Optimum coil shielding method/material

[woodbob123]

The other metod i was thinking about uses graphite powder.

Lay out a length of wide tape sticky side up sprinkle graphite powder on thickly either fold over trapping graphite between, or use 2 lengths of tape stuck together with graphite between. Copper shielding tape with conductive adhesive at each end to make a connection and confirm continuity. May be a good idea to go over with a hot iron to fuse glue and powder. If it works would be a clean way to use graphite.have to buy a pack of pencils to grind up before i can try this.

[Farside]

Farside, there will be metal.
It is just between two layers of thin plastic///
S

Yeah, the problem is connecting to it... I tried alligator clips and just pressing the multimeter probes into it but they just go through eventually.

I just went back and had another play with the space blanket. I folded the mylar end over a few times, clamped it with a crocodile clip, pressed it shut with a pair of pliers and with a bit of jiggling, I got a resistive connection :-) All I need to do now is figure out how to make a reliable and permanent connection...

[Farside]

Apparently, not all VHS tape is conductive

[simonbaker]

Yeah, the problem is connecting to it... I tried alligator clips and just pressing the multimeter probes into it but they just go through eventually.

I just went back and had another play with the space blanket. I folded the mylar end over a few times, clamped it with a crocodile clip, pressed it shut with a pair of pliers and with a bit of jiggling, I got a resistive connection :-) All I need to do now is figure out how to make a reliable and permanent connection...

Maybe try a different brand space blanket.

I had no problem connecting to the one I used, but the conductivity was limited in distance because creases in the blanket caused serious continuity breaks. These might not be too disasterous if you will be winding the ground wire around the shield anyway, but I did worry that some sections might not be shielded.

Maybe some brands are better to use than others.

-SB

[Farside]

Instructable on making conductive glues and paints:
http://www.instructables.com/id/Cond...nt-and-Conduc/

[satdaveuk]

If you going to buy graphite use synthetic type as its 99,5% min carbon unlike pencil lead which is about half the amount.
Up to now 1k spread over a meter aprox hospital paper bandage 50/65 mix with bog standard PVA bond with a dash of Acatone to thin it, messy job as the powder is so fine even breathing on it sends it everywhere until mixed with the other substances. (wear gloves cause gets into paws of your skin, then you end up conductive lol)
Shielded a project case made out of styrean, copper strips at each corner plus where ever mounting pots etc, brushed 3 coats of graphite mix all over and seems to do the trick.
I think the 1kohm overhall resistance from end to end is the magic figure for the TGSL coil.
For one reason here where i live once going much above that the detector starts reading the ground, lower starts reducing sensitivity.
The hospital paper type tape is the ticket because 1 it absorbs the shielding mix secondly it can be easily taken off if not sucessfull.

Its early days lots more tests to do, up till now its the best ive had far as shielding.
The rubber type glue I read in various places suposed to be ok, but not tryed it because over here its to exspensive to buy just for experiments.

Regards

[eaglewings35]

No Shielding?

Monk, I think your term "PI coil" means "Monocoil". I have a wideband (PI) metal detector, which operates much better than the conventional PI with Monocoil sensor, because my search head contains IB (induction balanced) TX and RX windings.
Coil shielding involves a serious problem for induction balance in wideband metal detectors (PI). The eddy currents in coil shielding and in cable shielding make impossible to adjust whole induction balance for all frequencies in the operating spectrum.
The best design solution for an IB search head is NO SHIELDING.
To avoid shielding in electronics is used circuit symmetry. For example we can make the RX winding with center tap and connect it to ground, but more easy is to use full differential front end or a single preamp with input symmetry. Look how simple is the circuit of front end in a TWIN LOOP metal detector when we use amplifier with input symmetry, for example LM386.

Mike, I like your explanation in this post. I am currently building my HammerHead PI Detector. I was looking for directions on making my search coils. I want it to be as optimized as possible, and detect as deeply as I can. I need to be able to also find
the small nuggets too. I know this is asking alot, but I also know that a large chunk of a detectors performance is in the coils. Could you email me? Thank you

[mikebg]

Mike, I like your explanation in this post. I am currently building my HammerHead PI Detector. I was looking for directions on making my search coils. I want it to be as optimized as possible, and detect as deeply as I can. I need to be able to also find
the small nuggets too. I know this is asking alot, but I also know that a large chunk of a detectors performance is in the coils. Could you email me? Thank you

Hi Kerry,

Designers use symmetry relative to ground to avoid shielding and interferences. The idea is not mine. For example, 19-th century starts use of circuit symmetry in telephone communications. I know two PI projects with coil symmetry: TWIN LOOP by Crone's:
http://www.geotech1.com/cgi-bin/page...loop/index.dat
and Twin coil PI by Gary Chemelec:
http://www.geotech1.com/forums/forumdisplay.php?f=15
Read them to understand how search head is made.

The idea for induction balance in PI metal detectors also is not mine. Even ancient wide band metal detectors designed in 19-th century use separate TX and RX windings placed in induction balance to increase modulation index of target signal.
Symmetry and induction balance are used in search heads for demining - see US patent 3,002,262.

[Davor]

IMHO the truly balanced Rx preamp with a truly balanced coil, either a center tap or otherwise, will surely be a better choice than shielding.
A balanced Rx frontend is hardly a well understood concept. There are a few implementations seen around here. One is mikebg's using LM386, and the other one is a Tinkerer's solution using SSM2019. Main point is having a finite, symmetric, and not too high input impedance for both common mode and differential signals.

Other possible options would include a center tap coil, THAT1510 family, cross-coupled (normal) opamps etc. Hardly any of these find their way into MD projects, and its a pity because it really frees you from shielding coils.

[Aiko]

Guys, what about no shielding at all? Do you know what shielding method is used from the leading companies? I do not see any shielding here: http://www.youtube.com/watch?v=h9iZkj81tmE

Check the NEL coil on the picture. Do you see a shield over the windings?

[Qiaozhi]

Guys, what about no shielding at all? Do you know what shielding method is used from the leading companies? I do not see any shielding here: http://www.youtube.com/watch?v=h9iZkj81tmE

Check the NEL coil on the picture. Do you see a shield over the windings?

Crossfire coils from Garrett have an electrostatic sheet over the coils. It's not wrapped around them. The Garrett Ace detectors are probably using a graphite shield, and I expect the NEL coil does the same.

[golfnut]

Most munufacturers of VLFs simply coat the shell with carbon ink / paint. Pop windings in, null etc. Pot with carbon/graphite epoxy then top up the fill with regular potting compound.

S

[Qiaozhi]

Most munufacturers of VLFs simply coat the shell with carbon ink / paint. Pop windings in, null etc. Pot with carbon/graphite epoxy then top up the fill with regular potting compound.

S

That's what I meant by "graphite shield".

[Aiko]

OK, but I did not saw anything of this in this video: http://www.youtube.com/watch?v=h9iZkj81tmE

[Qiaozhi]

OK, but I did not saw anything of this in this video: http://www.youtube.com/watch?v=h9iZkj81tmE

Not everything is revealed in the video, as they don't want to give their secrets away. For example, did you see them balance the coils?