PI coil shielding.

[DanZ]

I'm starting a PI build and doing a lot of reading concerning the shielding of the coil. I'm going to try a 9.5" coil first that I just wound and try others later. I'm seeing where some folks go without shielding and are fine while others suggest a Faraday shield and the multiple ways of applying them (wrapped foil, spray conductive coating, etc.). I understand the reasons for needing a shield (for the rx'r) but never having and using a PI just wondering how/if the performance is affected from a grounded shield around a pulsing coil. Wouldn't this reduce the transmitted pulses strength?
Also, looking through dozens of schematics I usually see shielding included with the BFO's and VLF's but haven't seen one (yet) drawn for a PI circuit.


Thanks and regards.

[baum7154]

The first question is what do you seek with your detector, small gold (1/2 gram or less), only coins, old artifacts? Small gold is one of the toughest to detect and requires a very fast coil likely less than 8" diameter. Where are you likely to be searching, at the beach, near saltwater, highly mineralized ground? again these conditions will influence the need for shielding. What coil pattern are you building, spiral, basket weave, spider coil, bunched wire. FYI my present coil does not have a separate shield but is built to search for small gold in mineralized ground.

The best shielding I have used is a painted on mixture of graphite applied at a resistance in the range of 1000 to 2500 ohms/inch. (Look at the threads by Satdaveuk) I have seen shielded coils lose some sensitivity even using this type of shielding. I have tried the thin metal foil shields applied properly with a gap ( Faraday shield) but they had severe impact on sensitivity. At this time I will only use graphite shielding on my coils until something better is discovered.

[DanZ]

Thank you. Good to know. My searches will mainly be inland, countryside, for coins and jewelry type of things. I have some experimenting to do.

Regards.

[baum7154]

Thank you. Good to know. My searches will mainly be inland, countryside, for coins and jewelry type of things. I have some experimenting to do.

Regards.

Knowing your targets and terrain it is clear that you don't need to squeeze every last microsecond out of your detector and coil. This frees you up on shielding a bit as well. You have probably heard that if your detector coil can 'see' the material you intend to use as a shield you should not use it. In this vein the Scotch 24 shielding tape made of fine thin wire is often recommended, but gold detectors operating at the minimum microsecond delays usually can see this stuff and that is not good.. Another test is that if your hand is detected by your detector it needs shielding.

Good luck

Dan

[bbsailor]

Knowing your targets and terrain it is clear that you don't need to squeeze every last microsecond out of your detector and coil. This frees you up on shielding a bit as well. You have probably heard that if your detector coil can 'see' the material you intend to use as a shield you should not use it. In this vein the Scotch 24 shielding tape made of fine thin wire is often recommended, but gold detectors operating at the minimum microsecond delays usually can see this stuff and that is not good.. Another test is that if your hand is detected by your detector it needs shielding.

Good Dan

luck

Dan,

Here is a little tip about using Scotch24. Scotch24 comes in a 1" wide flat, 2-layer tube-like shielding that should be cut to accomodate your coil design. If you cut the Sccoth24 along it's two folds, you will have 2 one-inch wide pieces and each piece should be long enough to make almost (there must be a space) a complete loop around the coil. A 1" wide piece will fit a coil bundle that is 1/3.14159 or .31831" in diameter. Use clothes pins to hold it on while applying a layer of spiralwrap. This will form a lower capacitance than a solid shield material as the area of the mesh is about half as much. Make sure that the Scotch24 mesh does not overlap or short when going around the wire bundle. This will minimize eddy currents from circulating in the shorted mesh area.

This should help.

bbsailor

[baum7154]

Joe,

Thanks for the tip. I'll look into it tonight. As I recall the material is a flattened tube of mesh and what I think you have said is to cut along the crease on each edge of the flattened tube to produce 2 each - 1" wide by X inches long pieces of material. In the case of my coil the cross section of the coil is approximately 1" diameter or 3.14" in circumference so I'd have to spiral wrap it with the cut material in order to get shield coverage. Maybe I'll just cut one of the creases to get a 2" wide piece and leave the gap to insure proper Faraday shielding.

Dan

[sinclairuser]

as have probably noticed when you get into winding and constructing your own coils, its just as easy to wind two as it is one, i would like to suggest you build one properly but with out shielding, still build your shielded coil, but make one without as well.
the reason i say this is some people on here seem to get hung up on adding a shield, and honestly there may be no need, when i started building pi coils worrying about emf meant i shielded my coils, but i am glad i had built one without and heres why, when i took the barracuda to the coast for the day, i put 2 coils in the car, the shielded one got the cable trapped in the car door and was damaged.
the outcome of this was i had to use the unshielded mono loop, and it worked fine it did not detect my bare hand and worked fine on the beach, this coil won't run stable in my shack, but on the beach in the middle of nowhere it was fine, i think sometimes we shield our coils and frankly in the real world its not allways needed.
ally.

[DanZ]

as have probably noticed when you get into winding and constructing your own coils, its just as easy to wind two as it is one, i would like to suggest you build one properly but with out shielding, still build your shielded coil, but make one without as well.
the reason i say this is some people on here seem to get hung up on adding a shield, and honestly there may be no need, when i started building pi coils worrying about emf meant i shielded my coils, but i am glad i had built one without and heres why, when i took the barracuda to the coast for the day, i put 2 coils in the car, the shielded one got the cable trapped in the car door and was damaged.
the outcome of this was i had to use the unshielded mono loop, and it worked fine it did not detect my bare hand and worked fine on the beach, this coil won't run stable in my shack, but on the beach in the middle of nowhere it was fine, i think sometimes we shield our coils and frankly in the real world its not allways needed.
ally.

Good advice. That's the idea I'm going with. When my PI is finished I'll try it with the first coil I made already (no shielding) and then other coils using the various types shielding to see the differences.

Regards

[sinclairuser]

dont get me wrong i think shielding is important, but it depends on the pi detector/setup and location.
i make my own graphite paint, search for a thread by satdaveuk in tools and techniques, the only difference in my mix to dave's is i use acrillic lacquer instead of polyureathane varnish.
another thing i do is only use enough epoxy to secure the coils, i then fill them with expanding foam when they are sealed, i like light coils and a balanced setup.

[baum7154]

dont get me wrong i think shielding is important, but it depends on the pi detector/setup and location.
i make my own graphite paint, search for a thread by satdaveuk in tools and techniques, the only difference in my mix to dave's is i use acrillic lacquer instead of polyureathane varnish.
another thing i do is only use enough epoxy to secure the coils, i then fill them with expanding foam when they are sealed, i like light coils and a balanced setup.

You and I have taken the same path with help from Dave and his graphite shielding technique. I worked with him to shield my first coil, a 410uh spider coil and I did notice some loss of sensitivity due to the shield. Probably I could have made it a bit more resistance and that would have mitigated the loss of sensitivity. I built a second spider coil with no shield but that provided by it's own geometry with the outermost turns tied to ground . This is the coil I use today and the one featured in the CHANCE PI COIL thread in this forum. It is the most sensitive coil I have for small gold at minimal delays. This coil convinces me that a shield is not always necessary. This 8" coil recently was used in a test to see a 10.8 grain piece of dental gold buried in the ground at my prospect site at a distance of 4.5".

Regards,

Dan

[baum7154]

luck

Dan,

Here is a little tip about using Scotch24. Scotch24 comes in a 1" wide flat, 2-layer tube-like shielding that should be cut to accomodate your coil design. If you cut the Sccoth24 along it's two folds, you will have 2 one-inch wide pieces and each piece should be long enough to make almost (there must be a space) a complete loop around the coil. A 1" wide piece will fit a coil bundle that is 1/3.14159 or .31831" in diameter. Use clothes pins to hold it on while applying a layer of spiralwrap. This will form a lower capacitance than a solid shield material as the area of the mesh is about half as much. Make sure that the Scotch24 mesh does not overlap or short when going around the wire bundle. This will minimize eddy currents from circulating in the shorted mesh area.

This should help.

bbsailor

I tried a 6" length of Scotch 24 cut down just one of the edge creases yielding a single layer 6" long X 2" wide. This was placed on the inner circumference of my 8" spider coil, a 6"ID X 8"OD toroidal shaped coil. The 6" length covered about 1/3 of the inner circumference and about 1/2 of the cross sectional circumference of the coil resulting in about 1/7 of the total coil being covered by the mesh sample. The detector sees this sample with a signal strength of 4. Alternatively I draped the sample over the top of the coil and the result was still 4 units of signal strength. I did not ground the mesh as a shield normally would be but the rule of thumb "If the detector sees it don't use it" applies here. I think the large quantity of Scotch 24 required to cover this coil form factor is likely the reason it is detected. FYI it is spaced a minimum of 6mm from the coil windings. Your success in using it on coils less than a diameter of .31831" leads me to think the smaller quantity of mesh makes the difference in being detected. The detector was run wide open for this test with a Guard Interval set to 10.

Regards

Dan

[DanZ]

Almost completed building my first PI when a thought popped up. Has anybody tried switching a Faraday shield so that it's grounded to the circuit during RX and "ungrounded" during the TX pulse? Perhaps a 4066 switch inline with the Faraday ground line and triggering it from the pulse timer circuit? I'm curious if a foil wrapping would still possibly decrease the TX pulse if it were not grounded.

[sinclairuser]

a pi would probably detect it, in my tests foil(alu) was a straight up no-no, that was connected to ground not connected would only make it more likely that the pi would see it.

[DanZ]

a pi would probably detect it, in my tests foil(alu) was a straight up no-no, that was connected to ground not connected would only make it more likely that the pi would see it.

I'm seeing many articles like http://www.silverdog.co.uk/images/HHv1p5.pdf (last paragraph page 9) Carl's Hammerhead PI using a shield and my question rephrased would be, would you get better performance if the shield ground was opened during the TX pulse event only? The receiver would still "see" a grounded, shielded coil.

edit: Thanks for the reply.

[sinclairuser]

no because its eddy currents in the foil that are detected, not the foil per se.