PI coils- Shielded or Unshielded?

[waltr]

I have been building and experimenting with a PI detector based on the Hammer head. See my thread about this project here:
http://www.geotech1.com/forums/showt...ake-on-the-HH2

Now to keep things simple to start I have not shielded the coils or used shielded cables (see pictures in my thread).
Whereas, in all coil build threads everyone adds shielding to the coils either by wrapping the coil with shielding material or applying a shielding material to the coil housing. Lots of threads on different shielding methods.

The shield is supposed to cancel the capacitive effect of the ground (and possibly reducing noise) and a test is moving your hand near the coil or moving the coil closer to the ground. If no response then the shield is working. However, I have no response without any shielding.

The biggest down side of a shield is the added capacitance which increases the decay time of the coil and needing a smaller Damping resistor value. So if a shield is not really needed then the coil can be "faster".

I did have issues with wobbly audio but this was solved by a fine adjustment of the Pulse frequency. I did try shielding but did not change this wobbling.
So this is simply AC mains beating with the pulsing frequency.

Is there something I am missing?

I am close to finishing coil builds which will be epoxied into shells. It will be next to impossible to make changes after applying epoxy so what to ensure I am not making a big mistake.

Is there anyone that has build un-shielded coils for a PI detector and not have issues?
Or is everyone just shielding the coils because every thing they read says to shield the coil?

[ripsdevala]

I have build clone, bara, chance, felzjo.... All without shield.
But idk, im not the expert

[Qiaozhi]

From my notes on sample delays:

8-10us: gold nuggets - coil must be shielded (eliminates electrostatic interference at low delays)
>15us: beach
30us to 50us: coin shooting
Coils for longer delays do not need a shield

[Digger429]

I have built shielded coils because everyone said you gotta do it, but never got anything other than reduced performance. Now, my coils are all unshielded. If there is any issue with capacitative effect of no shielding then it is small enough for me to not care.

[green]

Tried an unshielded coil. GEB off, 10usec target and EF sample. Test: hold hand about 1inch from coil, then remove quickly. Monitor integrator out change with oscilloscope. Hold a US nickel over coil, then lift quickly, find distance that nickel causes the same signal change that the hand caused at 1inch. First delay, time between coil off and target sample. 6usec nickel 12inches, 8usec nickel 14inches. Removing the hand didn't cause a change with delay over 10usec. If I touch scope ground with my other hand while lifting my hand from coil, integrator out goes opposite polarity from not touching the scope ground.

Looked at amplifier out while moving hand close and away.I have an IB coil and can see what is happening during Tx decay. Looks like lifting the hand only effects the signal during Tx off decay. Shielding my coil prevents the signal change when moving the hand near the coil. I've tested signal strength, integrator out volts vs target distance before shielding and after shielding and haven't seen a difference in signal strength. Results from my test, would guess someone else doing the test would get different results depending on Tx and amplifier turn off decay.

[Ferric Toes]

I would never build an unshielded coil; full stop. Properly designed, grounded, and using the correct shielding material it can only benefit the detector. You can get away with no shield in the workshop or doing air tests, but on wet grass, seaweed, or wet beach it can give rise to a lot of falsing.

A shield does reduce rf pickup and also radiated rf harmonics from fast switching devices in the detector. Without a shield the detector is more likely to fail an emc test, which all commercial detectors have to pass.

Eric.

[bbsailor]

I would never build an unshielded coil; full stop. Properly designed, grounded, and using the correct shielding material it can only benefit the detector. You can get away with no shield in the workshop or doing air tests, but on wet grass, seaweed, or wet beach it can give rise to a lot of falsing.

A shield does reduce rf pickup and also radiated rf harmonics from fast switching devices in the detector. Without a shield the detector is more likely to fail an emc test, which all commercial detectors have to pass.

Eric.

Eric is right on with his comment about shielding the coil I would like to add some additional information based on my own coil design research.

1. As the delay gets lower, between 5uS and 10uS, shielding material and shielding techniques become more important. Just choose a shielding material that is not detected or very minimally detected at your lowest delay setting. This includes how you wrap the shield around the coil. Try to avoid making conductive shielding metal to metal contact around the circumference of the coil cross section wire bundle. If you must overlap put a tape insulation to keep from forming a conductive loop.

2. Use wire insulation that has the lowest dielectric constant to minimize coil turn-to-turn capacitance. Thicker insulation causes less capacitance but lowers the coil inductance slightly so you may need to add an additional coil turn or two to get the desired coil inductance.

3. Use a low dielectric spacer between the coil wire and the shield to minimize the coil-to-shield capacitance.

4. When operating at very low delays the wire thickness may be holding eddy currents. Try to use single strands no thicker than AWG 30 or use marine tin plated stranded wire. The tin plating puts a slight resistance between strands and tends to minimize the formation of eddy currents in the wire itself.

5. Use the lowest capacitance coax wire to connect the coil.

6. Make sure that the peak fly back voltage is below the rated MOSFET voltage to prevent clamping and speed up the potential receive time.

7. Use a two stage low gain first and second amplification stage to minimize the amplifier lock up time.

Here is a good point to help you form a simple mental model about PI detectors and delay time. It takes 5 time constants for a targets fully stimulated eddy currents to decline to near 0%. If you have a 2uS small target and attempt to detect it at 10 uS, there will be no signal left to detect, as during the delay, all the energy in the target has dissipated. If you have a PI detector that can go down to 7.5 uS delay you may be able to pick up the tail end of the dissipating eddy currents. Thus smaller targets need lower delays to detect. But there is more.... lower delays pick up more ground effects also. Good shielding is more critical at lower delays!!!

Now here is is where it gets more complex. Theory tells us that a target must be stimulated 5 times faster than its own time constant.

Let’s use our 2uS example from above that needs to have a 0.4uS coil discharge pulse time constant. The coil discharge time constant is calculated by dividing the damping resistor value into the coil inductance. A typical 300uH coil would need a 750 ohm damping resistor to fully stimulate that 2uS target. Now here is where it all comes together. Less coil, cable and TX circuit capacitance allows higher damping resistor value to be used. Thus the term “fast coil”.

Eric would you please add anything that I have missed or rank the importance of the points I have attempted to explain.

Thanks

Joseph J. Rogowski

[waltr]

Thanks for all the answers.

Since I will not be looking for tiny nuggets on the USA east coast I don't need the Fastest coil/delay. Maybe small gold jewelry but no nuggets.
The 33cm DD coil can run minimum sample time of 8-9usec whereas the 25cm mono is 10-12us.
I can not get either to response to my hand- tested this again tonight with the scope on the integrator out.

I will get coils in Temporary shells outside to see if I have issues as described with wet grass, etc. This will determine if I shield or not before I epoxy them into nice shells.

[WM6]

7. Use a two stage low gain first and second amplification stage to minimize the amplifier lock up time.

Joseph J. Rogowski

Thanks for your coil shielding advices overview.

What universal two stage amplifier do you suggest?

[bbsailor]

Thanks for your coil shielding advices overview.

What universal two stage amplifier do you suggest?

Anything in the NE5532 family of dual opamps. Maybe some forum members can report on their experience with more modern or advanced dual opamps.

Joseph J. Rogowski

[bbsailor]

Thanks for your coil shielding advices overview.

What universal two stage amplifier do you suggest?

Anything in the NE5532 family of dual opamps. Maybe some forum members can report on their experience with more modern or advanced dual opamps.

Joseph J. Rogowski

[baum7154]

It’s been a while since I’ve replied here. Try the self shielding 3DSS coil detailed in the Chance PI Coil thread for small gold nuggets and small gold chains. Use Litz wire if you want fastest decay.

I like the performance of the OP37 amps over the NE5534. I used this in the 2 stage amp upgrade detailed in the Chance PI Build thread.

Also I like the use of a continuation of the PTFE insulated coil wire as a twisted pair feed line of much lower capacitance than coax feeds.

Regards,

Dan

[ionut_mtb]

is this a good wire for making PI coil?

https://www.ebay.com/itm/20m-SILVER-....c100290.m3507

[baum7154]

In my opinion this wire is not the best for a very fast coil because of its very thin insulation of about .008”. In order to get the inter wire capacitance down the insulation needs to be about 600 volt or more. In addition the wires are very fine and I think have more silver plating in the wire bundle than is desired to keep eddy currents to a minimum. I have not used Litz wire yet but have heard it is very good to use for fast coils.

Dan

[ionut_mtb]

In my opinion this wire is not the best for a very fast coil because of its very thin insulation of about .008”. In order to get the inter wire capacitance down the insulation needs to be about 600 volt or more. In addition the wires are very fine and I think have more silver plating in the wire bundle than is desired to keep eddy currents to a minimum. I have not used Litz wire yet but have heard it is very good to use for fast coils.

Dan

The problem i have is that the coil shell i have (ebay from bulgaria) does not allow me to use thicker insulation, as the bundle become to thick and is not fitting inside the housing. I'm not making coil for prospecting, i'm aiming only gold rings targets.