Aluminium Coil shielding does not reduce sensitivity?

[Claude]

I saw in this forum a recommendation to shield the coil winding with aluminium tape. Why to do this? It will not reduce the sensitivity? How that a metal enclosure let the coil transmit and receive the radiation?
Could anybody explain this?
Thank you.

[Carl]

Re: Aluminium Coil shielding does not reduce sensitivity?

You can shield coils using several different methods. Aluminum tape is one, but all involve surrounding the coil with something conductive.

I don't completely understand why this does not affect radiation, but apparently a shield does not support eddy current generation as long as the shield does not form a complete ring. I suppose that locally, in a small "dL" section of the shield, eddies cancel and the shield appears magnetically as a high impedance.

EM gurus, of which I'm not, would have to answer this.

- Carl

[SID]

Re: Aluminium Coil shielding does not reduce sensitivity?

Hi Carl, I think that a perfect ELMG shield is parallel to lines of magnetic force and has no influence to the emitted field. This condition is, of course, impossible to keep track of, so in some places of the shield are eddy currents generated and the energy of ELMG field is reduced proportionally to the thickness and conductivity of the shield. The more, the ordinary shield behaves as a transformer open single turn, so the screenig effect is not perfect. In the sum - the sensitivity will be always slihgtly reduced and there always will be slight groung influence. Sorry. Sid

[Claude]

If this reduces sensitivity is there a good reason for it?

Hi guys,
Thank you for the answer. If I understand, this have some negative effects, like reducing sensitivity; then why use it at all? Only for electrical power lines interference or some other reasons also? What are the pro arguments?
Thanks

[Eric Foster]

Reasons for Shielding

Hi Claude and all,

Shielding is essential for PI detectors if they are to have good sensitivity for small metal targets. Careful choice of the material for the shield, and also the positioning of the shield with respect to the coil, will result in no loss of sensitivity, in fact I can’t think of any negative effects that proper shielding has.

As has been said, if the shield is parallel to the lines of force generated during the transmit period, and the material is thin enough, then there will be no measurable eddy currents induced in the shield. The other important factor is the shield material’s conductivity. The lower this is, the faster any eddy currents will decay, and provided they decay before the sampling period of the detector, then they won’t be seen.

A flat sheet of shielding, either above or below the coil, is not the best, although it is often used for convenience. A field plot of the coil will show that only immediately under the winding is the field parallel to the shield. Most of the shield therefore has eddy currents generated in it and it has to be made of a relatively poorly conducting material. This is why graphite and nickel paints are often used here. Again, it depends on how thick a coat you put on as to the decay time of these currents.

If you wrap the coil with a metallic tape, then all points on the tape are parallel to the field. The field being coaxial with the coil cross section when close to it. You can then use a more conductive material, and as it is completely enclosing the coil, the shielding efficiency is much greater. You do, of course have to leave a small gap between the start of the wrapping and the end, otherwise the shield will form a complete ring of the same diameter as the coil. This would give a very strong signal as the flux is totally threading this ring.

Aluminium tape, or copper tape will certainly work, provided it is thin enough so that cross sectional eddy currents are not developed. I prefer lead tape as it has a lower conductivity for a given thickness. Copper and lead also have the advantage that the shield grounding wire is readily soldered to it.

Why is shielding needed? The primary reason is to prevent the capacitance between the ground surface and the coil giving false signals. For an unshielded coil, this effect is particularly severe on a wet salt water beach. Touching the wet surface, seaweed, and on land wet grass, can all cause problems without the shield, particularly with sample delays that are less the 25uS. With a shield that is connected to the electronics ground, the capacitance that the coil sees is only that of the shield, and is therefore constant. PI detectors such as the Pulstar and Superscan that are designed for finding large objects at depth, do not necessarily need a shielded coil. This is because the minimum pulse delay is greater than 25uS where the effect becomes much less noticeable.

The second purpose of the shield is to attenuate r.f. interference from various broadcast and other transmitters. It doesn’t get rid of it completely, as the shield is nowhere near as efficient as an aluminium enclosure, as it has to be thin enough to not cause attenuation of the wanted object signals. Also, low frequency r.f. signals from about 500kHz will not be attenuated to any degree. If they were, then you would start to lose sensitivity. Power line interference will also not be attenuated, and be just the same as for an unshielded coil.

A third purpose of the shield, which has a greater importance in these days of EMC, is to prevent any spurious emissions from the detector electronics causing interference with other electronic equipment. In the European Union, radiated emissions are measured from 30mHz upwards, to at least a 1GHz. With an unshielded coil a standard PI will likely fail this test, not because the transmitter itself is generating frequency components of this order, but other parts of the circuit, such as the clock generator, do have fast edges that leak into the coil circuit. I had the problem once where a 555 timer on the board was radiating sufficient energy via some over long pcb tracks on the output pin, to cause the detector to fail the test. A properly shielded coil and a well laid out pcb should have no trouble passing existing emissions tests, even when using a fairly high power pulse transmitter.

Eric.

[Pepso]

Re: Reasons for Shielding

Strange, I have Tracker PI without coil shield and the sensitivity for small targets is so high, that I have sometimes problem to see and find target in upcast. The coil diameter is 18 cm.