northwest Coil shielding
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australian electronic gold prospecting forum.com  |  Detector Technology and Electronics and new detectors  |  Detector Coils  |  Topic: Coil shielding 0 Members and 1 Guest are viewing this topic. « previous next »
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Author Topic: Coil shielding  (Read 4296 times)
Mechanic
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« Reply #20 on: Sunday February 27 2011 15:54:10 AEDT PM »

  
  
  
On an 18" mono coil using SS mesh I get a coil to shield capacitance of around 100 PF.
Regards, Ian

Geez, that's awesome! Who came up with the ss mesh?

Cheers Mick

oorr dont know is ss mesh same as the silver mesh I was talking about ?

SS mesh is stainless steel mesh, it would be less likely to hold eddy currents than silver.

Mick
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gef12
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« Reply #21 on: Sunday February 27 2011 16:17:57 AEDT PM »

ooooo kay  happy face  where do ya get it  Toilet reading
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« Reply #22 on: Sunday February 27 2011 16:25:52 AEDT PM »

When you get off the loo Look this up!   
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Doug
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« Reply #23 on: Sunday February 27 2011 16:40:59 AEDT PM »

  
When you get off the loo Look this up!   

One problem! I believe that Woody has an innovation patent (not examined?) on the use of SS fabric or mesh for coil shielding.
doug happy face
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« Reply #24 on: Sunday February 27 2011 16:56:10 AEDT PM »

  
  
When you get off the loo Look this up!   

One problem! I believe that Woody has an innovation patent (not examined?) on the use of SS fabric or mesh for coil shielding.
doug happy face

Until the patent is published, examined and then certified (which could be more than 3 months) then anyone could use this SS fabric!
So if anyone wants to use it now go for it!
doug  happy face
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« Reply #25 on: Sunday February 27 2011 17:04:38 AEDT PM »

Well it is my belief that if you want to use a patented method for your own purpose and not to sell there is nothing anybody can do! ::38::
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« Reply #26 on: Sunday February 27 2011 17:22:37 AEDT PM »

My reading of the law is if you do it for yourself and not for sale there is no problem it is expensive the biggest problem is set up cost of production and my supplier does not hold stocks it is made to order but if enough people got together the cost would be reasonable I have tried all the silver products by Less EMF and others and none come close to the SS mesh I can put a whole 10 meter roll over the coil and touching the coil and it is not seen by the coil even with the 2000 running a 4.5 MHz crystal.

Regards, Ian.
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Regards, Ian.
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« Reply #27 on: Sunday February 27 2011 19:42:45 AEDT PM »

yeh tis a bit exy
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« Reply #28 on: Sunday February 27 2011 21:05:05 AEDT PM »

  
My reading of the law is if you do it for yourself and not for sale there is no problem it is expensive the biggest problem is set up cost of production and my supplier does not hold stocks it is made to order but if enough people got together the cost would be reasonable I have tried all the silver products by Less EMF and others and none come close to the SS mesh I can put a whole 10 meter roll over the coil and touching the coil and it is not seen by the coil even with the 2000 running a 4.5 MHz crystal.

Regards, Ian.

Hi Ian,
100pf is really excellent, will be very hard to beat.
What is the distance between the coil wires and the shield?
Stainless steel is often difficult to get the solder to stick on. How is the mesh soldering?

All the best

Tinkerer
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Stefan
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« Reply #29 on: Sunday February 27 2011 21:56:40 AEDT PM »

Hi Tinkerer,

SS 316 will solder if you use zinc chloride flux. An old trade name was Bakers Soldering fluid.  Just need to clean off the residue flux.

Stefan
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bbsailor
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« Reply #30 on: Monday February 28 2011 04:29:13 AEDT AM »

  
On an 18" mono coil using SS mesh I get a coil to shield capacitance of around 100 PF.
Regards, Ian

Ian and all interested,

Back in December 2006 I published the following article based on my independent research.   

My goal was to find practical ways to minimize coil capacitance and find construction methods that would enable PI experimenters to make their own "fast coils".  Based on the discussion in this thread, it seems that others have stumbled on to using wire mesh as method to minimize coil-to-shield capacitance.

Here is an experiment that you can do that will allow you to easily see the difference between using various shielding materials and methods.

Obtain a small metal tube about 30cm long about the same outside diameter as the coil wire bundle diameter. Place an insulator/spacer over the metal tube. Put a piece of alumium foil over that insulator. This represents a solid shield. Next, measure the capacitance between the metal tube and the foil using a capacitance meter.   Remove the foil and replace it with a mesh shield and measure the capacitance again.

What you are looking to do is see the amount of capacitance difference between the solid foil shield and the mesh shield. Since the mesh has less surface area then the solid foil shield, the capacitance of the mesh shield should be less.  Change the spacer material from PVC to PE or other material to see the difference that the dielectric has on capacitance. The spacer thickness also has an impact on the capacitance. See the mogami web link in the "Shield Material Experiments" section of this article for an on-line calculator that lets you do some desktop "what-if" experiments with different materials and construction techniques.

Ian's 100pf coil-to-shield capacitance for an 18" coil is very good and seems to indicate a thick spacer with a low dielectric constant and a mesh shield with a lower surface area.

The reason why low capacitance coils are important is that you want to coil discharge time constant to be at least 5 times faster than the target time constant to fully stimulate it. The coil discharge time constant is primarily determined by the value of the damping resistor which controls the steepness of the coil discharge curve. A perfectly vertical discharge curve would be ideal but is impossible, so getting the discharge as steep as possible will be the best design for very small, low time constant gold targets.  

When posting about coil construction techniques, it would be helpful to also indicate the coil self resonance. This allows others to compare their results as the coil self resonance summarizes all the following capacitance adders to the coil circuit.
1. Wire gauge and outside diameter
2. Insulation thickness
3. Insulation dielectric constant
4. Coil inductance
5. Coil spacer thickness
6. Coil spacer dielectric constant
7. Shield area
8. MOSFET capacitance (COSS)
9. Coax length

Mass produced could have some variance in any or all of the above factors. That is why optimizing the damping resistor value to the individual coil allows you to squeeze the maximum potential out of your coil, but this is mostly visible on very small targets with low time constants.

I hope this helps.

bbsailor
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Tinkerer
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« Reply #31 on: Monday February 28 2011 06:18:41 AEDT AM »

  
Hi Tinkerer,

SS 316 will solder if you use zinc chloride flux. An old trade name was Bakers Soldering fluid.  Just need to clean off the residue flux.

Stefan

Thanks Stefan,

ah, I remember now, used to use that stuff 50 years ago.

Take a small flask, fill it with zinc shavings and then add some hydrochloric acid.
Let stand open until it does not bubble anymore. (Careful, highly explosive hydrogen/oxygen mix)
Then apply a drop on the part to be soldered, with a tooth pick.

Tinkerer
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« Reply #32 on: Monday February 28 2011 06:41:10 AEDT AM »

Low conductivity, like that of stainless steel or graphite paint, has a very short decay time. If the decay time is much shorter than the first sample delay, the shield is not seen.

With graphite paint, the usual problem is to find a way to solidly connect the shield to the ground.

My usual bench test for the shielding, is to approach my hand to the coil while observing the signal on the scope. No signal change = good shielding.
The human body is an excellent antenna, if I touch the probes of the scope, I can get up to 200V signal from my hand.
when I run an FFT of my hand antenna, I got high levels of spectrum, up to 400kHz.

Interestingly, a coil with a self resonant frequency of 200kHz, also attenuated the noise frequency spectrum above it's frequency.

Do any of you have similar observations to share?

Tinkerer
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WM6
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« Reply #33 on: Monday February 28 2011 07:59:43 AEDT AM »

  


Back in December 2006 I published the following article based on my independent research.   

bbsailor

Thank you for all your efforts and kindness and be aware that your mentioned article are widely welcome between detectorist till now. 
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« Reply #34 on: Monday February 28 2011 09:13:18 AEDT AM »

  
  
Hi Tinkerer,

SS 316 will solder if you use zinc chloride flux. An old trade name was Bakers Soldering fluid.  Just need to clean off the residue flux.

Stefan

Thanks Stefan,

ah, I remember now, used to use that stuff 50 years ago.

Take a small flask, fill it with zinc shavings and then add some hydrochloric acid.
Let stand open until it does not bubble anymore. (Careful, highly explosive hydrogen/oxygen mix)
Then apply a drop on the part to be soldered, with a tooth pick.

Tinkerer

Yep, You have have it. Old memory pathways opened.

Stefan
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gef12
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« Reply #35 on: Monday February 28 2011 11:11:44 AEDT AM »

  
  
Hi Tinkerer,

SS 316 will solder if you use zinc chloride flux. An old trade name was Bakers Soldering fluid.  Just need to clean off the residue flux.

Stefan

Thanks Stefan,

ah, I remember now, used to use that stuff 50 years ago.

Take a small flask, fill it with zinc shavings and then add some hydrochloric acid.
Let stand open until it does not bubble anymore. (Careful, highly explosive hydrogen/oxygen mix)
Then apply a drop on the part to be soldered, with a tooth pick.

Tinkerer

Ye tinkerer now your bringing back the years .. me too did the same ting
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« Reply #36 on: Monday February 28 2011 14:00:18 AEDT PM »

Hi all,
The best flux to use on the SS mesh for soldering is Comweld 965 in my previous life it is what we used commercially for soldering stainless steel it comes in a 125ml bottle and the bottle I have will last me out.

Joe, I use a method almost the same as you to check capacitance characteristics and my insulation is Styrofoam and on the 18" mono is 7mm thick all around the coil bundle , I read your article when you first posted it  and has been the basis of my building although found early on that Minelab coils need different parameters and shielding than single pulse length machines and have concentrated on coils for these machines.

Regards, Ian.
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Regards, Ian.
bbsailor
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« Reply #37 on: Monday February 28 2011 14:32:14 AEDT PM »

  
Hi all,
The best flux to use on the SS mesh for soldering is Comweld 965 in my previous life it is what we used commercially for soldering stainless steel it comes in a 125ml bottle and the bottle I have will last me out.

Joe, I use a method almost the same as you to check capacitance characteristics and my insulation is Styrofoam and on the 18" mono is 7mm thick all around the coil bundle , I read your article when you first posted it  and has been the basis of my building although found early on that Minelab coils need different parameters and shielding than single pulse length machines and have concentrated on coils for these machines.

Regards, Ian.

Ian,

I do not have a Minelab machine. Could you please summarize the unique things that the Minelab coils require?  Most of my experience is with single pulse length machines like the Hammerhead and the C Scope CS6-PI.

Thanks

bbsailor
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« Reply #38 on: Monday February 28 2011 15:14:37 AEDT PM »

BBsailor,
Due to the Multi Period Sensing  utilized in the Minelab PI Machines the Minelab coils have a narrow range to work in for maximum performance the ideal for a Mono coil is 300 uh and .4 ohms The inductance can be changed 40 uh either way depending on the characteristics that you want from the coil and because of the MPS the shielding must not carry eddy currents which is why you can not use Scotch 24 for shielding or silver plated Teflon wire as coil wire unless each strand is.2mm or less and insulated from one another hence we use either Litz wire or tin plated wires which do not have the eddy current problem and I strive for the lowest coil to shield capacitance and use a coax that is 17 pf a foot and try for a self resonance of around 700 KHz for an 18" coil my 18" Tri-Field has a self resonance of 725 KHz which I consider not bad.

Regards, Ian.
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Regards, Ian.
bbsailor
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« Reply #39 on: Monday February 28 2011 15:54:56 AEDT PM »

  
BBsailor,
Due to the Multi Period Sensing  utilized in the Minelab PI Machines the Minelab coils have a narrow range to work in for maximum performance the ideal for a Mono coil is 300 uh and .4 ohms The inductance can be changed 40 uh either way depending on the characteristics that you want from the coil and because of the MPS the shielding must not carry eddy currents which is why you can not use Scotch 24 for shielding or silver plated Teflon wire as coil wire unless each strand is.2mm or less and insulated from one another hence we use either Litz wire or tin plated wires which do not have the eddy current problem and I strive for the lowest coil to shield capacitance and use a coax that is 17 pf a foot and try for a self resonance of around 700 KHz for an 18" coil my 18" Tri-Field has a self resonance of 725 KHz which I consider not bad.

Regards, Ian.

Ian,

Thanks for the prompt reply.  When you say plus or minus 40uH inductance based on the characteristics you want from the coil, what characteristics are you talking about?

What characteristic does a plus 40uH or a minus 40uH inductance change favor?

Since you are using a wire bundle with AWG 18 plus a 7mm spacer on each side of the wire bundle, you must be using a coil housing that is about 1 inch thick?

What is the AWG 18 wire bundle diameter?


Thanks

bbsailor

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