Another PI coil shield method.

[kt315]

Wow, you're a trip, Mickstv.

[Carl-NC]

KT, Mick is right about ground capacitance.
KT, you are also right about electrostatic charge.

[kt315]

thank you Carl. I suggest for the people to open 246 page of INSIDE THE METAL DETECTOR and read ELECTROSTATIC SHIELDING chapter.

[kt315]

[mickstv]

How about you post up that complete section that you have purposely blurred out, so everyone can see/read the whole story.

[kt315]

sorry Mickstv. I have not possibility to post complete text because the book is copyrighted.

[Carl-NC]

KT, Mick is still right about ground capacitance. I didn't specifically spell this out in the book but it is one of the problems shielding helps minimize. Sorry if you don't understand this, so perhaps it's best to let it go.

[mohandes]

KT, Mick is still right about ground capacitance. I didn't specifically spell this out in the book but it is one of the problems shielding helps minimize. Sorry if you don't understand this, so perhaps it's best to let it go.

hi
this shielding is ok or no ??tinned copper wire OK or this tiny foil ??guide me
regards

[6666]

I have just opened a length of outside cable tv cable to see whats inside, the shielding could be worth a try, not sure what metal it is yet because the inside of the cable has this thick sticky goo which has gunked it all up, need to find a way to clean it off.

[joop]

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

[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.

[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

[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 ?

[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.

[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.