Coil Technology is no secret

I am a licenced Radio amateur and have studied electronics since I was 10 years of age, What I am writing here is not rocket science it is just basic electronics.
The first Question is what is a coil.
The Coil that hangs off your detector is nothing more than a coil of wire, but it has to be designed in a certain way so that it works at the right frequencies so that when metal upsets its inductive reactance it will be heard as a signal in your headphones created by the electronics in the detector.
A double "D" coil uses two coils of wire that overlap. A mono is just ,that a single coil of wire.
When You add a capacitor across a coil it becomes a tuned circuit, and the early model detectors, that came on to the Market years ago, were simply a tuned circuit, Which changed frequency when a metal object was detected within the range of the coil,
Coil Technology has not changed much over the years and I doubt if it can be improved on. It is true tha Lizt wire makes a coil work better but that is only because it is less affected by any sort of interference outside of the range of the operating frequency(ies) of the electronics .
However, because of the use of modern silicon chips and varicaps (or variable capacitors) it is now possible to create metal detectors that operate on multiple frequencies . Pulse induction ( Which Minelab Use) is not new it has been used for years in some ultrasonic cleaners etc. However it is new to Metal detectors. Seta Technology is different again but the signals are still controlled by the inductive reactance caused by metal passing within the range of the tuned circuit of the coil capacitor combination.
A coil must be made so that it is very rigid. Any movement in the coil itself will make a coil very noisy, because if you hit something with a loose wound coil or poorly designed coil it will change its inductive reactance , causind a change in frequency, which will be detected as noise.
Most of the coils today work very well when the inductance is around 26 millihenry.
I used to make my own simple detectors, and a trick I learned, when making coils was to wind a coil around a 12 inch plasic tube, and with every turn run a very small piece of lead within range of the coil to get the loudest signal at the furthest distance from the coil, and basically that is how coils are still made. coils are not rocket science, but if they are not well designed they will be noisy.
A coil must be very rigid to be stable, must use the best conductive wire available, and designed to work within the range of frequencies of the electronics of the detector. Some coils use a faraday shield, I never found it necessary.

thanks for the enlightenment billsymo. I always wondered what was in there. What sort of cap is it, like .047uf or similar?

Panther

there is no capacitor oranything else in the coil other than wire, and maybe a faraday shield which can be as simple as alfoil. All other components are part of the detector circuit.

Billsymo wrote:there is no capacitor oranything else in the coil other than wire, and maybe a faraday shield which can be as simple as alfoil. All other components are part of the detector circuit.

Hello Billsymo,seems your expertise has scared of all the cyber tech guys , great bit of information Billsymo.

One actually asks the forbidden question what is actually inside a mono or a double D, "DD" coil with this new space age technology LOL. Which has all been around for decades LOL. LOL Litz wire is a trade secret.

At this stage I haven't personally opened up any of my expensive coils that i have bought but at some stage i hope to acquire some faulty cheap coils and have a look inside myself to satisfy the cat. Yes your probably right $20 worth of materials and $430 profit LOL in the actual cost of some of the bigger coils.


Inside a Minelab Metal Detector Coil for P.I
https://www.youtube.com/watch?v=BY7WFyCVO_A - woooooooooooo that is world renowned technology and now i can see the justifications for their expensive price tags LOL.

How to Repair Faulty Metal Detector Coils - https://www.youtube.com/watch?v=0OwrkEOpPiQ

Metal detector prototype test (PI MD) - https://www.youtube.com/watch?v=M-H6OZlrblg - maybe a coat hanger wire as someone suggest in the forum would work fine LOL for a coil construction. Maybe perform better than a car aerial LOL

https://www.youtube.com/watch?v=TNXaxceyFsM

http://en.wikipedia.org/wiki/Litz_wire

"Litz wire - From Wikipedia, the free encyclopedia

Litz wire is a type of cable used in electronics to carry alternating current. The wire is designed to reduce the skin effect and proximity effect losses in conductors used at frequencies up to about 1 MHz.[1] It consists of many thin wires strands, individually insulated and twisted or woven together, following one of several carefully prescribed patterns[2] often involving several levels (groups of twisted wires are twisted together, etc.). This winding pattern equalizes the proportion of the overall length over which each strand is at the outside of the conductor.

The term litz wire originates from Litzendraht, German for braided/stranded wire[3] or woven wire[4]."

How to put on solder for the Litz wire - https://www.youtube.com/watch?v=iEhOW3dOAAM

Litz Wire - https://www.youtube.com/watch?v=2sMz1jwWWPw


Cheers

Hi billsymo...........Thanks for the interesting post.
A question.....If coils are so simple, does this mean you can use any coil on any machine if you have the correct plug fitted on for the detector box and the wires correctly aligned on that plug ?
Cheers.

sandy2010 wrote:Hi billsymo...........Thanks for the interesting post.
A question.....If coils are so simple, does this mean you can use any coil on any machine if you have the correct plug fitted on for the detector box and the wires correctly aligned on that plug ?
Cheers.

The simple answer to this is yes, but.
A mathematical formula is used to match the inductance of the coil to the capacitance at the output of the detector, to operate in a frequency or range of frequencies ,so they may not work as well as one specifically designed for your detector, however using any coil, by changing the plug will not hurt your detector, and with a bit of luck may work better.
Be aware that a mono coil only really needs two wires connected, a double D may need four.
It serves no purpose pulling a coil to pieces to see what is in there. They are all basically the same.
You will only find one coil of wire in a mono and two in a dd. The dd should have the two coils overlapping in the centre of the coil.
The only other thing you will find in some coils is aluminium foil which is used as a faraday shield, I have never found a shield really necessary.My belief is that if you do need a faraday shield in the coil, the detector itself may be badly designed an subsceptable from interference from other sources.
Some Lizt wire is of better quality than some manufactured by diferent companies (Countries). So if a coil says it is made using Lizt wire, it does not necessarily mean that the quality of that wire is any better than normal unstranded copper wire,

Thanks for your answer billsymo......
I don't want to take up your time with what might appear to be trivia, but whilst on the subject of coils, I carried out a continuity test on some of my coils and I'm curious about the result:
The coils from my 2100.....the old dustbin lid mono(5 pin)
11" mono m.lab "
11" DD " "
14" DD c/tek ".................
All 5 pins are interconnected on the continuity test.

On a coil from my m/lab 17000 only 2 show continuity (DD I believe).

As Prof. Sumner Miller used to say,"Why is this so"?

sandy2010 wrote:Thanks for your answer billsymo......
I don't want to take up your time with what might appear to be trivia, but whilst on the subject of coils, I carried out a continuity test on some of my coils and I'm curious about the result:
The coils from my 2100.....the old dustbin lid mono(5 pin)
11" mono m.lab "
11" DD " "
14" DD c/tek ".................
All 5 pins are interconnected on the continuity test.

On a coil from my m/lab 17000 only 2 show continuity (DD I believe).

As Prof. Sumner Miller used to say,"Why is this so"?

Yes there is conductivity between all pins on the gp series, but 1 pin on the coil will show a higher resistance when connected to any of the other four. On the gpx 4000-4500 that pin is pin 4.
Set your multimeter to a low ohms range above 20 ohms. most meters lowest value is 200 ohms so set it to this.
Place one of the probes in pin 4 and another pin and you should get a reading of somewhere near 20 ohms.
If you try all of the other pins with pin 4 you should get the same reading of around 20 ohms.
If you try measuring all the other pins, with each other , but not pin 4, you will get a zero ohms or very close to zero reading.
This test is for a mono coil. so in reality pin 4 can be connected to any of the other four to complete the coil circuit. Any other combination will not work.
I suspect the reason for this type of wiring is to minimise faults, and interference, and probably confuse anyone who wants to make their own coils.
I haven't tested a dd coil yet but it is a similar test but there should be two pins that have a higher resistance than the rest when combined with other pins.
There is a more complicated test using a grid dip oscillator but this shows resistance, and frequency and can be
used to find out the frequency range and work out the capacitance and inductance of the coil in millihenries.
A word of warning, never turn your detector on without a coil connected,when the battery is connected, it may damage the control box. You may get away with it once . but it does not do the electonics any good operating without a load. The load being the coil.

Hi Guys, Billsymo very interesting thread & thanks for your time doing it. You mention using the best conductive wire available. How would gold wire be? Expensive maybe but they reckon an ounce can be drawn out to 50 odd miles, but would it be a lot better than copper or litz wire?

Regards

JW

Hi JW. I am lead to believe that gold is nowhere near as good a conductor as silver or platinum etc. It is mainly used to plate connectors as it is anticorrosive. Platinum is used on space shuttles etc rather than gold I am told because of it's superior conductivity. I doubt gold wire would be tough enough to take the punishment we dish out.
How you been anyway. We are still coming to grips with the flooding over here
Cheers
Panther

I spoke to Trevor at Coiltek several weeks back about the gold wire theory....he explained that it probably was'nt viable because of it's "resistance"....it might be possible in an alloy but it did'nt warrant the time and expense in research.
My other theory is silver which is a better conductor than gold.

Maybe billsymo could comment here.

kiwijw wrote:Hi Guys, Billsymo very interesting thread & thanks for your time doing it. You mention using the best conductive wire available. How would gold wire be? Expensive maybe but they reckon an ounce can be drawn out to 50 odd miles, but would it be a lot better than copper or litz wire?

Regards

JW

I doubt if gold wire would be much better than copper wire, and in my opinion not serve any purpose, other than overkill, it would have one advantage only. it would never corrode. There are more conductive metals than gold. If a coil is made of copper or Lizt wire and of very rigid construction there is no need to go to any other metals.
The coil has to match the electronics of the detector, perfectly. to get a perfect response. I use mainly minelab or coiltek coils and I don't have any problems. Minelab have the right idea and make their coils very rigid, but their main fault is that the larger coils are very heavy.
I have a couple of Coiltek coils which are much lighter, because they are the open type, and are very light and pleasant to operate. They have one slight fault which I really do not find annoying, and that is if you hit on something while swinging you will sometimes get a false signal, however because I know what the prolem is it does not bother me.
There is a cheaper version of wire than pure copper wire and that is copper-coated steel wire, the steel gives more strength to the wire, and because it is coppercoated, has the same conductivity as copper. Telstra has been using this type of wire for years.
I noticed somewhere else in the thread, someone mentioned the price of coils. The biggest cost of coils is not in the wire or components. The biggest cost is in labour. Metal detecting is a niche market and on a world scale not very large, the companies do have to make a profit. The worst part about this is that Minelab (now Codan) contracts most of its work out to Malaysia, and there is not a lot of Australian content in them any more.

How I repaired a Coiltek Goldstalker Coil
https://www.youtube.com/watch?v=UaHeqIXDKaM&feature=related

This seems to be a common issue with Coiltek's coils loose litzy wire????

cheers

24kt wrote:How I repaired a Coiltek Goldstalker Coil
https://www.youtube.com/watch?v=UaHeqIXDKaM&feature=related

This seems to be a common issue with Coiltek's coils loose litzy wire????

cheers

Minelab have a problem too, but it is not with the wire.
Minelab use a very cheap and nasty Faraday shield which is nothing more than paper coated with carbon powder as a faraday shield. Because this is connected to the common side of the coil, it sometimes causes problems, when the paper flexes when hitting an object with the coil, normally it does not cause a problem, but once that connection on the negative side becomes loose, then you do have problems. I may be the odd man out but I believe that a faraday shield around a coil, is not necessary. One day I will get around to building one myself, to see if I am wrong or not. In the past I built a coil to suit a Garret, and it worked a hell of a lot better than their own.

I think I saw a you tube footage from Woodypoopoo that he reckond the faraday shield was really necessary either. I didnt think coiltek used litz wire & that it was only minelab & nugget finder coils. Unless coiltek went to it with their goldstalker range. I have sort of gone now for the minelab commander range & a couple of nugget finder coils. I arent concened with the bit of extra weight with the minwlab coils as I use a hip stick. Man that hip stick is the bee's kness. I love it.

Happy hunting

JW

I have a NF 16" mono coil that i wore the shielding of the bottom from not using a skid plate. (old f/glass model) Coil is now not usable because if you touch the ground or a blade of grass it sounds off.

sandy2010 wrote:Thanks for your answer billsymo......
I don't want to take up your time with what might appear to be trivia, but whilst on the subject of coils, I carried out a continuity test on some of my coils and I'm curious about the result:
The coils from my 2100.....the old dustbin lid mono(5 pin)
11" mono m.lab "
11" DD " "
14" DD c/tek ".................
All 5 pins are interconnected on the continuity test.

On a coil from my m/lab 17000 only 2 show continuity (DD I believe).

As Prof. Sumner Miller used to say,"Why is this so"?

Hi again Sandy, when I replied to your post before I neglected to add some information. Here it is.
On the GP series 4000 , 4500, 4800, the pins on the plug are wired in this order. Pins 1 and 5 are soldered together and are one side of the coil. pins 2,3,4 are soldered together and are the other side of the coil. The resistance of a coil is very low so set you meter very low when testing. I don't know the pin configuration for the 17000, but if it is a 5 pin I would imagine that it would be the same. On a grid dip oscillator test, my coils dip at between 10 and 14 Mhz. I do not know if the same frequency for the 17000 applies. regards.

Although I am electronically trained there is something confusing about the use of Lizt wire in detector coils.

Lizt wire is only effective up to a frequency of 2 Mhz and is most effective around 5oo Khz. I have always maintained that its use in detector coils is not the best way to use lizt wire and its effectiveness at the frequencies that Detectors operate at, renders it useless and ineffective. It would not operate any better than normal copper wire.
Its real use is to reduce "skin effect" in alternating current circuits and reduce interference to other equipment by that alternating current. It is very effective at frequencies up to 1 Mhz and is rarely used for any effect above 2Mhz .
A metal detector coil is an inductance which creates a magnetic field around itself to aid in the detection of any metal object that passes within range of that magnetic field.
Because of the fact that Lizt wire reduces skin effect in an alternating circuit, in my opinion, that would make it useless as a detector component up to 2 Mhz.
Most metal detectors work in frequencies much lower than 2 Mhz. but in my opinion, Lizt wire at the frequencies the detectors work at would work no better than normal enamel coated copper wire. If there are any technophiles out there that can technically explain why Lizt wire is used in metal detector coils at all, I would appreciate your input.
Another point that makes me believe that it is ineffective is the fact that some minelab and other coils use a Faraday shield.
If Lizt wire worked well at lower frequencies than 2 Mhz, I would think that a Faraday shield would actually reduce the effectiveness of the coil. Back to you.

Some more info about the use of litz wire for PI detector coils. PI detectors are all about eddy currents in the target material. Having said that, eddy currents can also be generated in the coil windings under normal operation if the wire is thick enough for the eddy currents to form. For PI coils, solid enameled copper wire is not the best choice for optimum performance. The resulting signal will be a composite of the target eddy current response and the coil eddy current response.

So to minimize this effect, litz wire or tin plated copper wire cable is used, essentially breaking up the total cross section wire of the solid strand into multiple strands. This way eddy currents will not form in the windings.

The benefits of using litz wire to counteract skin effect is minimal at the frequencies used in the Minelab detectors.

Regards,

Stefan.

Stefan wrote:Some more info about the use of litz wire for PI detector coils. PI detectors are all about eddy currents in the target material. Having said that, eddy currents can also be generated in the coil windings under normal operation if the wire is thick enough for the eddy currents to form. For PI coils, solid enameled copper wire is not the best choice for optimum performance. The resulting signal will be a composite of the target eddy current response and the coil eddy current response.

So to minimize this effect, litz wire or tin plated copper wire cable is used, essentially breaking up the total cross section wire of the solid strand into multiple strands. This way eddy currents will not form in the windings.

The benefits of using litz wire to counteract skin effect is minimal at the frequencies used in the Minelab detectors.

Regards,

Stefan.

Metal detectors need eddy currents to work properly, The magnetic field produced by the coil causes an Eddy current in the target which in turn is detected by the coil, identifying a target. Cancel out the Eddy currents and you get a null response.

Billsymo wrote:

Stefan wrote:Some more info about the use of litz wire for PI detector coils. PI detectors are all about eddy currents in the target material. Having said that, eddy currents can also be generated in the coil windings under normal operation if the wire is thick enough for the eddy currents to form. For PI coils, solid enameled copper wire is not the best choice for optimum performance. The resulting signal will be a composite of the target eddy current response and the coil eddy current response.

So to minimize this effect, litz wire or tin plated copper wire cable is used, essentially breaking up the total cross section wire of the solid strand into multiple strands. This way eddy currents will not form in the windings.

The benefits of using litz wire to counteract skin effect is minimal at the frequencies used in the Minelab detectors.

Regards,

Stefan.

Metal detectors need eddy currents to work properly, The magnetic field produced by the coil causes an Eddy current in the target which in turn is detected by the coil, identifying a target. Cancel out the Eddy currents and you get a null response.

You are quite correct, however I was refering to the possible actual eddy currents generated in the coil windings not the eddy current signal from the target. So my statement "The resulting signal will be a composite of the target eddy current response and the coil eddy current response. " leaves the target eddy current response intact if the the coil eddy current is canceled.

Stefan

Hi folks,
I have a question relating to this thread,
I am not the smartest person in the converation but it seems to me that to wind any coil you would first need to decide on its operating frequency in order to choose [ or make as stefan does ] the correct litz wire ? [ 57/30AWG stefan ? ]

So my question is, at what frequencies do the 2 channels operate at on the SD & GP series ?

I am returning to this sport after a 17 year absence, things have changed a little since my then state of the art XT17000 which gave the awesome choice of 6.4 or 32 Khz operating freqencies.

Any info appreciated, just keep it simple for me.

Thanks
DD

woops

Some good reading here on building fast PI coils http://www.geotech1.com/pages/metdet/projects/fastcoils/FastCoil.pdf

bubbazoo wrote:Some good reading here on building fast PI coils http://www.geotech1.com/pages/metdet/projects/fastcoils/FastCoil.pdf

Thank you so much. That is the most detailed explanation of Outer Diameter, and Inside Diameter with pictures I've seen.
I'll be using that as a reference foresure.