Theories about coil-shielding

[rickb]

Carl:
In reference to graphite paint, is there any published data on how low the sheet resistance must be to generate enough eddy current to cause the shift?

Likewise, is there any published data that shows how high the sheet resistance must be before electrostatic shielding becomes ineffective?

Rick

[Krzychu_]

The theory is that with perpendicular coils they should not interfere with each other so maybe a spiral wound screen from a thin, non-magnetic wire would be the best solution in this situation (?) - A bit time-consuming and laborious, but I think I need to check it ...

[Carl-NC]

Carl:
In reference to graphite paint, is there any published data on how low the sheet resistance must be to generate enough eddy current to cause the shift?

Likewise, is there any published data that shows how high the sheet resistance must be before electrostatic shielding becomes ineffective?

None that I know of, for either.

[Hyena]

Correct, and you want to minimize that. Ideally the shield should be perpendicular to the flux so that the cos(Θ) in Faraday's Law is 0. But there will be small losses, and this is evident with VLF coils when the graphite-coated bottom is applied and the null shifts. It is evident in PI coils when the decay curve shifts.

Should it be really perpendicular? I thought that electric field is always perpendicular to magnetic. So I suspect that shield should be parallel to magnetic field to get less energy. This way it should be bettet to wind tape around each coil, because each shield will get only field from oposit coil. But from your comment it is oposit and it mean, that is actualy better to shield entire coil with two flat surfaces (covers).

[Carl-NC]

Sorry, yes, parallel not perpendicular. Duh. You want no flux lines cutting through the surface of the shield.

[Krzychu_]

I think that the key is the fact that in closed loops they induce current and due to their flow, power is lost, while in open ones only voltage is induced and power is not lost. The smaller the conducting area through which the field permeates - the smaller the chance of inducing eddy currents in it, but this is only my theory that is not supported by practice
- Perhaps the best shielding would be a thin insulated winding cable around the main receiving coil and connected to the ground only on one side or in the middle, if the winding of the screen was wound on the contrary in two symmetrical cuts (?)

[Melbeta]

Carl, your response regarding perpendicular placement, puzzles me a bit. Hard to grasp. Here is the definition of perpendicular.

Quotation: "Perpendicular, a line at right angles to another line or surface, at right angles to a horizontal line or plane, upright, vertical.''

Now I can understand a shield placed on top of the bottom cover of the coil. It appears to be parallel to the coils. Am I wrong? Perpendicular to my thinking would be vertical not horizontal. Could you explain this concept so I can move on with the perpendicular?

Now I know that Compass Electronics did extensive experimentation with placement of shields, some being under the coils, some being on top of the coils. I do not understand the placement on top of the coils. Then I even have one coil of theirs, and it has a dual shield, one above the coils, the second below the coils. I think both shields have to be interconnected into one large sheild. Right?

How does one place a shield so its conductive lines are not at variance with the lines of the coils??? I never thought of control of the lines inside a shield...

And I do not know the proper definition of FLUX. Could you elaborate on that word?
Melbeta

[Hyena]

Parallel to coils means perpendicular to magnetic field.

So better to directly shield perfectly circular section of coil. But with graphite or something low conductive. Any inperfection in shape is worse becaufe distance from coil is quite small and in fields distance is often squared. Any inperfection in coil-equidistant shape is multiplied by 1/distance squared.
It is nearly impossible in DIY enviroment. Easier to graphite-shield cover. Is my guess any close to reality?
Is possible to measure loss anyway?

[Carl-NC]

Carl, your response regarding perpendicular placement, puzzles me a bit. Hard to grasp.

Hyena pointed out the error, I meant to say parallel to the lines of flux. Magnetic flux encircles the wire bundle, lots of info on this in your favorite search engine.

For parasitic capacitance, a top shield might help in tall wet grass but not much else. But shielding also helps with EMI, so 100% shielding is generally better than 50% shielding.

[Carl-NC]

Parallel to coils means perpendicular to magnetic field.

So better to directly shield perfectly circular section of coil. But with graphite or something low conductive. Any inperfection in shape is worse becaufe distance from coil is quite small and in fields distance is often squared. Any inperfection in coil-equidistant shape is multiplied by 1/distance squared.
It is nearly impossible in DIY enviroment. Easier to graphite-shield cover. Is my guess any close to reality?
Is possible to measure loss anyway?

Joe Rogowski's method of wrapping the coil with spiral-wrap spacer followed by shield material seems to be a popular DIY method. It's easier for a one-off but harder for mass production. IMO, graphite shielding is harder.

You can measure loss by building 2 coils, one with shielding and one without.

[Krzychu_]

Some theory about eddy current loses:

http://www.ti.com/lit/ml/slup197/slup197.pdf