Giant Coil - Peanut Brain!

[Helikiwi]

Hi everyone,

I’m new to this forum and new to detecting, I want to build a large mono coil and operate it on an old Minelab SD (I’m thinking SD2000). I have a mechanical trade within the aviation industry so I’ve got a broad range of skills and generally work to a high standard. So given some guidance I think I have the skills to build one, however I really lack the intelligence to design one. I’ve been reading articles and the forum here and a lot of the information goes well over my head.
I’m hoping someone can direct me to an article on basic mono coil building please, I’ve seen the article “Building a fast pulse induction mono coil” but not sure if this is what I’m after.
I’ll give you more background info on what I’d like to do with it and happy to take any feedback or criticism. Perhaps what I am proposing won’t work at all - happy to hear thoughts…

I recently visited a gully that was very gold rich prior to being worked 3 or 4 times (most recently early last century). It is very thick in vegetation and pretty hard to move around in. In recent times I would say it’s had minimal attention compared to most places - no doubt plenty of people have been into the more accessible areas of the creek to get alluvial gold but I doubt it’s ever been thoroughly run over with a detector as this would be difficult.

So my idea is to build a large (lightweight) coil and rather than scrubbing the ground I want to sweep above the ground cover and even project on angles to get in and around bushes, under logs and up steep banks etc.
So having a large coil is not so much to get maximum ground penetration but more to enable me to operate from a raised position.
I envisage making a carbon fibre housing and telescopic arm.

If there is anyone out there who is interested in providing guidance that would be greatly appreciated. Thank you.

Regards,

Phil

[Carl-NC]

Hi Phil, the article you found is exactly what you need. Basically you want to replicate the inductance and resistance of the stock Minelab coil at whatever size you choose. Shouldn't be difficult.

However, a large coil will only be sensitive to large nuggets. If you make, say, a 50cm coil don't expect to find a 1-grammer with it, especially with the coil elevated. Do some tests with fake targets to get a feel for what it can do.

[Helikiwi]

Thanks Carl,

Ok, I’ll re-read and study that article but I guess I’ll have to wait until I get my hands on a detector before I can really make a start.
Copy that on the large nuggets. That’s what I’d like to chase anyway.
Thanks again, good to get a positive reply! I know so little about detecting I thought my idea might be laughable.

Regards,

Phil

[Carl-NC]

Lots of people have done what you intend, including me. All of the SD and GPX models have basically the same coil design so you can start on the coil now. I think it's 300uH and 1Ω but someone may correct me on that.

[Altra]

One thing to consider is the self adjusting threshold. You need a curtain amount of coil movement or your targets will fade out. Also depending on the ground mineralization you need to maintain the coil height at the same level. This will be difficult in brush and rough terrain with a large coil. The large SD coils I have seen on YouTube were mounted to sleds and towed behind ATVs. The sled maintains the height and the ATV the constant motion.

Believe it or not, the opposite strategy may be the only way. Scan the area with small coils that can go under the brush and around rocks.

I would post a photo of your terrain on a prospecting forum and ask what the best approach for that kind area.

[pito]

to eliminate ground effect the coils are lifted 1m above the soil.

[Helikiwi]

Thanks Altra and Pito,

I believe (well hope) ground mineralisation in the area is pretty low. I think I’ll give this a go. If it doesn’t work I’ll probably convert it to a skid mount and try using it elsewhere.

Does enclosing a coil in carbon fibre have any detrimental effect on its operation?

Thanks again,

Phil

[Altra]

Phil, what size coil were you planning to make?

[Helikiwi]

Good question, I really don’t know. I’m thinking perhaps 30” to begin with. But if my interest develops I think I’ll go bigger, particularly if I go with the sled eventually. What is the governing factor when considering size?

Thanks,

Phil

[Altra]

Okay, 30" is large. It seems the 18" coil is popular and still manageable on a standard GPX setup. There was a carbon fiber coil about 30" being marketed. It was selling for over $1k. So it seems possible

[IBGold]

Lots of people have done what you intend, including me. All of the SD and GPX models have basically the same coil design so you can start on the coil now. I think it's 300uH and 1Ω but someone may correct me on that.

The Specifications we build for SD to GPX is 300uh and 0.4 ohms I have built up to 1800mm Mono coils the largest I use while swinging is a 32"concentric or Mono.

Regards, Ian.

[Nightryder]

I assume you're after small nuggets not big chunks since it's a worked gully. If so, do the opposite and go small. Working with detectors optimized for gold nuggets needs you to be sharp and focused for the slightest change in tone. And you need patience, lots of it because the key here is low and slow sweeping wise. It's important to do what Carl said about testing your machine even with fake targets to get familiar with it.

Good luck!

[Helikiwi]

Thanks Ian! 1800mm wow, what detector would be used with a coil this size?

Thanks Nightryder, I’m not particularly interested in really small stuff, I believe there were many +10oz nuggets taken from the area. I’m hoping there are still some similar there. I guess it’s hard to know how thoroughly it was worked, as I said terrain is pretty challenging.

[Nooom]

It's easy and difficult at the same time What you need: Polystyrene insulation boxes are used to insulate heat in buildings. You can choose the thickness, diameter, and pressure required It is cut in a circle or left as square as you want, and a path is made for the wire at the ends of the polystyrene, and the wire is wound in the duct until it reaches the correct size, which is 300 microhenry, and the resistance is less than 1 ohm. What remains for you is insulation. You need to paint graphite, then protect the coil with strong adhesive tape or fiberglass intended for treating fractures. Also use it sometimes. and what else You can use a coil that can be carried by two people, similar to the Lorenz coil and the like. I think it is lighter And good luck