northwest Slow coils for ML
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australian electronic gold prospecting forum.com  |  Detector Technology and Electronics and new detectors  |  Detector Coils  |  Topic: Slow coils for ML 0 Members and 1 Guest are viewing this topic. « previous next »
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Dontbstme
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« on: Monday February 18 2019 00:04:33 AEDT AM »

Hi folks.

I have been trying to build a slow coil for GPX, so it wouldn't be sensitive to small stuff (fly poo) and so on. It appears ML does not accept well slow coils.
Have anyone else here tried to make coils for GPX that are insensitive to the tiny targets?
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WM6
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« Reply #1 on: Monday February 18 2019 20:51:38 AEDT PM »

  
Hi folks.

I have been trying to build a slow coil for GPX, so it wouldn't be sensitive to small stuff (fly poo) and so on. It appears ML does not accept well slow coils.
Have anyone else here tried to make coils for GPX that are insensitive to the tiny targets?

Why quick coil disturb you?

Smal stuf signals are able to cover (damp) deeper big stuff signals.

So it is recommended to first clean all shallow small stuf signals, to get more clean and whole big stuff deper signals.

From that point of view, there is no reason to abandon quick coils?

What are your reasons?
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Dontbstme
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« Reply #2 on: Tuesday February 19 2019 01:39:37 AEDT AM »

  
  
Hi folks.

I have been trying to build a slow coil for GPX, so it wouldn't be sensitive to small stuff (fly poo) and so on. It appears ML does not accept well slow coils.
Have anyone else here tried to make coils for GPX that are insensitive to the tiny targets?

Why quick coil disturb you?

Smal stuf signals are able to cover (damp) deeper big stuff signals.

So it is recommended to first clean all shallow small stuf signals, to get more clean and whole big stuff deper signals.

From that point of view, there is no reason to abandon quick coils?

What are your reasons?

Two reasons.
Litz wire is expensive to use and relatively difficult to obtain.
My customers want to go easy and not having to clean small signals first.
So a coil that is not sensitive to small signals would sell better for may treasure hunters as it will be simply more productive on medium and larger object that they are after.
People simply do not want to dig small targets around here and I can not blame them. Digging small targets is a waste of time and profits from professional metal detecting are calculated on the basis of man-hours.
So if you dig small stuff all the time (and 99.99% of them have no value) you are wasting enormous amount of time, because I am talking about having to clean useless trash, not detecting small gold nuggets, which is a different topic.
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Dontbstme
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« Reply #3 on: Tuesday February 19 2019 03:58:57 AEDT AM »

My last experiment from today is demonstrating the the GPX detectors will not work with any slow coil.

I could be mistaken and will be grateful if anyone else has managed other wise.

I have compared two loops. One from 11" ML mono and another that I have made from solid thick copper wire, two strands and the same diameter as the 11' ML mono.
The connecting cable was for both coils the original cable from the 11" ML mono.
No screen to eliminate the margin of error from different screens.
Both coils have the same inductance and resistance, one to one.

The GPX4500 worked well with the original ML coil made with Litz wire, but it does not accept the same coil made from solid copper.
Any clues to why?
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IBGold
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« Reply #4 on: Tuesday February 19 2019 16:16:50 AEDT PM »

Hi Dontbstme,

With the Minelab Pi's you will find as I did many years ago they do not like wire with strands any heavier than .2 mm as heavier strands carry eddy currents which upsets things and they are not optimized to run large coils as far as coil wires Litz is best but you can also use similar constructed wires with tinned strands.

Regards, Ian. happy face
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Regards, Ian.
WM6
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« Reply #5 on: Tuesday February 19 2019 19:57:50 AEDT PM »

  

With the Minelab Pi's you will find as I did many years ago they do not like wire with
strands any heavier than .2 mm as heavier strands carry eddy currents which upsets things


Coils self detecting at thicker wire used could be here core problem as you say.

But what about sampling?

Apart from ML PI, if early sampling in MD is fixed, even with Litz wire, coil could be still (to) slow,
if wound with to big interwire capacitance.

Using Litz wire alone, does not guarantee that coil suit best to very early sampling.
Litz wire still should be properly wound to avoid interwire capacitance and by this unnecessary
capacitive reactance.

Capacitive reactance is frequency dependent. Not so by frequency of square pulses,
but by frequency composition of sine signals to which square pulses fall apart, by its
joint with coil tank circuit.

According said, slow coil using proper selected Litz wire is possible. Main question could be,
if selected detector sampling system is set (constructed) to use with such slow coil.

Still remain question of blanking small gold pieces viability. By blanking small surface pieces
signals, deeper weak signals of bigger pieces of gold are blanked also.

Better approach is to clean surface pieces first (usually by smaller coil) and then go for deeper
big pieces using bigger coil.

Bigger (bundle wound) coils are in effect similar to slower coil, it will overlook small gold pieces.

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Dontbstme
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« Reply #6 on: Wednesday February 20 2019 01:15:37 AEDT AM »

  
Hi Dontbstme,

With the Minelab Pi's you will find as I did many years ago they do not like wire with strands any heavier than .2 mm as heavier strands carry eddy currents which upsets things and they are not optimized to run large coils as far as coil wires Litz is best but you can also use similar constructed wires with tinned strands.

Regards, Ian. happy face
Hi Ian.
Thank you for the info.
I did arrive to some disappointing conclusions regarding the GPX and big coils, but I am also a stubborn pig and will have to do more. No way around this I am afraid.
I have established a contact with a reliable supplier for Litz wires from the UK and will proceed further to this project. Well the coils will not be slow as I wanted, but they will definitely be big - 30" and 41" CC for sure. I have the shells, only got to push the bloody wires in.happy face
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Dontbstme
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« Reply #7 on: Wednesday February 20 2019 01:23:37 AEDT AM »

  
  

With the Minelab Pi's you will find as I did many years ago they do not like wire with
strands any heavier than .2 mm as heavier strands carry eddy currents which upsets things


Coils self detecting at thicker wire used could be here core problem as you say.

But what about sampling?

Apart from ML PI, if early sampling in MD is fixed, even with Litz wire, coil could be still (to) slow,
if wound with to big interwire capacitance.

Using Litz wire alone, does not guarantee that coil suit best to very early sampling.
Litz wire still should be properly wound to avoid interwire capacitance and by this unnecessary
capacitive reactance.

Capacitive reactance is frequency dependent. Not so by frequency of square pulses,
but by frequency composition of sine signals to which square pulses fall apart, by its
joint with coil tank circuit.

According said, slow coil using proper selected Litz wire is possible. Main question could be,
if selected detector sampling system is set (constructed) to use with such slow coil.

Still remain question of blanking small gold pieces viability. By blanking small surface pieces
signals, deeper weak signals of bigger pieces of gold are blanked also.

Better approach is to clean surface pieces first (usually by smaller coil) and then go for deeper
big pieces using bigger coil.

Bigger (bundle wound) coils are in effect similar to slower coil, it will overlook small gold pieces.



You see WM6. The GPX just doesn't fire right if the parasitic capacitance of the coil plus coil cable is above certain level. GPX simply refuses to work with slow coils.
So I'll go for the Litz wire gimmicks, but at least my big coils will offer the power output of mono combined with the discrimination of DD (in this case with CC).
So if one can have at least good discrimination on a huge loop and at the same time low weight (800 grams for 30" CC) this should be something good.
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australian electronic gold prospecting forum.com  |  Detector Technology and Electronics and new detectors  |  Detector Coils  |  Topic: Slow coils for ML « previous next »
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