Theories about coil-shielding

[Krzychu_]

Maybe such material would meet the requirements. (unfortunately I do not have a sample of checking):

https://utekcomposites.en.made-in-ch...Dedusting.html

[bbsailor]

Here are a few tips to follow for those tinkering with the shielding of their coils.

Here is a mental model to think about. Consider the shield as being one plate of a capacitor with the wire coil bundle being the other capacitor plate. The space between them will determine how much capacitance those two plate areas will represent. The dielectric constant of the spacer material will vary: Dry air is 1; PVC is 4 to 6;Teflon is 2. A lower dielectric constant is better for less capacitance between the shield and the coil. To measure the capacitance between the coil and shield just use a capacitor meter with one lead attached to the shield and the other lead attached to either coil wire. There may be a slight difference of a few pf depending on which coil wire you use due to how the wire is laying inside the coil bundle relative to the shield.

The material you choose for a shield should not be detected by your coil operating at the lowest delay. However even if the material is not detected how you wrap the shield on the coil may create a loop that is detected. Try this experiment to see what I mean. Take a piece of straight wire about 2.5 inches long and see if it is detected at your lowest delay. If it is detected, advance your delay just enough to not detect it. Now form a loop with the wire with the ends twisted together or lightly soldered to form a conductive loop. This loop will seem like a larger target and will produce a stronger signal than the straight wire. You do not want to wrap your shield around your coil to form a cross section conductive loop around the coil bundle. Leave a slight space on the top of the coil almost like the letter "U" with the top of the shield material not touching so as to form a conductive loop. You can use electrical tape or other insulator to allow the shield to overlap but not touch. The shield also needs the classic space, where the coil leads enter and exit the coil, to not form a shorted loop around the circumference of the coil.

I tried using Scotch 24, a thin wire mesh that has much less area than a solid shield such as a foil. Scotch 24 comes in a two layer, 1 inch wide size that opens to form a tube. Cut this to one layer and only use a wide enough single layer piece to go around the cross section of your coil including the shield spacer thickness that you use. Keep the Scotch 24 from forming a shorted loop around the cross section circumference of your coil.

If you use a shield that is not solder-able, attaching the shield to the ground wire of the coil can be a challenge, you will want to use a conductive adhesive or glue. If your shield material has a high resistance, just weave a thin wire around the coil circumference, but not forming a continuous loop, to keep the lowest resistance to most of the coil shield area.

When you attempt to get delays below 10uS, you are now in the area where coil construction techniques, wire size and stranding, shielding techniques, and solder joints to coax wire become detectable.

The net result of all this will be a coil with a higher self resonant frequency and having the potential to operate at lower delays and using a higher damping resistor value to better stimulate smaller targets.

Just keeping these tips in mind when making your coil will help you extract the maximum performance from your coil.

I hope this helps?

Joseph J. Rogowski

[Melbeta]

Compass Electronics, when they were testing various kinds of coils, put various test markings on the coils. Now I know what some markings mean, which we re the dots above the coil number, and the dots below the coil number. They meant shield put on below coil, shield put on above coil, and the last was a dual shield, one above the coil and one below the coil. However there were no reports on what the experience was on the various shields.

Now I have to assume they must have connected the sheilds together in some way, but again, no reports on what they found out...

Now other markings such as "XX's" were stamped between the ears of the coils. Now again, no reports on the tests.
Melbeta

[dbanner]

The first and foremost PRIMARY reason for shielding coil housing is to prevent electrostatic effect. So static discharges via the shielding back to ground, and not into the coils themselves( false target response).
I don't know why people decide to shield whole coil housing, when they could shield the coil itself. Much easier to wrap shielding around the coils than to put over inside of housing. But for mass production, easier to spray coil housing than to wrap coils, totally unnecessary for self made.
There is big misconception about what is best.

[waltr]

The first and foremost PRIMARY reason for shielding coil housing is to prevent electrostatic effect. So static discharges via the shielding back to ground, and not into the coils themselves( false target response).
I don't know why people decide to shield whole coil housing, when they could shield the coil itself. Much easier to wrap shielding around the coils than to put over inside of housing. But for mass production, easier to spray coil housing than to wrap coils, totally unnecessary for self made.
There is big misconception about what is best.

My understanding is putting the shield on the housing it is farther from the coil thus less capacitance so faster coil.

Whereas, when the shield is on the coil the C increases and speed decreases (longer delay need) and loss of detecting very short TC targets.

Of course, one needs to know what targets and environment to design for. Many do not need very short sampling delay so no need for fastest coil.

[dbanner]

My understanding is putting the shield on the housing it is farther from the coil thus less capacitance so faster coil.

Whereas, when the shield is on the coil the C increases and speed decreases (longer delay need) and loss of detecting very short TC targets.

Of course, one needs to know what targets and environment to design for. Many do not need very short sampling delay so no need for fastest coil.

For pulse induction coil, sure. But what about induction balance?, I have read elsewhere on forum where a member suggested that no shielding was required whatsoever if the Rx coil were balanced with center tap driven from balanced oscillator. And that shielding of TX coil was totally unnecessary. Except for EMC compliance. Sometimes confused as to what is fact and what is fiction.

[dbanner]

So it is, but finally, after a year trying, I have a superb concentric head.What has this to do with the thread?
Well, I made a DD - very easy, and I wrapped the Ds in foil and copper tape as an RFI shield - yes,we need the magnetism.
For a year now, I have made at least 20 concentric coils that were crap - always I lined the shell with Alu and copper.
This one now is different. (I will be posting full details). The null is about 0.3 mV (point 3!!!)
When put into the foil lined case - CRAP!
I am waiting for an RF graphite aerosol to arrive - notice that ALL DIY concentrics use this method and NOT foil.
DDs are quite different to Cons which is why foil works for them.

This quote is very curious to me. Wrapping coils will not work for concentric search head, WHY not?

[Hyena]

This quote is very curious to me. Wrapping coils will not work for concentric search head, WHY not?

For me it did not work, because aluminium shield shifted inductance so much, that I was not able to null it after shielding even with big free loop. So I needed to use graphite spray. I did not try copper tape yet. I do not understand why copper tape change inductance so low. Change in inductance on D with copper tape was not noticeable. I would like to test copper tape on concentric, because I hate graphite spray. I am afraid, that spray will separate coil cover from epoxy in case of bigger impact, which it is able to survive without spray between these materials.

[Altair]

Mylar tape coming from cable shield works great for me.

[kt315]

you have to use NOT good conductor but BAD conductor. mylar tape is GOOD conductor. guys study what is elecrostatic charge and discharge BETTER.
you have to have RESISTIVE conductor, 10-100 kOhm on 1 cm of lenght.

[Krzychu_]

It seems to me that electrostatic shielding (ESD) to prevent false signals from being discharged is important in both cases (PI and TR-IB), but it is more important to limit and stabilize the variable capacity that appears between the coil and the earth during the probe movement. The capacity affects the time constant in PI detectors and the amplitude and frequency characteristics (resonance) in the TR-IB detectors. The material from which the screen is made should have properties such that it does not generate too large eddy currents (average conductivity), decisive for the loss of energy but at the same time would be an "effective capacitor electrode" between the receiving winding and the mass of the detector.

[Koala]

Mylar tape coming from cable shield works great for me.

Check my posts in the IDX thread. Foil over or under drain wire connected in the search head or separate wires and only grounded on the PCB its was never 100%
Even then it deteriorated over about a year
Tried many times.

[dbanner]

you have to use NOT good conductor but BAD conductor. mylar tape is GOOD conductor. guys study what is elecrostatic charge and discharge BETTER.
you have to have RESISTIVE conductor, 10-100 kOhm on 1 cm of lenght.

Ah, so as to prevent static charge from building up in the first place, and only secondarily as a means of discharging static. A poor conductor implies resistance.
But now who can say what is ideal resistive properties of the graphite shielding. How many ohms per such an such. And how long(or short) drain wire make contact with the graphite shielding, longer drain wire contact make overall less resistance of the shielding.. And what about the gap? Is the shielding material frequency dependant(the limits to how much resistance per square area.
Seems like no exact science.
Conclusion: buy off the shelf coil and make life easier. Keep workshop clean.
"Excessive exposure to graphite dust over extended periods of time can cause a chronic and more serious condition known as Graphitosis, which is a form of pneumoconiosis. This condition arises when inhaled particles of graphite are retained in the lungs and bronchi"

[Krzychu_]

I present (I do not know if it's right) the illustration of what I wrote earlier, but when it comes to the conductive screen, in two attempts I managed to get a mixture of solvent-diluted wood varnish and powdered graphite, which, after a little careful application and drying, has resistances in depending on the thickness of the row layer 0.5 ... 1.5kOhm between the points of the meter spaced apart by 1 cm. The attempt to mix graphite with vicol wood adhesive, which is a water dispersion of polyacetate resin, ended in failure - the layer after drying almost does not conduct electricity and has a resistance of 20..35MOhm / cm, and polishing does not reduce this result. In the next step I'm going to apply a smooth, even layer with a paint roller and check the effect of detecting it with a metal detector.

[Altair]

you have to use NOT good conductor but BAD conductor. mylar tape is GOOD conductor. guys study what is elecrostatic charge and discharge BETTER.
you have to have RESISTIVE conductor, 10-100 kOhm on 1 cm of lenght.

It could be but I think we need to avoid Eddy current flows into the shield; this is the main goal. Shield material should not interfere with the EM field. It's make like a capacitor armatures; that are made always of a good conductor!