Another PI coil shield method.

[6666]

Hi, no I have not tested the copper foil on a coil yet, maybe in next few weeks, but give it a try your self.

[punagold]

6666,
Have you have success with the copper tape as a shield on your pi coil? I was going to order some when I came across this thread.

[6666]

Hi John
no have not tested the amount needed to wrap around coil yet, only the 50 mm strip, and whole coil.
this stuff is very thin.

[johnandles]

Just tested a 2 inch or 50mm strip with PI, and not detected, however the WHOLE roll could be.
more to come.

Did you test the amount you you would need to wind around your coil?
I wonder if that would be detected.
Cheers
John

[6666]

I wonder if the cross section shorted section is what is causing so many people to have trouble with foil shielding ?

I am thinking maybe a thin copper tape spiral wind around coil but winds not touching ?
http://www.ebay.com.au/itm/New-Arriv...item2333329955

Just tested a 2 inch or 50mm strip with PI, and not detected, however the WHOLE roll could be.
more to come.

[6666]

Cut a piece 5mm, 10mm, 20mm, 50mm and 100mm long and mount on a piece of cardboard and wave under the coil.

Hi Joseph, I have ordered some of that copper tape and when it arrives will test using your method thanks.

[bbsailor]

I wonder if the cross section shorted section is what is causing so many people to have trouble with foil shielding ?

I am thinking maybe a thin copper tape spiral wind around coil but winds not touching ?
http://www.ebay.com.au/itm/New-Arriv...item2333329955

The ultimate shield test is to make a small, unshielded, fast, test coil to see if various length samples of the shield are detected. Cut a piece 5mm, 10mm, 20mm, 50mm and 100mm long and mount on a piece of cardboard and wave under the coil. The cardboard prevents your hand from being detected on an unshielded coil. Typically, when you want to use delays below about 10us you will need to use a thin coil solid wire or very thin stranded coil wire (about AWG32 stranding). Tinned stranded wire is better than silver coated wire as silver is too conductive between strands and looks more like a small target.

I hope this helps?

Joseph Rogowski

[6666]

Cut the Scotch 24 mesh 1.18 inches wide. Wrap around the coil but make sure that the wire mesh does not touch and form a cross section shorted section that will cause eddy currents to form.

I wonder if the cross section shorted section is what is causing so many people to have trouble with foil shielding ?

I am thinking maybe a thin copper tape spiral wind around coil but winds not touching ?

http://www.ebay.com.au/itm/New-Arrival-5mm-x30M-Copper-Foil-Conductive-Tape-EMI-Shielding-Adhesive-Barrier-/151182809429?pt=AU_Instrument_Accessories&hash=item2333329955

[6666]

the target used was a uk pound coin, the coil shells used are dons(idx dd type), so i assume 9" round, the barracuda tested was the original anchor spec 9v, my other barracuda(silverdog spec) with the fet and running at 11us(from the pre amp out) running on 12v can detect the pound at 10.5" with the shielded coil, with the unshielded one 11" but the unshielded one is terribly unstable.
though to be fair the two were tested on different days, and the anchor barracuda is non motion, where as the silverdog one is motion, this may effect the results, either way i'd be happy with 10 inches on my favourite beach, as it has remains of a crude oil spill in the hard pack at about those depths and deeper.

Thanks for the info with the pound coin, I now have 2 in my collection will try them next time at the beach.

[mickstv]

Try using Scotch 24 a tubular wire mesh shield that is 1 inch wide but two layers folded flat
Joseph Rogowski

Hi Joseph, thanks for the reply and extra info. I should point I used the shielding material from my first post because I couldn't find an local supplier for Scotch 24.

For Everyone

I have two coils here now both wound exactly the same, one with the mesh the other without any shield. Both coils decay at the same time.



I should also mention I did try aluminum foil once and it was hopeless, the aluminum foil added at least 2-3us to the decay, which isn't very good when your looking for small targets.

[sinclairuser]

Interesting comments on the barracuda and Andy's coil
just so we can compare apples with apples
You say you can get 12 inches(30cm) on a buried target at the beach, what target did you use ? what diameter was the coil ?
Is your barracuda modified or stock kit with fet ?
what voltage are using, 12V ?

the target used was a uk pound coin, the coil shells used are dons(idx dd type), so i assume 9" round, the barracuda tested was the original anchor spec 9v, my other barracuda(silverdog spec) with the fet and running at 11us(from the pre amp out) running on 12v can detect the pound at 10.5" with the shielded coil, with the unshielded one 11" but the unshielded one is terribly unstable.
though to be fair the two were tested on different days, and the anchor barracuda is non motion, where as the silverdog one is motion, this may effect the results, either way i'd be happy with 10 inches on my favourite beach, as it has remains of a crude oil spill in the hard pack at about those depths and deeper.

[6666]

Interesting comments on the barracuda and Andy's coil
just so we can compare apples with apples
You say you can get 12 inches(30cm) on a buried target at the beach, what target did you use ? what diameter was the coil ?
Is your barracuda modified or stock kit with fet ?
what voltage are using, 12V ?

[bbsailor]

Hi Thomas, well all I can say is it isn't tinned copper, I checked the suppliers website but it doesn't mention the braid type. I suspect its aluminum braid.

All I know is I can't detect it with my PI, but metering it, it has zero ohms resistance for the length I used.

Also I tested the coil before and after, adding the shield didn't increase the decay time, but it did stop hand capacitance.

Try using Scotch 24 a tubular wire mesh shield that is 1 inch wide but two layers folded flat. Cut it along the seam to make a length that is 2 inches wide. Measure the cross section circumference of the coil. A 1/8 inch wire bundle should have a foam spacer with a low dielectric constant. Assume the spacer is 1/8 inch thick so now the cross section wire bundle with spacer is 3/8 inch or 0.375 inches. 0.375 times pi or 3.14159 equals 1.18 inches. Cut the Scotch 24 mesh 1.18 inches wide. Wrap around the coil but make sure that the wire mesh does not touch and form a cross section shorted section that will cause eddy currents to form. Use electrical tape along one edce even if the mest overlaps a little the tape will keep the ends form touching. Ensure that there is a small opening where the coil wires exit the coil to prevent making a full conductive ring around the total coil circumference.

Make very small solder joints using thin wire or be creative and make the joints a few inches above the coil housing.

The benefit of using the wire mesh over a full area shield is that the mesh has less coil-to-shield capacitance and thus makes a potentially faster coil. Faster coils only work on PI machines that allow low minimum delays at or near 10uS. Typically a faster coil will use a higher value damping resistor due to the coil making techniques that reduce the total capacitance in the TX circuit including: the coil itself, the shield, the coax, and the MOSFET COSS.

Joseph Rogowski

[sinclairuser]

before i tested silverdogs pi coils, i would have said don't use aluminum foil in pi coils, but that's what he used in his pi coil, and i also have another identical coil exactly the same except this one does not have the foil shield (he wanted them both testing to see if it was needed).
my conclusions are that it works perfectly fine, it does reduce the depth on the detector tested, the depth reduction was 2.5 cm, now on a vlf design a 2.5 reduction would matter, but here's the thing, the actual depth on the barracuda is-
unshielded silverdog coil- 12 inches(30cm),
shielded coil- 11 inches(27.5cm),
so, on the designs were are working on for mostly beach work, it does not really matter, since at 11 inches its way down into the dirty hard pack anyway, the depth reduction in this case is worth it, the barracuda is like a different detector with the shielded coil.
now the unshielded coil works perfectly fine, on a beach in the middle of nowhere, but in an urban environment like your workshop its as skittish as hell falsing, random beeps etc.
the shield removes all that making the detector real easy to setup and fine tune, once done the unshielded coil can be used at the beach for instance, so in terms of andy wanting to know if a shield was a good idea on coils made to be sold along side the pi kits, its a no brainer especially on the barracuda, so my advice is add a shield, what we need to do is stop arguing about the best material to use, and find a decent repeatable method, so noobies can simply add it when they build coils, my tests have shown me that:-
1 a shield IS needed,
2 the depth reduction in a PI is neither here nor there,
3 setting up a home built pi is easier when the shield is present,
4 super fast low ohms unshielded coil does work well but only away from civilisation.
so to sum up a shield is needed, foil works(even on a pi), the only caveat is the connection to the wire(fine copper foil may make connection easier).
i dont know how andy connected the foil to the wire, as the coil is potted, but it works well, my only niggle with his coils has nothing to do with the operation of them, its the fact that they are fully potted, they need micro balloons or packing under the compound, as for me i find them a little heavy.
thanks,
ally.

[joop]

Thanks for sharing the pdf, mickstv. I will have a go at it!