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 4295 times)
Doug
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« on: Saturday February 26 2011 13:58:15 AEDT PM »

 How much effect can coil shielding have on the sensitivity and detection depth of a coil/detector? eg Does very good shielding from EMI and electrostatic effects (ie a very quite coil) necessarily mean a reduced sensitivity/ depth to targets particularly those with fast decays where one needs early sampling?
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authere
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« Reply #1 on: Saturday February 26 2011 16:19:41 AEDT PM »

Hi Doug,

You have opened up a can of worms there, the way i see it is that the coil needs a shield to effect a Ground Balance as well as reduce the effects of EMI and all.

The coil would see the shielding first and semi tune onto it to help smooth out the ground interferences, the biggest problem as we all know is capacitance and the lower it is the better, as to how low we can go i'm not sure but by the way the new SS mesh behaves i would say lower is good.

If cables are lain and tied in tri-foil they do reduce the effects of interference to other cables or to themselves a bit like the old tv aerial wire, twin wires spaced apart.

One of my projects is to wind a coil with twin wire to reduce the ohms in a large dia. coil.

we would get more penetration from a coil without any shielding but in most cases we do need it, similarly G/B is req'd mostly but can be done without.....

So, To a point we need shielding and G/B but what do we get... fixed in concrete,unadjustability!!!



Ron
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Doug
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« Reply #2 on: Saturday February 26 2011 16:26:08 AEDT PM »

 It has been suggested that if we are looking solely for large conductive objects with long decay times eg big nuggets and we can delay the sampling long enough then we may be able to use unshielded or minimally shielded coils?
By the way has anyone tried this for coil shielding?
Introducing the 3M™ Copper Nickel-Plated Conductive Woven Fabric Tape CN4190
  
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Stefan
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« Reply #3 on: Saturday February 26 2011 17:09:33 AEDT PM »

I have tried the copper fabric type shielding tap  and found that the detector will detect the tape.   I did a test by placing a 300 mm length on the ground and my SD2000 was able to detect the tape.  I did the same test on the shielding tape that I make and the detector as unable to detect the tape.

When I make large coils I use the minimal shielding required to minimise the ground capacitive effect.  What is clear is that as the diameter of the coil increases the shield capacitance increases if you use the same shield method as used for a much smaller coil.  I manage this by measuring the shield capacitance of the coil and adjusting the shield grid pattern to give me optimum results.

Stefan
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gef12
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« Reply #4 on: Saturday February 26 2011 17:12:34 AEDT PM »

Or even this High performance Silver Mesh
  
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Doug
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« Reply #5 on: Saturday February 26 2011 17:12:55 AEDT PM »

  
I have tried the copper fabric type shielding tap  and found that the detector will detect the tape.   I did a test by placing a 300 mm length on the ground and my SD2000 was able to detect the tape.  I did the same test on the shielding tape that I make and the detector as unable to detect the tape.

When I make large coils I use the minimal shielding required to minimise the ground capacitive effect.  What is clear is that as the diameter of the coil increases the shield capacitance increases if you use the same shield method as used for a much smaller coil.  I manage this by measuring the shield capacitance of the coil and adjusting the shield grid pattern to give me optimum results.

Stefan

What sample delay did you use when you detected the tape?
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Stefan
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« Reply #6 on: Saturday February 26 2011 18:12:24 AEDT PM »

Well the SD2000 was running at 2Mhz and based on the timing details for this freq, the delay time for the shorter pulse length of 60 us was 15 us. The response was quite clear.

Stefan
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gef12
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« Reply #7 on: Saturday February 26 2011 18:19:45 AEDT PM »

  
Or even this High performance Silver Mesh
  

Using 3.2Mhz with this and nil response using aprox 300mm x 20mm strip of fabric.
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« Reply #8 on: Saturday February 26 2011 18:40:46 AEDT PM »

  
Or even this High performance Silver Mesh
  

This is promising. Have you used this shielding material  on a coil.

Stefan
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gef12
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« Reply #9 on: Saturday February 26 2011 20:05:27 AEDT PM »

Yep use it on a 11inch mono .. .. have not tryed it out bush yet .. couple o months to go
Air tests seem ok
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Stefan
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« Reply #10 on: Saturday February 26 2011 20:24:34 AEDT PM »

  
Yep use it on a 11inch mono .. .. have not tryed it out bush yet .. couple o months to go
Air tests seem ok


Gef,

Can you remember what your shield to coil capacitance was.

Stefan
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gef12
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« Reply #11 on: Saturday February 26 2011 20:49:22 AEDT PM »

nah .. sorry om ..forgot .. hell am over 45  ::38::
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« Reply #12 on: Saturday February 26 2011 20:53:47 AEDT PM »

  
nah .. sorry om ..forgot .. hell am over 45  ::38::

and I'm an old fart at 60  rolleye 1

Stefan
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Aziz
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« Reply #13 on: Saturday February 26 2011 21:18:50 AEDT PM »

  
It has been suggested that if we are looking solely for large conductive objects with long decay times eg big nuggets and we can delay the sampling long enough then we may be able to use unshielded or minimally shielded coils?

Hi Doug,

this applies to induction balanced (IB) coils, where the high voltage from the TX coil is almost not seen by the RX coil. The high capacitance slows down the dI/dt (switch-off current slope) as dt gets longer.

On the mono coils, you have to achieve 0 volt as quickly as possible as you usually have a high voltage gain at the amplifier. This affects the dynamic range. A higher capacitance will cause less dynamic range therefore. At the end, a higher capacitance configuration looses much signal response due to longer delay time.

Aziz

PS: regarding the capacitance (shielding effect not taken into account)
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« Reply #14 on: Saturday February 26 2011 22:33:31 AEDT PM »

Hi All,

About my twin wire project or a tri-foil arrangement....it becomes self shielding....big coils.....maybe!!!!

Ron
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« Reply #15 on: Sunday February 27 2011 12:30:09 AEDT PM »

On an 18" mono coil using SS mesh I get a coil to shield capacitance of around 100 PF.
Regards, Ian
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Regards, Ian.
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« Reply #16 on: Sunday February 27 2011 12:42:03 AEDT PM »

Hi Ian,

Thats fairly good as it is always a prob to keep it below 350pf, that would have to give a good percentage increase in depth, is there any other advantages

Ron

PS:The nylon mesh coated with silver would have to be a lot less of a load as the silver coating would be very thin,certainly sounds as if it would be worth a try
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« Reply #17 on: Sunday February 27 2011 14:31:03 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
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« Reply #18 on: Sunday February 27 2011 14:48:40 AEDT PM »

100 pico farads...i would be happy with that.Ian does that include the coax as well ?

Zed
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« Reply #19 on: Sunday February 27 2011 14:59:56 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 ?
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australian electronic gold prospecting forum.com  |  Detector Technology and Electronics and new detectors  |  Detector Coils  |  Topic: Coil shielding « previous next »
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