Carl, Joe, and any other experts, thoughts please.

[Kev]

Hi,

The other day I made a post about the reactivity of the carbon shaft on my GPZ suggesting that maybe this is the reason why coils smaller than the 14x13 were not produced by Minelab.

https://www.geotech1.com/forums/foru...ls-for-the-gpz

After watching the video I linked in that post, (sorry for the dreadful audio in that) I noticed something that makes me wonder if it's actually my fault and not the carbon shaft.



The image is from timestamp 3:12 in the video. At this point you will notice that when I change hands and grip the upper carbon shaft, touching the lower one doesn't cause a response, only after moving my hand to the grip does it sound off.
There is no direct conductive connection between the upper and lower carbon shafts, so considering the displacement of the upper shaft from the coil field, is it possible that the anomaly is caused by the fields produced around the cable passing within the shafts, or is it just capacitive coupling between the shafts?

I make the coil cable myself, I use Litz wire for the Tx so that I can tailor the number of conductors and achieve exact total coil resistance. These are separately wrapped in PTFE tape and a ground/shield wire of gas injected Teflon runs between them. The Rx wire is another Teflon coated conductor wrapped in an unravelled Litz wire shield, also wrapped in PTFE, and runs opposite the ground wire. The whole is wrapped in PTFE tape and encased in two layers of polyolefin shrink tube. I have made cables like this for years and never before encountered any issues. All the plastics are electron acceptors, the only donor being the acrylic wrapping around the Litz, but with no contact-separation or frictional movements triboelectrification will not be an issue, besides the cable is completely static when these disturbances occur.

I thought the alternate fields from the cable Tx wires and the use of Litz wire would cancel each other out? By not using the best of dielectrics am I extending the cable field too far and involving the carbon?
Or is the detection field being bent up the shafts as I originally thought.

Any input would be great.
Cheers
Kev.

[Carl-NC]

IMO, it's more likely proximity effect with the cable. If the carbon shaft was causing this, I think it would be even worse with the larger stock coil. When you grab the upper shaft with your left hand, it creates a proximity effect (capacitance) and then your right hand has very little additional effect. The conductive carbon shaft may exacerbate the proximity effect by increasing the conductive area around the cable; perhaps you don't see the problem (or it's far weaker) when using the fiberglass shaft. Probably better cable shielding would solve this. It would be interesting if you could get a hold of a dead GPZ coil to salvage the cable and use it on your coil.

[Kev]

Thanks Carl for your thoughts.
I forgot to mention, actually I only just thought of it while reading your post, the effect does exist with the larger stock coil, but to a much lesser degree, in fact almost imperceptible until you submerge the coil up to the first connection point where the lower carbon shaft begins. The Minelab cable shields the Rx only.
The first part of the video shows how strong this response is with a more sensitive coil when I submerge or exhume the plastic/carbon connection point.

Additionally, as you continue to submerge the large stock coil it begins to get very noisy as it approaches the upper connection point between the two carbon shafts. When I first got this machine I wasted a lot of energy digging in hip deep pools for big nuggets that were non existent.

So in the case of the body of water, it is acting like my body? Actually I think the effect is most pronounced when I'm wearing antistatic boots or standing in water.

I'll see if I can improve my cable construction since I've not heard of any X-coil users having problems so it probably is the issue.

Thanks again
Kev.

[Altra]

Hi Kev,

Can you share the specs for the GPZ coil?

Thanks

[Kev]

Hi Altra,
The waterproof one is a DOD it is detuned a little as the ground response is intensified under water, iron rich cobbles congregate more than on slopes and gutters, and the rock in general runs hotter.
Tx is bundle wound,
305uH, 0.38 Ohms wire 0.1mm x 150

Rx coils are spiral wound
374uH combined, 6.2 Ohms 0.1mm x 9

The land hunting one is a concentric Tx is axial wound,
300uH, 0.38 Ohms 0.1mm x 150

Rx is spiral wound
480uH pre bucking 435 post 6.3 Ohms 0.1mm x 9, buck 0.1mm x 10

capacitance between coils and shield all below 320pF

Stock coils are around
300uH, 0.365R,
and
420uH, 6R,

[Altra]

Thanks, hope you discover the source of the false signals.

[Detectorist#1]

Hi Kev,

We thing that search coils are the "sensor" and connecting wires to the electronics are other part of the MD. But maybe all connecting wires (screened or not screened) are part of the "sensor". One participant of geotech1 forum said "I trayed 100 types of connecting wires and no one worked without problems". My efforts are pointed to situate as more electronics near to the search coils as this is possible. I continue my efforts and no 100% good answer for this moment. I thing that capacitance influence is a main reason for the problems - not the exact type of the long non-metal pipes.

[Kev]

Hi Detectorist#1,
Thanks for your feedback. I agree the cable is an integral part of the detection circuit and the ultimate is to embed a micro preamp in the coil and use a differential circuit to the secondary stages in the handle. With the right ferrite material the circuitry in the coil can be stealthily concealed, and actually aid in focusing the field. Something I experienced, but don't have the instruments to fully develop the idea, only blindly imagine what the results are.

The problem occurs only with the high sensitivity homebrew coils, and not the ones designed for larger targets.
Because the cable travels inside the shaft it has to have a helical coil wound into it, so that the shaft can be extended and retracted without the cable getting all jammed up. I'm beginning to wonder if this is what is detecting the carbon in the shaft.

With my next build I'm going to try winding a longer spread out spiral in the cable, just to see if it helps.

The shielding paint I use has carbon in it, and passing a coil sized painted disk over the coil makes the machine sound off like with a large solid target. Remember, my problem disappeared with the fiberglass shaft. So I'm convinced shaft composition can alter performance in some situations.

Cheers
Kev.

[Detectorist#1]

Hi Kev,
I also have some experiments for PI MD with high sensitivity for very small objects and moderate max distance of 30-35cm for coins. See please my post - https://www.geotech1.com/forums/foru...able-fe-reject
I agree with you - the problem occurs only with the high sensitivity homebrew coils, not for ones designed for larger targets. After many experiments, I start to use bipolar TX pulses, two Rx coils and final Tx stages and First Rx stage in small PCB in the coils ( in the sensor). Before using of inside PCB, any near hand to the cables make strong "target" signal.

[Detectorist#1]

I have not use ferrite screen for inside PCB, but strongly symmetrical disposal for the two Tx parts of the schematic and for two Rx channels of differential Rx input.

[Kev]

Hi #1,
Very interesting project. It would be excellent for a local beach that's absolutely swamped in bottle tops. Somewhere there was lost a $50,000 diamond ring, and that was over 30 years ago, so it would be a nice nest egg by todays prices.
I tried GPX4500 in DD disc mode, but it still sees all the little scraps of iron.

Maybe I will build your design and see how the PI/Grad works. Finding time is the issue at present. Need to keep the wife happy too!


Cheers
Kev.