would lower the self resonance. Thinking need more than 50 feet to get 300uH. Maybe split and splice together. Interested in what FatBob comes up with.

[kt315]

If coiled like the cable is in the package, it will create a coil with high parasitic capacitance. Coil windings need spacing between them and minimal shared surface area. Could work for a slower-coil PI detector of course.

use ordinary rope/thread winding it around the flat wire. diameter of the rope 3-4 mm is enough. wind the rope around and wind the
coil simultaneously. you will get the needed gap.

[ODM]

use ordinary rope/thread winding it around the flat wire. diameter of the rope 3-4 mm is enough. wind the rope around and wind the
coil simultaneously. you will get the needed gap.

Yes, or just get proper PTFE insulated wire in the first place for a more mechanically stable coil.

Do you know of any detectors that would use flat copper tape for fast coils instead of, for example, round litz wire? Low capacitance was the supposed advantage.

[green]

I would think the coil as wrapped in the package wouldn't be worth much but could lay the scope probe close to the test coil and excite it with a PI coil and measure the self resonance for some information.

[kt315]

Yes, or just get proper PTFE insulated wire in the first place for a more mechanically stable coil.

Do you know of any detectors that would use flat copper tape for fast coils instead of, for example, round litz wire? Low capacitance was the supposed advantage.

whites - not
tesoro - not
first texas - not
garrett - not
minelab - yes, round litz

[kt315]

http://savepic.org/1558887.htm

[kt315]

http://www.23hq.com/metaldetectorsse.../story/1674201

[kt315]

GP coil http://i43.tinypic.com/2nhorys.jpg