VLF coil size vs target size

[Koala]

I know there's a relationship as when you use a small coil you find more smaller artefacts such as .22 cartridge cases and buck shot with a 5.5" coil than with a 10" coil

Doesn't seem to be much difference when air testing

Is there a formula for the optimum size of coil. Say X times the target diameter for example.


Or is it more the small coil sees less of the ground. So there is less chance of something like a piece of coke masking the weak signal from a small target

[Skippy]

You've no doubt read this thread ( you have contributed to it ):
http://www.geotech1.com/forums/showt...etection-depth

I was intending to continue the thread with more VLF-related stuff. One obvious factor is the way DD and CC differ, and how they relate to mono-coils. And VLF's pick up the ground signal more than PI's, and the level of pickup varies with coil size, another significant difference.
And also quite significant is the difference between IB and mono coils on very shallow targets.

I suspect there are other hard-to-analyse features of VLF ground pickup, too. A bigger coil will pick up more ground signal,..... but it's an average of a larger area of ground, so it's likely to fluctuate less than that produced by a small coil.
And then there's how you sweep the coil : do you sweep the larger coil proportionally higher above the ground, or are you going to maintain the same height as a small coil?

And then there's overall system gain/amplification. While it's true that nearly all machines make no changes to their signal processing to cater for coil characteristics, some do, some may do, and some could be modified in order to produce such changes.

Some of the newer Minelab machines can 'read' the coil size etc, so could use this info in a technical way. The XP Deus coils obviously have most of the detector built-into the coil, so each coil could be designed differently. Didn't Minelabs RelicHawk have a switch you adjusted to suit different coil choices?

[Koala]

Mmmmm not easy to test. I have found things like a copper gilt bead that's 2 mm x 1 mm with the small coil. Even though waving it in front of a 10" coil picks it up. You just don't find them.


The finds pouch shows more of a difference than air testing does.



Again I hadn't thought about DD as I only use concentric.

[Skippy]

Not easy to test, and very difficult to mathematically model, too. You may have read the threads about ferrous target modelling, and ground pickup simulation. Some ideas floated, but whether they work in practice is tricky to assess. And modelling just an airtest is quite challenging, though achievable.
Even if you could make good tests on real coils, you have to be sure you're comparing data properly. For example, I have a 5" 'circular profile' DD and a 11" x 7" heavily elliptical DD for my Fisher F75. If I got hold of their big 15" coil, it's 'circular'. So only the 5" and 15" would compare properly.
And there's no point comparing a 10" with an 8", there's such a trivial difference you'd struggle to show anything useful.

[dbanner]

Small coil for smaller, shallower targets.
Bigger coil for larger, deeper targets.
That's good enough rule for me.
Somewhere in between gives best of both worlds.

[Koala]

Small coil for smaller, shallower targets

Are you sure about shallower targets.

On my Bandido a badly corroded and thin pre decimal half penny buried at 10". Is the same with a 5.5 or 10" coil.
There is a difference as to how easy it is to latch onto the target though.

Air testing a pre decimal penny is the point at which you get more depth with the larger coil. So for 90% of what I am interested there's no difference except loss in ground coverage

[green]

Are you sure about shallower targets.

On my Bandido a badly corroded and thin pre decimal half penny buried at 10". Is the same with a 5.5 or 10" coil.
There is a difference as to how easy it is to latch onto the target though.

Air testing a pre decimal penny is the point at which you get more depth with the larger coil. So for 90% of what I am interested there's no difference except loss in ground coverage

Interesting. Hyperphysics calculates about 4inch distance for a 5.5 and 10inch coil to have the same Rx signal strength if coil inductance and Tx current profile are the same. Wonder why you get 10inches? Probably something I'm missing.

[Skippy]

Green: You're missing the fact Koala is using a VLF machine.
So, amongst other things, his 5.5" and 10" concentric coils probably mimic 4" and 7.5" mono's. And his small coil picks up half the ground signal of the big one, so he can resolve targets half the strength, ie. +15% deeper.
Though I can't quite match his impressive in-ground performance, I agree with the general idea he's stating. With my F75, the 5" DD (probably more like 4.5" windings) achieves 80% the depth of the 11" x 7" DD (which behaves like an 8"). This roughly matches "theory" - the return signal from the smaller coil is about 1/6th that of the large one. Double the gain because of ground pickup as explained. This allows detection of targets about 1/3rd signal strength, matching the 80% depth figure.

[Koala]

Green: You're missing the fact Koala is using a VLF machine.
So, amongst other things, his 5.5" and 10" concentric coils probably mimic 4" and 7.5" mono's. And his small coil picks up half the ground signal of the big one, so he can resolve targets half the strength, ie. +15% deeper.
Though I can't quite match his impressive in-ground performance, I agree with the general idea he's stating. With my F75, the 5" DD (probably more like 4.5" windings) achieves 80% the depth of the 11" x 7" DD (which behaves like an 8").

They are a true 5.5 and 10 as I built both coils. Both concentric with the RX half the size of the TX


At the moment I am using a 1265x with a factory 7.5" (measured Size) concentric.

Air tests on this were surprising which is what made me think.

A cartwheel penny which is a massive copper coin gave little improvement over a pre decimal penny

Also I was quite surprised just how close small silver coins were to the large copper and bronze coins.

[dbanner]

Are you sure about shallower targets.

On my Bandido a badly corroded and thin pre decimal half penny buried at 10". Is the same with a 5.5 or 10" coil.
There is a difference as to how easy it is to latch onto the target though.

Air testing a pre decimal penny is the point at which you get more depth with the larger coil. So for 90% of what I am interested there's no difference except loss in ground coverage

I see your point.

[Skippy]

"They are a true 5.5" and 10" as I built both coils. Both concentric with the RX half the size of the TX"

You misunderstand me. I'm comparing your concentric coils with an equivalent MONO-COIL, which is what Mr. Green is using in his simulations. While your TX may be 10", the RX is only 5". The net result is your coil as a finished item "behaves" like something in-between, 7", 7.5"? I can't say precisely, and I don't have a concentric for my F75 machine, so haven't got any relevant measurements. I can say that my 11" x 7" elliptic DD behaves like a 165mm ( 6.5" ) mono-coil, which surprised me.

[dbanner]

For coin targets and rings etc, there must be an optimal size(diameter of coil), beyond which you get diminishing returns. Then the target would have to be larger to make sense using a bigger coil. ( Effect of ground)

[Koala]

https://www.youtube.com/watch?v=niFbf6Am23g

Not the best video. The internal diameter of the TX is 5.5"

This is the coil I tried to copy.

I would probably run slightly more sensitivity, less threshold and discrimination and use headphones.

[Koala]

"They are a true 5.5" and 10" as I built both coils. Both concentric with the RX half the size of the TX"

You misunderstand me. I'm comparing your concentric coils with an equivalent MONO-COIL, which is what Mr. Green is using in his simulations. While your TX may be 10", the RX is only 5". The net result is your coil as a finished item "behaves" like something in-between, 7", 7.5"? I can't say precisely, and I don't have a concentric for my F75 machine, so haven't got any relevant measurements. I can say that my 11" x 7" elliptic DD behaves like a 165mm ( 6.5" ) mono-coil, which surprised me.

Ah got it now. Thought you were talking the fact Tesoro call the outer diameter as its size where as the TX is often 1/2" from the outside.

[Koala]

For coin targets and rings etc, there must be an optimal size(diameter of coil), beyond which you get diminishing returns. Then the target would have to be larger to make sense using a bigger coil. ( Effect of ground)

That's how I see it.