turns comparison

[green]

Wondering if I've been using ideal number of turns for my coils. Tried a spice analysis with Tx 20 and 25 turns. Not much difference in signal strength. 20 turns higher Tx current. Does the spice analysis make sense? Would I get a similar result if I made and tested the two coils?

[lucifer]

Wondering if I've been using ideal number of turns for my coils. Tried a spice analysis with Tx 20 and 25 turns. Not much difference in signal strength. 20 turns higher Tx current. Does the spice analysis make sense? Would I get a similar result if I made and tested the two coils?

Depends what "ideal" is for you. If we consider monocoils with the same wire size, more turns means slightly better sensitivity but at the same time increased coil resistance and less current through the coil. Less number of turns gives you more current through the coil and less parasitic capacitance hence a faster coil but less sensitivity. For this reason detectors have separate Tx and Rx coils.

[Op04]

I've tried it, triple turns get double sensitivity... almost forget, not the same diameter (also three times smaller)

[Carl-NC]

Assuming a mono coil design, if there is no wire resistance then it's a wash. When you double the turns the TX ampere-turns drop by half but the RX induced EMF doubles. However, when wire resistance is included then more turns helps the RX faster than it hurts the TX. However... more turns also hurts the SRF and therefore you can't sample as early, and therefore you lose signal gain. Also, more turns decreases the SNR due to EMI because you are trying to use add'l turns to compensate for a weaker TX so while you are keeping the target signal about the same you are meanwhile boosting the EMI signal.

[bbsailor]

Assuming a mono coil design, if there is no wire resistance then it's a wash. When you double the turns the TX ampere-turns drop by half but the RX induced EMF doubles. However, when wire resistance is included then more turns helps the RX faster than it hurts the TX. However... more turns also hurts the SRF and therefore you can't sample as early, and therefore you lose signal gain. Also, more turns decreases the SNR due to EMI because you are trying to use add'l turns to compensate for a weaker TX so while you are keeping the target signal about the same you are meanwhile boosting the EMI signal.

Carl,

The skin effect of solid wire may hold enough eddy currents to be detected as a target at low delays. Stranded wire tends to reduce this effect but Litz wire with insulated strands reduces the skin effect the most. The main point is that coil builders need to ensure that the wire they use for the coil will not hold any eddy currents longer than the lowest delay they use to find their desired targets. Is this a valid point worth discussing?

Joseph J. Rogowski

[Carl-NC]

The skin effect of solid wire may hold enough eddy currents to be detected as a target at low delays. Stranded wire tends to reduce this effect but Litz wire with insulated strands reduces the skin effect the most. The main point is that coil builders need to ensure that the wire they use for the coil will not hold any eddy currents longer than the lowest delay they use to find their desired targets. Is this a valid point worth discussing?

Yes, that's all true but is independent of green's question. It seems that most gold PIs have settled on 300uH. Why is that? Will performance improve with more turns? Fewer turns? It turns out for a mono coil system 300uH is about right for sampling at 8-10us.

Once you've decided the diameter & turns, the next question is wire gauge & wire type. This can get even more important if you decide to go for fewer turns as that translates into a higher peak current and a need for a heavier wire gauge. Then Litz becomes more important.

Somewhere there is metal detector project that uses a 1-turn coil made of copper tubing as I recall. I think it was a BFO, but you could design a 1-turn-coil PI detector. Probably wicked fast.

[lucifer]

One should not forget the coil cable. It's often ignored but affects the coil performance, especially fast coils because it adds not only resistance but capacitance as well.
And for really fast coils, the preamp might become the bottleneck as a huge gain bandwidth product would be required.

[green]

One should not forget the coil cable. It's often ignored but affects the coil performance, especially fast coils because it adds not only resistance but capacitance as well.
And for really fast coils, the preamp might become the bottleneck as a huge gain bandwidth product would be required.

Agree. Cable capacitance is higher than coil capacitance for the coils I build.

[dbanner]

I always think of the coil itself and the coil wires going all the way to the PCB, as being part of the coil parameters, taken as a whole, includes solder joints too.
So the coil really ends where the wires enter the PCB.
So cable is part of coil actually, so LCR of cable is impacting.

[Melbeta]

Carl, I once had a friend, was Chief engineer at Tranex, they made transformers, he made his own coils, all sizes, I never made one coil myself. He told me he hooked coil up to detector, began winding the coil, when the audio sound came in, he stopped winding, added some additional wire for the shaft winding up the case. Does that make sense to you what he told me? Is it better his way? Or another way?
Melbeta

[Skippy]

Quote:"Somewhere there is a metal detector project that uses a 1-turn coil made of copper tubing"

Not forgetting several VLF two-box units, that have a single turn of thick wire for each coil, and they couple via a step-up (RX) or step-down (TX) transformer to more conventional oscillator / pre-amp circuitry.

( I still plan on home-building a two-box one day. Quite recently, I salvaged a 4 metre length of lightning-conductor wire from a construction site skip ( dumpster ). It's electrical-grade aluminium, about 4mm x 22mm, with PVC insulation. It seemed a good candidate for these single-turn coils. )

[Carl-NC]

Carl, I once had a friend, was Chief engineer at Tranex, they made transformers, he made his own coils, all sizes, I never made one coil myself. He told me he hooked coil up to detector, began winding the coil, when the audio sound came in, he stopped winding, added some additional wire for the shaft winding up the case. Does that make sense to you what he told me? Is it better his way? Or another way?
Melbeta

Maybe for a BFO detector.

Quote:"Somewhere there is a metal detector project that uses a 1-turn coil made of copper tubing"

Not forgetting several VLF two-box units, that have a single turn of thick wire for each coil, and they couple via a step-up (RX) or step-down (TX) transformer to more conventional oscillator / pre-amp circuitry.

This is how the White's TM-808 works. The aluminum rod coil is inserted through a toroid with windings. But I vaguely recall that there was once a BFO or a PI project in a magazine article with a copper tubing coil that was direct-drive.

[Qiaozhi]

But I vaguely recall that there was once a BFO or a PI project in a magazine article with a copper tubing coil that was direct-drive.

Today, while searching for something else, I stumbled across a different single-turn BFO design in Charles D. Rakes' book: "Building Metal Locators, A Treasure Hunter's Project Book" - 1986 - ISBN 0-8306-0506-1. The project starts on page 9 and initially uses a 9" diameter former with 0.25" diameter copper tube for the coil. Then on page 18 there is a second design for a large loop BFO locator with a 24" diameter search coil, also made with copper tubing. This second design is claimed to be able to detect an automobile above ground at a distance of 6 feet.

[Carl-NC]

Must be where I saw it. I have that book, but it's at my office in Oregon.

[Melbeta]

Some years ago, I got a Wire Table chart, and it had varsious things on it, I guess for importance. One of two things that struck me was, one was OHMS per 1,000 feet, at 20 degrees Centigrade.

The second was, CURRENT CARRYING CAPACITY at 7000 C.M. per Amp. So I am going to ask this question. Is one, or both of these, or just one of these of what I call of PRIMARY IMPORTANCE??? Now it also has an area, describing TURNS per Linear Inch (25.4 MM). It also rates the wire, as A.W.G. with (B&S) in brackets.

Is the OHMS more important then Current???

Any explanations that I should know about anything as far as winding the coil???
Melbeta