coil shielding

[Spyke]

I have a question about coil shielding.
I plan to use copper-tape for shielding.
My question is, should I cover the complete coil or only surfaces not faced to ground?

[ripsdevala]

Complete coil with a gap

[Spyke]

Ok, thanks.
Whats the gap for?

[Qiaozhi]

Ok, thanks.
Whats the gap for?

If you don't have a gap, the shield will act like a shorted turn on a transformer, and your detector will lose all sensitivity.

[Ferric Toes]

I have a question about coil shielding.
I plan to use copper-tape for shielding.
My question is, should I cover the complete coil or only surfaces not faced to ground?

If you are looking for delay times of less than 20uS, copper tape might not be the best, as even with a gap, you may get excessive signal from the shield. Properly damp the coil with no shield and if you have a scope, look at the output of the preamp and see how fast the waveform recovers to a flat baseline. Then put on your shield and then observe how much the recovery time has increased. You can get some improvement if you have the shield tape flat around the side of the coil, fold one edge around the coil to the inside and then a strip of insulating tape on the first fold, so that when you fold the second edge over the first, the two edges are insulated from each other. If you wind the tape helically around the coil, then normal copper EMC tape will be too conductive and ruin the recovery.

A flat shield should be on the ground side as it eliminates the coil to ground capacitance effect, particularly on wet ground or wet beach. It will do little to reduce r.f. pickup, whereas a shield around the coil as described above, will do both. The best material for a flat shield is graphite of nickel conductive paint. A solid metallic flat shield, even with a gap, will give a large signal.

Eric.

[ripsdevala]

Mr.eric sir, for easy understanding, do pi detectors need a shield? Some says yes and no.
My xperience with shielding is always reduce the deep, and what would happend if i shield my coil but i not connect it to gnd side of the battery? In this case i found it has no different, whatever i connect to the gnd or cut the line, all test is do above the ground(air).
I put shilding (alu foil) roll and roll over the coil, not make a long strip from the outside and inside of coil circle, is this the right way of shielding?

.....Should I shield it?

Arief

[Spyke]

If you are looking for delay times of less than 20uS, copper tape might not be the best, as even with a gap, you may get excessive signal from the shield. Properly damp the coil with no shield and if you have a scope, look at the output of the preamp and see how fast the waveform recovers to a flat baseline. Then put on your shield and then observe how much the recovery time has increased. You can get some improvement if you have the shield tape flat around the side of the coil, fold one edge around the coil to the inside and then a strip of insulating tape on the first fold, so that when you fold the second edge over the first, the two edges are insulated from each other. If you wind the tape helically around the coil, then normal copper EMC tape will be too conductive and ruin the recovery.

A flat shield should be on the ground side as it eliminates the coil to ground capacitance effect, particularly on wet ground or wet beach. It will do little to reduce r.f. pickup, whereas a shield around the coil as described above, will do both. The best material for a flat shield is graphite of nickel conductive paint. A solid metallic flat shield, even with a gap, will give a large signal.

Eric.

Thanks, very interresting!
graphite of nickel conductive paint - does it mean ESD paint?
If so, I have tons of ESD paint at work.

Marc

[Spyke]

or better this one:
http://www.kontaktchemie.com/koc/KOC...=all&plang=all

[CDI]

or better this one:
http://www.kontaktchemie.com/koc/KOC...=all&plang=all

I have that Graphit 33, must say it's realy good.

[baum7154]

The best material for a flat shield is graphite of nickel conductive paint.

Eric.[/QUOTE]
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I am wondering what 'graphite of nickel conductive paint' is or is this a typo? Perhaps it was intended to be 'graphite or nickel conductive paint'?

Thanks,

Dan

[Ferric Toes]

You are correct. Yes, it is a typo.

Eric.

[woodbob123]

I have used graphite on my last coil but due to a house and country move not been able to test, first I covered sticky tape (sticky side) with brushed on graphite wrapped coil (mono for chance) graphite side up over this a (spiral spacing 1inch) winding of a single strand of wire thinnest I could find from cable shielding, over this another wrap of graphite covered tape graphite on inside then a wrap of insulating tape. As I've said not had chance (pun) to test it yet but would be interested on your thoughts.

[Sean_Goddard]

Nickel paint works VERY WELL but is a pain as it is HORRIBLY TOXIC to work with and eats steel if you get it on it so BEWARE. One coat of THIN nickel on a Surfmaster PI coil make the coil fully immune to just about everything. I'm getting 14" in air on a 1p (1c) and around 16" on a £1, 18" on a £2. I also added a snumbber to the SMPI to kill the overshoot, tweaked the delay time and sample seperation times (1st sample to second sample). This is now a seriously good machine so I've sold all my commercial beach machines (except my ETrac and the 18X15 SEF). That is just WAY too good!

[Ferric Toes]

Nickel paint works VERY WELL but is a pain as it is HORRIBLY TOXIC to work with and eats steel if you get it on it so BEWARE. One coat of THIN nickel on a Surfmaster PI coil make the coil fully immune to just about everything. I'm getting 14" in air on a 1p (1c) and around 16" on a £1, 18" on a £2. I also added a snumbber to the SMPI to kill the overshoot, tweaked the delay time and sample seperation times (1st sample to second sample). This is now a seriously good machine so I've sold all my commercial beach machines (except my ETrac and the 18X15 SEF). That is just WAY too good!

Was it not so in early Surfmaster coils that the plastic shells were themselves conductive and acted as a shield?

Interesting about the Nickel paint being toxic. I presume it is the solvent MEK. One small problem I noticed was that there was a low level decay curve similar to that from weak magnetic ground. Then I remembered that Nickel has magnetic properties.

Eric.

[totob]

What about using a copper braided mesh as shielding in VLF metal detector?