GPX homemade large coil

[GoldPandemic]

Hi, I'm wanting to make a 1 metre PI single loop mono coil for GPX 5000.
I believe I need to match the Inductance of my new coil to a original coil.


For my original 11" coil the Coil Calculator gave me 0.93 mH Inductance.
(Inner Radius 270mm, Wire Thickness 0.8, Number of Turns 23)

And for the 1 metre coil I'm creating I got 0.864 mH Inductance.
(Inner Radius 1000mm, Wire Thickness 0.8, Number of Turns 10)


I think this is close enough.
But I'm a novice and would really appreciate some advice?

[Geo]

GPX is a motion machine so you must move directly the large coil to have indicator (not so easy...).
Also GPX works at low delay ( i think 8us), so it is very difficult to construct a coil 1x1 m with ability to work at so sort delay.
My 1x1m coil for TDI works ok after 15us.

[GoldPandemic]

My coil housing will be constructed of a plastic tube - so weight will not be an issue (I also have a harness).
I do not understand what you mean by "low delay", or how I can solve it, could you explain in more detail?

[greylourie]

Hello, theres lots of info on required coil specs for older models, if you do a forum search. Your calculated inductance value for your 11" coil, seems to be a bit out. Probably best to peruse posts by Stefan and I.B.Gold. As for building larger coils for minelabs pi, theres not much info about. It would be interesting to hear what type of hunting you plan to do with such a large coil. You might be better off with another make and model of detector.

Here are a few posts:

http://www.geotech1.com/forums/showt...285#post122285
http://www.geotech1.com/forums/showt...728#post175728
http://www.geotech1.com/forums/showt...8803#post68803

[GoldPandemic]

Thanks for the links.
Since I've spent thousands on my GPX 5000, I'll be sticking with coils for this.
Coiltek make a 20" mono, and a 40" DD, 40" = 1 metre.
I know that RF interferance is an issue with coils of this size, but I plan to go 50km+ deep into the wilderness. After Gold nuggets of course, in Fiordland New Zealand.

Can you explain how I got my calculations wrong, and/or correct them?
11" converted to centimetres is 27.94cm - I removed the remainder to get the Inner Diametre of the coil.
I have a multimeter, and Oscilloscope (40MHz) - unsure if these will be useful.

[greylourie]

Does your multimeter have an inductance measuring facility? When using the coil calculator, I guess that any wire encapsulation/insulation be factored into your calculations when deciding on a figure to input into the coil calc wire thickness window. Using Q's coil calculator would be an iterative process. You could also play with the inner radius (not diameter) figure. Also bear in mind that whatever wire you have chosen, has a low enough resistance so that your total windings/length does not exceed 0.4ohms. That 0.4ohms (max) figure is for any coil made up to be connected to the tx side of your detector. Ignoring that could result detector downtime and repair costs.

A quick look on the mentioned vendor/manufacturer website shows their 40 x 20 aka "Mothership" is listed as an A.I. coil, not DD. And comes with 3.5 metres of cable, probably for dragging behind an atv on relatively flat surfaces. When you mentioned Fiordland, google showed me images of some very arduous terrain. It will be a challenge using a large coil in such an environment. Maybe some other members of the forum can offer advice. Coil building can end up being a whole new hobby/pastime, and end up detracting from valuable time actually spent detecting. Fingers crossed, I.B.Gold or maybe Kev see your posts, and chip in with advice and helpful suggestions.

[Mechanic]

Hi Goldpandemic,

You need to aim around the 300uh mark for inductance and .4ohms.
ML machines are very fussy with coils, there is not much room for error. Capacitance is one thing that you need to be careful with, especially with a coil so large. The capacitance needs to be as low as possible. From memory, last time I measured the resonant frequency of a commercial coil, I think it was about 750khz, hmmm or was it 250khz??
Have you considered a commercially available 25" mono?

Good luck with it!

Cheers Mick

[GoldPandemic]

Awesome, thanks for all the helpful info. I'm unsure what my multimeter & oscilloscope can do - will do some reading up - and test my destroyed Minelab GPX 11" mono coil:

My intention is to copy (or closely match) all of the measurements that I take from this coil - except for the size & perhaps windings (if they need to be adjusted).
It is still working - so perhaps I could put it in a pipe similar design to my new detector - to see if that changes anything.
As for shielding my new coil I was thinking of doing a thin card wrap along the entire length with the over lap being on the top - then spraying it with carbon spray.

Yes, I realise that the country will be rough.
Once I make a successful coil, I intend to sell them on ebay.
It is a month before I leave on my trip to the South Island, I also need to construct a Dynamo (just attach unicycle style pedals) & make waterproof case for my detector box & battery (I'm going into some rivers too).

[Mechanic]

Oscilloscope can be used to check resonant frequency, as well as checking inside the detector to make sure that the signal from the coil looks right and to test that the coil has settled before the first sample is taken.
Multimeter can check the resistance.
You will need an lcr meter for the inductance.

The wire is litz and as far as I know it is not commonly available. You can't use solid and normal wire is a bit iffy.
The coax is specially made litz, low capacitance with the correct amount of strands to deliver the current required. As far as I know there is only one company that makes it and I don't know who that company is.

My suggestion for a start would be to re-assemble the coil you have dismantled, or re-assemble it to the same size and see how it works
You might be biting off a bit more than you can chew... Coil making is a black art, one that I am happy to leave for others!!

Cheers Mick

[GoldPandemic]

Ah ahahah

I'm in contact with the company that makes the said litz wire - just working out shipping details now.

As a last resort I would have to purchase a US$500 18" mono, I'd rather not spend that much.
Brilliant idea - I could simply change the loop size on my current coil - to make it 1m.

[Mechanic]

You could do that however I think you will find that you will need more wire length to achieve the correct inductance, then you may need another strand or 2 to get the correct resistance again.

Cheers Mick

[Achillion]

Hey GP,

Seems ML is using 72 strands of 36 AWG litz. According the site from New England Wire this would equate to a equivalent 18AWG solid wire with an approximate DC resistance of 6.8Ohm per 1000 feet.

Is this the one you're after?

Wonder why a more common type like 660/46AWG litz would not be suitable. Same equivalent 18AWG solid wire and DC resistance therefore in the same range. More individual strands make a faster coil.

Cheers,

Jacob

[Mechanic]

I think there becomes a point where more strands = more inter-wire capacitance, thus adding to the coils total capacitance, slowing it down. If you go the other way and go for thicker strands, there becomes a point where the eddy currents formed in the wire become problematic. As a good man here once said "There is no free lunch" Well he said it many times, and it is so true! Also a larger coil has a larger shielding surface area. Surface area = capacitance...

I must point out, I am not trying to poo poo the idea. ML make excellent detectors, built on edge to get the best possible performance. To make a coil for them the coil needs to be very carefully made to suit the detectors needs.

Cheers Mick

[Davor]

In the rest of the world extra strands, being tightly packed, do not increase the inter-wire capacitance, and we are free to use those in our projects

[Mechanic]

In the rest of the world extra strands, being tightly packed, do not increase the inter-wire capacitance, and we are free to use those in our projects

And also 1k preamp input resistors don't create noise in the rest of the world Must be something in it!