How to judge shield effectiveness?

[satdaveuk]

Hi
Shall try sandwich Graphite on paper.
In the past Ive been using rubberized tape wound directly round the windings to reduce internal capacitance then hospital paper tape then applying graphite mixed with varnish giving constant good results.

[satdaveuk]

The only accurate way to check a coils shielding effectiveness is get it out into a field where its going to spend its life, you can use flashing LEDSs, dunk it in water, put it between bricks, and anything else you can think of but theres only one way and thats in are soils introducing it to the minerals that have been there for thousands of years.
With shielding effectiveness there only a basic formula available which is of little use in the metal detector industry as theres so many other variants to consider.
Also bare in mind that what may be classed as a good shielding in one country may be near useless in another due use and history of the land.

[bklein]

The only accurate way to check a coils shielding effectiveness is get it out into a field where its going to spend its life, you can use flashing LEDSs, dunk it in water, put it between bricks, and anything else you can think of but theres only one way and thats in are soils introducing it to the minerals that have been there for thousands of years.
With shielding effectiveness there only a basic formula available which is of little use in the metal detector industry as theres so many other variants to consider.
Also bare in mind that what may be classed as a good shielding in one country may be near useless in another due use and history of the land.

Thus, my mind remains boggled.
George and Carl's book describe the shield only briefly-saying it reduces EMI as well as reduces capacitive coupling between the coil and ground that changes as the coil moves. So the EMI effectiveness can be measured with a spectrum analyzer and a RF comb generator I would think. The capacitive part of it I'm not sure. Is this only an issue with detectors that have no ground balance circuitry or all detectors? I'm wondering if we need to then create a 3-column "golden" reference table of shield approaches that would show how such a material performs for EMI reduction, capacitive coupling, and finally - detection of the material itself by fast coils.
I think possibly some of these materials are being applied to detectors with no ground balance circuits, found ineffective, and then criticized as not helping - when in fact they do for other detector designs. If you have a surf PI detector, there is no ground balance circuitry. That may really be what we should be spending our time working on.

[Davor]

EMI refers to Tx emissions and would bother you only in case if you manufacture detectors and wish they are EMI certified, while capacitive coupling to the ground is what you are up against, and refers to E-field. Ground balance deals with magnetically coupled mineral responses, and has nothing to do with shielding.

Regarding E-field, think about her Einstein and the effect he was granted a Nobel prize. In effect your coil picks electrical changes exactly as an electret microphone does. It could also be approach to measure shield's effectiveness.

IMHO a good differential mode Rx front end fixes this problem better than shielding because E-field effects are solely in common mode.

[satdaveuk]

EMI refers to Tx emissions and would bother you only in case if you manufacture detectors and wish they are EMI certified, while capacitive coupling to the ground is what you are up against, and refers to E-field. Ground balance deals with magnetically coupled mineral responses, and has nothing to do with shielding.

Regarding E-field, think about her Einstein and the effect he was granted a Nobel prize. In effect your coil picks electrical changes exactly as an electret microphone does. It could also be approach to measure shield's effectiveness.

IMHO a good differential mode Rx front end fixes this problem better than shielding because E-field effects are solely in common mode.

I was under the understanding that shielding performs two things, one of which is to minimize capacitance effect of the ground which in turn will effect the performance of ground balance whether auto or Manual which effects ground signal intensity among other things. so in turn shielding property's need to be varied considerably from one location to the next unless your happy with middle of the road like manufactures do.
The second is what we all agree with EMI fields.
In a ideal world with what I have learned doing practical experiments with Faraday coil shielding, optimum performance you would need different shielding property's for different permissions let alone different country's.
A prime example is SEF coils, over here in England they are a big hit, selling like hot cakes to Minelab owners all models, the reason why because the factory coils cant handle UK soil, where as with the SEF coil there getting better depth and far more stability.
Now go onto any USA site incl Minelab owners forum and they don't rate the SEF coil at all.
So the saying goes the proof is in the pudding and that there is a tie up between coil shielding and ground minerals, why that is I don't really know but seriously needs to be addressed .
Regards

[Henrikras]

interresting reading you posted Satdeveku.. Could it be the same with NEL coil and Minelab ? I can read that a some european Minelab 305/705 owners are happy with the NEL 13" dominator. (I think SEF dont build to the 305/705).One of mine Detector is a Teroso Vanquero. I changed to a SEF coil (10x12"), and was very surprised with deept and stability, but when I said that in a US forum, they didn't agreed..
The reason must Be what you said... Different countrys, different shielding, to optimum performance.
Ps. Sorry to interrupt this thread again.
Henrik

[satdaveuk]

interresting reading you posted Satdeveku.. Could it be the same with NEL coil and Minelab ? I can read that a some european Minelab 305/705 owners are happy with the NEL 13" dominator. (I think SEF dont build to the 305/705).One of mine Detector is a Teroso Vanquero. I changed to a SEF coil (10x12"), and was very surprised with deept and stability, but when I said that in a US forum, they didn't agreed..
The reason must Be what you said... Different countrys, different shielding, to optimum performance.
Ps. Sorry to interrupt this thread again.
Henrik

Hi Henrik
Your comments and opinion are valid so feel free to comment at anytime, the more feedback we get the more understanding we all get, Shielding and coil construction in general are a good subject because its the route of a successful build, if your coils are wrong then you have a third rate detector, if you have a excellent coil you can make a third rate detector that much better so its a win win situation.

[Midas]

EMI refers to Tx emissions and would bother you only in case if you manufacture detectors and wish they are EMI certified, while capacitive coupling to the ground is what you are up against, and refers to E-field. Ground balance deals with magnetically coupled mineral responses, and has nothing to do with shielding.

Regarding E-field, think about her Einstein and the effect he was granted a Nobel prize. In effect your coil picks electrical changes exactly as an electret microphone does. It could also be approach to measure shield's effectiveness.

IMHO a good differential mode Rx front end fixes this problem better than shielding because E-field effects are solely in common mode.

The full phrase from Carl and Georges book is "Shielding can also help reduce external EMI-induced falsing" so they can't be referring to solely reducing EMI emissions for regulation purposes. Perhaps one of them would be so kind as to elaborate further.

Midas

[Qiaozhi]

The full phrase from Carl and Georges book is "Shielding can also help reduce external EMI-induced falsing" so they can't be referring to solely reducing EMI emissions for regulation purposes. Perhaps one of them would be so kind as to elaborate further.

Midas

Also see Chapter 15, page 246, "Electrostatic Shielding".

[multieagle]

http://www.thebigger.com/physics/ele...tic-shielding/

[Midas]

Also see Chapter 15, page 246, "Electrostatic Shielding".

Yes, I realize the capacitive effect is usually by far the most dominant reason for a shield, its been well covered. But assuming the capacitive effect can be negated with a differential amplifier as Davor is advocating is there still an EMI reduction benefit to having a shield? The passage I quoted seemed to indicate that perhaps there is and that is specifically what I wanted the elaboration on.

[Carl-NC]

Depending on how it's done, a differential coil might cancel far-field EMI. As I briefly think about it, a differential coil that cancels far-field EMI probably would do poorly on ground electrostatics, and vice-versa. But there may be a design that cancels both nicely.

[Ferric Toes]

Depending on how it's done, a differential coil might cancel far-field EMI. As I briefly think about it, a differential coil that cancels far-field EMI probably would do poorly on ground electrostatics, and vice-versa. But there may be a design that cancels both nicely.

Only E field though with centre tapped bifilar mono. As I mentioned some while back H Field component gets through two layers of wrap around shielding on a 12in detector coil and it makes a good antenna for my workshop radio up to 7MHz. Above that it falls off rapidly. In my book, shielding has three functions 1) reduce emissions to comply with EMC regulations 2) Reduce rf pickup outside the frequency band required for detection 3) eliminate capacitive effects from the ground - particularly wet salt water beaches. I had an industrial PI detector that failed an emission test with an unshielded coil, but passed by a reasonable margin with a shielded one, without any loss in performance.

Eric.

[bklein]

I started searching loop and e-field topics with Google and came across this site:
http://www.w8ji.com/magnetic_receiving_loops.htm
How much of what he suggests applies to our coils?
Note what he says about noise source coupling distance from the loop, how the shield is an acting antenna, how the shield gap must be specifically placed so there is differential signal balance. He doesn't seem to cover shield material choice. I wanted to ask him about that but couldn't find his email.

[Davor]

a differential coil that cancels far-field EMI

True, but we have a small terminology problem here that must be rectified.

A term "differential" refers to a difference between ... whatever. However, speaking in two-port network terms, there are a common mode and a differential mode signals, as referred to the ground connection, or the lack of it.

So in effect we have a differential coil, which consists of two coils in a differential combination, and a differential mode of a two-port network connection.

I try hard not to miss the "mode" word when I refer to the differential (balanced) preamp inputs.

There are in fact two kinds of differential mode inputs, a simple and a split differential mode that uses a ground tap. A simple differential mode presents a high impedance for a common mode signal, while the split differential mode provides a short to the ground for the common mode. Considering the sources of common mode signal in metal detectors' coils is of capacitive origin, and high in impedance, the ground tap reduces them significantly prior to entering the preamp, and that's why I like it so much.

Perhaps there is a simple enough way to test the efficiency of a shield. Find a large enough conductive plate, place enough insulation around it to be safe, and connect a live wire from the mains. Maybe with a 100k resistor in series to be extra safe. As it is referred to the ground, it will produce a 50/60Hz E-field. As most of the detectors are not too happy with large chunks of metal in front of them, such plate could be a salty water soaked cardboard - should work just fine. You don't need much larger voltages than the mains if you place a coil a cm or so in front of a plate. If your detector hums - your shield sucks.