GPZ homebrew coil

[Kev]

I want to make some coils for my GPZ.
I was holding out to see if Nugget Finder came out with what I want but with the global viral response, everything has gone mad.
The GPZ-19 is potted in copious amounts of heavy epoxy and really is a mission to operate, once you get it swinging it's hard to change direction, no fun. Also, from my experiments making DD coils I understand that the overlap not only produces a null but can also be adjusted to ignore smaller surface items and hit on deeper bigger stuff better and with improved surface mineral ignorance. I think with the molded forms Minelab use to fabricate these coils quickly, they are less than ideal as far as overlap "tuning" is concerned, and the 19" version I believe is engineered in this respect more for really big stuff down deep.


Anyway, I've been gathering some info to see if I can do it and all is good so far except I've encountered a wire that appears to be floating, I suspect it is the shield drain wire that is steered to the control box for analysis or simply earthing to the front-end without interfering with the TX and RX circuits. It measures 689pF in relation to the TX coil, so I guess it's the shield drain?


Here's what I've found so far on the 19" assembly complete with the cable (below the security # IC).

TX:

equivalent diameter = 15.5"

Inductance = 300uH

Resistance = 0.4 Ohms

Capacitance = 472pF
SRF = 423kHz
Q = 4.71
Wire (cable) 70 strands of 0.1mm enamelled

RX:

equivalent diameter = 11.4" each

Inductance = 375uH (both)
Resistance = 6.1 Ohms
Capacitance = 244pF
SRF = 526kHz
Q = 0.393

Wire (cable) coax - outer wrap = 50 strands of 0.1mm enamelled, inner 14 strands of tinned.



I'm not willing to dismantle my 14" nor destroy the 19" to see what the actual coils are made of, but a couple of images give some indications. On the Minelab x-ray image the RX coils are possibly simple enamelled wire, and the TX a woven enamelled wire or litz (can be seen best above the upper ferrite).
The Russian coil appears to use enamelled for the RX and it looks like a litz for the TX (the silicon soaked clothe wrap could give an enamelled look)



Not sure now if I should make an enamel pseudo-litz winding jig or get some real litz from Alibaba. Maybe real stuff might stuff it up?


Just wondering if anyone else has attempted this?

Cheers
Kev.

[6666]

Thanks interesting pictures do you have any more of the mono x coils

[Kev]

Thanks interesting pictures do you have any more of the mono x coils

No, but I sure wish I did have.

I recognise the flooring vinyl in that image, it's the same anti-static vinyl we have in our Lab.
I suspect that image could be from a tear-down by Minelab.

The real sensitive x-coils I believe have flat wound (spiral) coils, it would be good to see if it's just the RX as I assume.
I wish Minelab had pulled finger, there's nothing magic here.

[6666]

No, but I sure wish I did have.

I recognise the flooring vinyl in that image, it's the same anti-static vinyl we have in our Lab.
I suspect that image could be from a tear-down by Minelab.

The real sensitive x-coils I believe have flat wound (spiral) coils, it would be good to see if it's just the RX as I assume.
I wish Minelab had pulled finger, there's nothing magic here.

it's the same anti-static vinyl we have in our Lab.

Hmmmmmm, is the floor grounded, we used to wear anti static straps on our shoes , made a huge difference, and anti static floors now I think about it, long time in retirement
Thanks hope you get one working

[Kev]

Yes you're on to it, no need of wrist straps you can walk all round carrying boards and devices, total freedom.
Thanks for your interest.

[ZED]

G'day Kev

I got some pics for you,dont know if admin is ok with one of them,we will see,if not they can delete it.Just so you know the GPZ coils have a little identifying chip in the coil plug,the GPZ wont run if it doesnt see this chip,see pics

[Carl-NC]

Probably the blue wire is a shield connection. I would use RealLitz as making your own is almost impossible. You could build a first-try coil using regular stranded wire for the TX coil, and compare to the original coil. I suspect GPZ performance is super-critically dependent on the coil quality so such an experiment might tell you what to expect when the Litz version isn't good enough.

[Altra]

In Kev's x-ray photos, first post. It looks like a ferrite tube shielding the solder joints for the tx coil? A few years ago we discussed the patent for this solder joint shield.

[Kev]

Wow thanks Zed, that's great. I knew about the hash chip, and assumed the pcb in the coil was just a junction box. I wish I could read the value of those damping resistors, I guess they'd be just enough to counter capacitance 1k2 to 1k5?

So that also confirms what Carl and I had suspected, the blue wire is a drain, the 4 corner wires diverging from it.

I wonder what the tinned links are used for, would they go to the trouble of shorting out each coil individually to critically dampen the other?

I suspect the Russians have used real Litz, funny thing is Minelab Litz is not "true" Litz, just 2 dozen 0.2mm enamelled wound up in no special order. Another thing that makes the large Russian coil superior to the 19" is that they used a conventional coil form and not the flimsy spider web affair that required potting the coil in epoxy to give it strength. Epoxy is a sensitivity killer, I've had real super sensitive coils on the bench go totally deaf after potting. The Russians have used just enough epoxy to attach the coil to the foam, but not to much to deaden it.

Cheers
Kev.

[Kev]

In Kev's x-ray photos, first post. It looks like a ferrite tube shielding the solder joints for the tx coil? A few years ago we discussed the patent for this solder joint shield.

Altra do you remember if the ferrite material was identified?
I know old VHF TV balun cores and ferrite caps from AM radio tuning coils are undetectable and mask metal from the detector, but I've not pinned down the material yet.

[Altra]

Altra do you remember if the ferrite material was identified?
I know old VHF TV balun cores and ferrite caps from AM radio tuning coils are undetectable and mask metal from the detector, but I've not pinned down the material yet.

No, but after posting I looked at the Russian clone photo and noticed they are using two of the common cable "clamp on" type ferrites. Not sure what material?


https://www.ebay.com/itm/20-PCS-Ring...4AAOSweWtcwtU6

[Kev]

No, but after posting I looked at the Russian clone photo and noticed they are using two of the common cable "clamp on" type ferrites. Not sure what material?


https://www.ebay.com/itm/20-PCS-Ring...4AAOSweWtcwtU6

Thanks for the link Altra.
I've tried a few snap on chokes that I have, I've got a kit with different frequency ranges and sizes, but they are all detectable.
Many of these chokes contain a lot of Nickel and so have a high permeability.
I think I'll get a few samples of different binocular ferrites to try #63, #67, #68 or #61, I found that the later is the same as M7 material and is used in old analog TV aerial baluns.
The material that is undetectable and masks metal has a matt grainy soft look as opposed to the smooth hard shiny look of those that give a strong signal response.

I've ordered some wire from China.
0.1mm x 100 strands for TX
and
0.1mm x 7 strands for RX

Busy now following Don's vac-forming tips towards construction of bucks and table to make my shells.
I know I should get a coil running on the bench first, but I don't see a problem with this, the real problem with Minelabs comes when you take the coil out to test on mineralized ground, I need a good supporting shell to do this.
Thanks to people on here like Stefan we know the importance of Q!

[Kev]

Funny, I just realised what may contribute to the secret of success of these X-Coils, besides overlap and wire selection.
Maybe the masking ferrites are made of the same material as the calibration toroid?
This would provide a continual balance feedback irrespective of the ground conditions.
Something to investigate anyway.

I also remember many years ago Eric mentioning that the most uniform field is produced when the coil length to width ratio is around 4:1, even 3:1 has more uniformity than a circular coil, elliptical being the worst.
I checked the ratios on the Minelab and X-coils TX loops, Minelabs' just over 2:1 and X-Coil 1.7:1, so this is not a factor it would seem, but more area for future experimentation.

[Altra]

Thanks for the link Altra.
I've tried a few snap on chokes that I have, I've got a kit with different frequency ranges and sizes, but they are all detectable.
Many of these chokes contain a lot of Nickel and so have a high permeability.

Same here, I had two different sizes and both were weakly detected. Maybe this does not matter, once they are fixed into place the detector should only see these as a slight dc offset? Assuming they are soft ferrite and have a lower permeability. If you find something let us know, I have been looking for the "pure ferrite" response sample piece for testing if it exists.

[6666]

Altra do you remember if the ferrite material was identified?
I know old VHF TV balun cores and ferrite caps from AM radio tuning coils are undetectable and mask metal from the detector, but I've not pinned down the material yet.

Interesting , maybe hollow Coil Former Ferrite Cores to ? I place a ferrite core on the gate lead of irf740's etc