Optimum coil shielding method/material

[Aiko]

I read somewhere in the russian forums that the shielding is necessary, only for verry dry conditions, where the ground is charged with static electricity.

[satdaveuk]

I read somewhere in the russian forums that the shielding is necessary, only for verry dry conditions, where the ground is charged with static electricity.

Hi Aiko
It really depends on the ground minerals and whats around you to whether you need shielding.
I can really only ever advise on whats required here in England as I have not been fortunate enough to be able to tests in other country's.
Maybe in Russia they don't need shielding, you need advise from some of the Russian lads here.
What I do know is here in England with are types of soil, with a typical induction balance format or pulse induction you need coils shielded .
Whats more with my experiments of late, if your in a location apart from the middle of no where, the back end of the detector IE electronics also need to be shielded for optimum results .
Regards

[maikl]

I read somewhere in the russian forums that the shielding is necessary, only for verry dry conditions, where the ground is charged with static electricity.

It is best to put the shield on the coil. If the shield done properly, it will not interfere with the detector. And if you run into the ground where the shield is necessary (which in most cases), detector will be useless.

[Davor]

Just careful intervention to Rx front end to be capable of accepting balanced input, and centre tapped Rx coil does the trick beautifully. It works for me. Unfortunately you can't expect any of the commercial rigs or coils to follow this path ... yet.

[Aiko]

Guys, is there any objective reason to make a graphite shield instead from a thin aluminum foil. I think that the foil shield is much more technological and clean method. Can we expect any improvement in the sensitivity of the detector if we use a graphite shield? If there is no any advantage from the graphite why we should make our live harder? As an objective reason I mean some real experiments and comparisons between two methods.

[Davor]

That's a kind of Schrödinger's cat thought dilemma. You can say that by introducing a load of material that you intend to measure into a measurement theatre, you'll lose some information of the very measurements objects. Because the shield is always stationary against the coil, you can also say that it's response is not changing with waving a coil (it is cyclostationary), and hence it will not make difference. In most IB rigs it will not, provided you shielded both Rx and Tx coils, and the net effect is just a few degrees rotation of phase response, which is fixed and hence easily compensated. In my simulations I found that foil can reduce response of foil targets by negligible percent - nothing to worry about.
In PI it makes the coil slower. Foil's tau is ~10us, hence your first sample is hurt by foil. Graphite is much more resistive and hence it's tau is below 1us.
Because shield is fighting electric fields, and there you have high impedances, the shield conductivity is irrelevant, hence graphite is a better shield.

[Aiko]

Just careful intervention to Rx front end to be capable of accepting balanced input, and centre tapped Rx coil does the trick beautifully. It works for me. Unfortunately you can't expect any of the commercial rigs or coils to follow this path ... yet.

Hi Davor, would you explain more detailed what exactly you mean. My English is not very good and can not understand well what exactly you want to say.

[Davor]

Static E-field pickup can be cancelled by shielding, by balanced signal path, or both. To learn more about balanced signal paths, see about professional microphones, and their preamps.
Most of the commercial coils are designed for single-ended operation (signal wire + ground connected to shield), and they can't be convinced to work properly as a balanced signal source (two signal wires in counter-phase, with or without a ground wire).

The advantage of single ended front end (and signal source) is ~3db less noise for same quality of components, and simple configuration, but there is no protection against static other than a shield.

A balanced front end has 3db more noise than the single ended one, and is a bit more complicated to design using same quality components, but it provides common mode (static) rejection. It can be even better at rejecting static by using a centre tapped coil, where a centre tap is grounded. It can be also shielded, but my whole point is that balanced front end + a centre tapped coil work just fine without a shield.

[Aiko]

Thanks Davor! What decreasing in the detection distance in percents, we can expect, if we use an aluminum shield instead a graphite one? I ask only for an opinion.

[Davor]

Just about 0%. I found some little percent in linear scale, and considering 6th power law in detection the difference comes to nothing. In PI machines it is a bit different though.

[Aiko]

Thanks Davor! I also suspected this, because I know for sure that some of the best Detech coils are shielded with an aluminum foil.

[dimovnis]

Hello. I bought the graffiti, and make a new coil. I wonder if spray it on the first on the wire or need first to make housing for the probe and then to inflict graphite spray. I apologize to my bad English

[ZBIQ]

Ive seen patent, where graphite was sprayed on some cardboard and then fitted in coil housing.
Once I used graphite shield with about 1k resistance from point to point in diameter.

[golfnut]

In post 55 in the Nel jpg you can see the graphited sticker form in the central region - its the mat black stuff that looks a bit wavy or rippled.
S