Crude coil sheilding paint

[DOOLEY]

hello all ,

sb , the combination of all the testing of different materials and on tx/rx or both would take a long time , but i'm sure among the collective on here most things have been tried , so we could simply post all our own views and go from there.thrn poss edit into a usefull list.

incidentaly , un-shielded PI coil , based on own experiance isn't affected that much from the ground , my signal goes slightly lower ( quieter ) as i aproach the ground , but it all depends on sample delay ,

long sample delay ( 30-50 uS) minor reduction in audio "click" / threshold setting

short delay (below 20 uS) big reduction in audio "click" / threshold setting ,slow click (5/sec) goes dead.

[maikl]

Link to Kev has set in post # 49 http://www.findmall.com/read.php?34,...370#msg-134370 provides answers for this topic.
Quote from Eric Foster:
"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 cant 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 materials 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 wont 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 doesnt 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 downwards, 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."

[Vortxrex]

If Lead works so well, why not use Lead Paint?

[DOOLEY]

hello vortxrex ,

not sure if lead paint is still available ,anywhere , something about it killing people , but i like your train of thought .

not sure that lead paint was ever conductive enough , amount of lead content was quite low .

[simonbaker]

Link to Kev has set in post # 49 http://www.findmall.com/read.php?34,...370#msg-134370 provides answers for this topic.
Quote from Eric Foster:
"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 cant 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 materials 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 wont 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 doesnt 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 downwards, 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."

This is a nice catalog of reasons which we can examine.

I think his points are good, but there is still a tendency to want to "have it both ways". We want our shields to let through our TX signals and RX signals, but also to block outside EMI.

I don't think you can have it both ways. I suspect that to transmit and receive signals, you also have to receive EMI of certain types.

He talks about two main characteristics of the shields that determine how they affect signals:

1. Shield geometry being parallel to signal "lines of force". (I think he means parallel to the "magnetic field lines", which are not what I think of as lines of force).

2. Frequency of the signals we are affecting. Blocked EMI is much higher frequency than the TX frequency.

Regarding (1), I suspect that if the shield's geometry does not block our TX signal, then it will not block EMI of a similar electromagnetic field orientation, and vice versa.

Regarding (2), the frequency argument (shields block much higher frequencies) sounds valid, but our tuned coils and MD electronics may block those frequencies anyway to a very high degree. Of course VLF and PI coils are tuned very differently, so his statements seem more accurate for PI coils.

Similarly, a tuned coil would not be expected to radiate any significant high frequency signal. Even a PI coil probably "chokes" high frequencies quite a bit. His radiation problem may have been directly from a circuit board trace, not the coil.

His argument about creating a constant capacitance to ground seems convincing, as different couplings to ground would probably modulate the TX frequency and amplitude somewhat. It is most convincing for the case of actually touching objects with the coils. Of course a Synchronous Detector is supposed to be somewhat impervious to phase/frequency changes in the TX signal, but clearly MDs are super-sensitive detectors, so it makes sense to suppress any disturbing influences.

If we are really serious about expecting shields to block EMI, then they should be highly conductive, because that is how shielding works -- induced currents canceling the electro-magnetic field of the EMI. However, if the EMI has a similar polarization to our TX signal, I wouldn't expect our shield to block it -- because the gap in our shield will prevent the needed current to block the EMI; except if the EMI is very high frequency, in which the gap is like a small capacitor and we get the current we need. But again, is high-frequency EMI ever a real problem?

Because high-resistance shields (graphite, etc) seem to be actually used in commercial coils, it implies to me that EMI blocking is not the real objective of coil shields. But perhaps high-resistance shields are only useful to one type of MD (PI or VLF).

It is probably advisable to divide shield discussions into VLF shields and PI shields until it is proven that both technologies benefit the same from any shield.

-SB

[maikl]

We could divide shield discussions into VLF shields and PI shields. Quote from Eric Foster refers to the PI detectors. Shield geometry being parallel to the magnetic field lines can be accomplished with enough thin strips lead, copper (suitable for soldered) ... Paint is not suitable for this.

[mikebg]

Graphite shielding

Graphite shielding
In the 1940s started the use of conductive water-based graphite dispersions for shielding. Minelab places inside search heads paper sheets covered with aquadag for coil shielding.
Search the WEB for "aquadag"
http://www.semicro.org/Aquadag%20E%20TDS.pdf

[detectormods]

When shielding your coils for Minelab detectors try to aim for 80 ohms per inch, this is the standard Minelab and coiltek values. If you look at the newer Nugget finder coils the resistance is close to 30 ohms per inch. The new range of commander coils are 700 ohms per inch. I suspect that it may be so high in order to get the fine gold mode working correctly on the gpx5000. The Q is fairly high on the NF 12" at 5 while the others range between 3.2 and 4.6

[petravka]

Whether this is good, and so on. I'm interested in how to apply the final layer of graphite?

[detectormods]

Why paint the coil shell? Paint the coil core as this will reduce noise as the coil is locked to its Faradat shield. Aim for 500pf or less shield to coil capacitance.

[petravka]

Whether the original MINELAB TS1000 graphite to the coil shell.

[Tinkerer]

Shielding capacitance

Why paint the coil shell? Paint the coil core as this will reduce noise as the coil is locked to its Faradat shield. Aim for 500pf or less shield to coil capacitance.

A spacer between the coil winding and the shielding greatly reduces the coil to shield capacitance.

What influence does the coil capacitance have?

Below is the coil current of a traditional PI, critically damped, with 100pF capacitance.

Take note of the coil current decay and the time it takes to reach near zero A.

Tinkerer

[Tinkerer]

coil to shielding capacitance

100pF capacitance

[Tinkerer]

Coil to shielding capacitance

Now, how does it look with 500pF capacitance?

Take note of the time it takes for the coil current to reach near zero A.

Also look at the coil current decay slope. See how it drops first and then goes up again. What this means is that the current generates eddy currents of one polarity in the target and then as the current direction reverses, it nulls these eddy currents and only when the current is on the final slope it starts generating the eddy currents that give us the target response.

Tinkerer

[detectormods]

Good graph Tinkerer,

500 pf including including the cable capacitance if a good yard stick for the Minelabs.

Have you opened up a ML coil to see that the winding is level with the bottom of the foam in order to place the windings as close as possible to the target.......but in doing so the ML winding is hard against the Shielding thus adding maximum c. Their testing must of told them that it is more important to gain 4mm of depth in relation to coil to target proximity than to reduce the coil capacitance and to optimize DV/DT.