New Concentric Co-planar Coil with Strong MF.

[simonbaker]

Hi Monolith --interesting analogy!

The larger coil will also have less rate-of-change when sweeping typically, which gives the smaller coil an advantage up to a certain depth.
-SB

I have to correct myself -- sweep rate-of-change is insignificant since it is the oscillator rate-of-change that is responsible for the target currents. The sweep rate however does affect the filter electronics in the detector - and the filter definitely can act like a bowl of jello -- or at least a car with lousy shock absorbers cruising over bumpy ground looking for a bump of a certain size.

Regards!

-SB

[Qiaozhi]

I have to correct myself -- sweep rate-of-change is insignificant since it is the oscillator rate-of-change that is responsible for the target currents. The sweep rate however does affect the filter electronics in the detector - and the filter definitely can act like a bowl of jello -- or at least a car with lousy shock absorbers cruising over bumpy ground looking for a bump of a certain size.

Regards!

-SB

You will have probably noticed that IB VLF motion detectors have an optimum sweep speed. Some detectors like to be swept very slowly, whereas others (such as the Tesoros) can be swept very quickly. So your analogy to driving a car with knackered suspension is quite apt. Only when your head hits the roof have you found a bump of the correct size.
Of course, driving more slowly would miss the bump. As would driving a lot faster.

[der_fisherman]

I like your way of making coils, I have been interested in the idea of using router to cut grooves in plywood (or other wood) for the coils with just enough room to adjust for nulling. Then cut away excess wood. Currently I make some coils just gluing coils to a flat platter of thin veneer plywood. Uglier than a mud fence but easy to make -- except attaching the pole requires some construction. But I have a long way to go at making a great MD.

I'd like to see some pictures of your coils.-SB

Hi,

Although I am far away from being an expert, I have made 6 or 7 coils using plywood and a router, the first time was at least 10 years ago and I still have that machine, though all the ones since have either been sold or given away or weren't mine in the first place.....and I haven't built a new detector in years.....but I will dig out the old Twin loop PI that I made and make some photos of the head......light, but strong, maybe not pretty, but prettier than a mud fence!!

After reading your reply, I do feel that such construction details need to be better documented and put on Geotech in a blog for construction details (assuming that nobody has done that up to now!!), we have a coil blog, perhaps that is the right place, but for electronics cases and the expanding leg (I am useless on this bit!!) ideas, maybe another blog should be started.....what do you all out there think?

When I make the coil (this year) for my IGSL, I will make pictures and post them here on the Coil Blog, its really simple and easy to do........the trick being to cut a slot only as deep and wide as needed as the epoxy quickly adds weight. I sand down and fill any tiny voids with car filler fiber glass, slightly flexible version.

It should not be used to actually hold the coil down itself though, then a good sand, followed by one coat of spray paint, sand lightly, then two coats.....it keeps moisture out of the wood. I have also sometimes coated with epoxy to seal.....it depends on how fluid the epoxy is, some are too thick.....

Where coils crossover, the depth of cut needs to be increased (doubled) to account for that so that the coils are not "sticking out" of the wood. I also try and make the coil winding itself a bit wider and therefore thinner at the point where they crossover in a Double D config (If I don't forget to do that when Super glueing!!), to stop major thickness buildup.....

Tools needed are an electric sander with various grades of sanding discs, a router with various different cutters, ones with a ball end are particularly good as you have a cut that is rounded at the bottom and this reduces the amounts of epoxy needed....eg. weight!
But even if you don't have a rounded cutter, a square one will do as well.......

Try and measure the thickness of the coils and use a cutter of that width, if the slot is then too tight, you can always run the cutter around again, slightly offset from the first time.....its usually helpful to cut a millimeter deeper than you think you need, then when sanding, you won't hit and damage the coil......

The biggest problem is that you need a piece of plywwod far greater than the coil, so that you can hold it down well, you also need some throwaway wood underneath, so that you can cut out areas to lighten the assembly up, and you need to cut deeper than the plywood. So there is a degree of waste that one has to accept as well, so plan carefully to keep that to a minimum......

I buy my plywood at a big company where they take peanuts for good big quality offcuts.....that keeps costs down.

The overlap areas need to be more open to allow the coils to be nulled easily, be prepared to cut some wood away to assist in that.....

Do try the coils in the head BEFORE YOU TAKE IT OFF THE ROUTER TABLE. (been there and got the T-Shirt!!), you may need to take off more wood and its easier when its still held fixed.....

As a rule of thumb, you need to find plywood at least twice as thick as the coil, but make sure that its not too thick (too heavy), though you can make it narrower where/when you cut out unwanted wood.......you can add another piece of ply on top of the nulling area, to add extra strength nif needed.....properly glued over the whole area......and clamped while setting. Use the router to make it look better and to blend it in.....

Now who here knows a great way to make a telescopic leg for the detector......????

Regards

Andy

[Monolith]

What about a small TX coil inside a large RX coil, would that improve matters more?

Regards

Andy

Hi Andy,

I never tried a second small TX in the center. The RX works best in a position of weak magnetic field density. There is a French patent where they call it a "magnetic vacuum", because the residual voltage from the TX coil coupled to the RX coil is a limiting factor.

What causes the residual voltage?

Can the residual voltage be calculated?

Monolith

[simonbaker]

You will have probably noticed that IB VLF motion detectors have an optimum sweep speed. Some detectors like to be swept very slowly, whereas others (such as the Tesoros) can be swept very quickly. So your analogy to driving a car with knackered suspension is quite apt. Only when your head hits the roof have you found a bump of the correct size.
Of course, driving more slowly would miss the bump. As would driving a lot faster.

Good point -- how many people know the optimum speed of their detectors?

Also gives some insight into ways in which a detector can be fooled -- such as small bumps spaced apart at just the right intervals might build up a signal, like a pendulum. Next time you sweep, it's gone -- then its back -- drives you nuts...

This does depend on the damping factor of the filter of course, some more sloshy than others I imagine.

-SB

[simonbaker]

Hi,

Although I am far away from being an expert, I have made 6 or 7 coils using plywood and a router, the first time was at least 10 years ago and I still have that machine, though all the ones since have either been sold or given away or weren't mine in the first place.....and I haven't built a new detector in years.....but I will dig out the old Twin loop PI that I made and make some photos of the head......light, but strong, maybe not pretty, but prettier than a mud fence!!

After reading your reply, I do feel that such construction details need to be better documented and put on Geotech in a blog for construction details (assuming that nobody has done that up to now!!), we have a coil blog, perhaps that is the right place, but for electronics cases and the expanding leg (I am useless on this bit!!) ideas, maybe another blog should be started.....what do you all out there think?

When I make the coil (this year) for my IGSL, I will make pictures and post them here on the Coil Blog, its really simple and easy to do........the trick being to cut a slot only as deep and wide as needed as the epoxy quickly adds weight. I sand down and fill any tiny voids with car filler fiber glass, slightly flexible version.

It should not be used to actually hold the coil down itself though, then a good sand, followed by one coat of spray paint, sand lightly, then two coats.....it keeps moisture out of the wood. I have also sometimes coated with epoxy to seal.....it depends on how fluid the epoxy is, some are too thick.....

Where coils crossover, the depth of cut needs to be increased (doubled) to account for that so that the coils are not "sticking out" of the wood. I also try and make the coil winding itself a bit wider and therefore thinner at the point where they crossover in a Double D config (If I don't forget to do that when Super glueing!!), to stop major thickness buildup.....

Tools needed are an electric sander with various grades of sanding discs, a router with various different cutters, ones with a ball end are particularly good as you have a cut that is rounded at the bottom and this reduces the amounts of epoxy needed....eg. weight!
But even if you don't have a rounded cutter, a square one will do as well.......

Try and measure the thickness of the coils and use a cutter of that width, if the slot is then too tight, you can always run the cutter around again, slightly offset from the first time.....its usually helpful to cut a millimeter deeper than you think you need, then when sanding, you won't hit and damage the coil......

The biggest problem is that you need a piece of plywwod far greater than the coil, so that you can hold it down well, you also need some throwaway wood underneath, so that you can cut out areas to lighten the assembly up, and you need to cut deeper than the plywood. So there is a degree of waste that one has to accept as well, so plan carefully to keep that to a minimum......

I buy my plywood at a big company where they take peanuts for good big quality offcuts.....that keeps costs down.

The overlap areas need to be more open to allow the coils to be nulled easily, be prepared to cut some wood away to assist in that.....

Do try the coils in the head BEFORE YOU TAKE IT OFF THE ROUTER TABLE. (been there and got the T-Shirt!!), you may need to take off more wood and its easier when its still held fixed.....

As a rule of thumb, you need to find plywood at least twice as thick as the coil, but make sure that its not too thick (too heavy), though you can make it narrower where/when you cut out unwanted wood.......you can add another piece of ply on top of the nulling area, to add extra strength nif needed.....properly glued over the whole area......and clamped while setting. Use the router to make it look better and to blend it in.....

Now who here knows a great way to make a telescopic leg for the detector......????

Regards

Andy

I like it.

I'm also interested in an alternate construction -- instead of routing, cut pieces and build them up like a sandwich, gluing pieces, leaving channels that would be same as routing. The idea is to do it with minimum tools, just a saw (hand scroll saw or jig saw or band saw).

The bottom piece would be a very thin veneer plywood or even cardboard since the stacked pieces would eventually make a rigid structure.

Always searching for extreme DIY techniques for those who just want to get an MD made no holds barred.

-SB

[der_fisherman]

That sounds like a great idea for anyone without a router. Maybe cutting bits of wood or ply with a hand fretsaw.....

For anyone who does not know what a fretsaw is or how to use it, it does finer work than an electric jigsaw, look here:-

http://en.wikipedia.org/wiki/Fretsaw

regards

Andy

[simonbaker]

That sounds like a great idea for anyone without a router. Maybe cutting bits of wood or ply with a hand fretsaw.....

For anyone who does not know what a fretsaw is or how to use it, it does finer work than an electric jigsaw, look here:-

http://en.wikipedia.org/wiki/Fretsaw

regards

Andy

Nice - I knew of coping saws, but this is better.

[der_fisherman]

Nice - I knew of coping saws, but this is better.

...and VERY cheap!!! Even new blades are really cheap.....a dozen at a time for pennies......

As a kid in the '50s, I made jigsaw puzzles for my younger brother with one!!!

There are electric ones around, some on ebay....liken here:-

http://www.ebay.co.uk/itm/FRETSAW-Dr...item3cbaf4e0fd

regards

Andy

[simonbaker]

...and VERY cheap!!! Even new blades are really cheap.....a dozen at a time for pennies......

As a kid in the '50s, I made jigsaw puzzles for my younger brother with one!!!

There are electric ones around, some on ebay....liken here:-

http://www.ebay.co.uk/itm/FRETSAW-Dr...item3cbaf4e0fd

regards

Andy

Thanks, I'll keep an eye out for something like that. I have a problem with space however, so will have to do some major organizing at some point.

Regards,

-SB

[satdaveuk]

...and VERY cheap!!! Even new blades are really cheap.....a dozen at a time for pennies......

As a kid in the '50s, I made jigsaw puzzles for my younger brother with one!!!

There are electric ones around, some on ebay....liken here:-

http://www.ebay.co.uk/itm/FRETSAW-Dr...item3cbaf4e0fd

regards

Andy

ditto we made jigsaw puzzles with a fretsaw, and my dad made them for us when we were knee high to grass hoppers, I thought most people owned one.
Talking of comparing your DD coils with other types, I can only comment on personal experience of owing and using many different brand detectors over the years that to be honest with you even in are uk soils it largely depends on the detector design on whats good or better on which particular machine.
For instance the Whites DFX which is not so popular over here and never was for what reasons im not sure but at a guess I think your avarage detectorist just found them to complicated to use, but if they had grasped the way it operates and had done some good field tests by people like myself with my own custom settings which took a good 18months and hundreds of hours to master then the tables would have turned because even by todays standards of machines on the market there still way past half way to the top end machines.
Coil wise they work at there best with the circular coil they came out the factory which was a 950cm, I have done exstensive tests with the Whites 14x10 DD coil which Whites themselfs told me should give me that extra depth which never happened.
All it did was make pinpointing less accurate and ruin the balance of a perfectly made machine.
So tech boys work that one out because that coil been so much larger should in theory have at least produced more depth .
On the other hand I have a Minelab Quattro which is multifrequency with a 10DD coil factory fitted ,it gives a tad more depth with silver finds, most detectors are good with copper and lead so not worth the mention, but gold its crap, and that no disc and all metals, even my Whites 1v pics up better again with the circular coil.
So my point is between the two types of coils you have been chatting about its horses for courses, I really have come to the conclussion that some work better than others on different machines and yes even maybe the same makes and models because two machine will never be identical due to lots of factors, and thats why different people use different settings, its not just the types of soils etc there detecting on, its also the machines, What do you think?
Ive never made a circular coil for a induction balance machine because they seem so complicated, has anyone here got some uncomplicated instructions to build one say for the TGSL most of us built here because it will be interesting to see the results, Ive made loads of DD coils with very good results, or at least I think so.
May a add ive been reading this thread with great interest like so many others on here, your such a clever bunch I thought I was good but now I feel like a apprentice again
So a big thank you to all of you and as ive said previously if you was girls id give you all a big wet kiss.
On a serious note wish I had known about this forum years ago its been a education and such a learning curve.
Regards
Dave

[der_fisherman]

ditto we made jigsaw puzzles with a fretsaw, and my dad made them for us when we were knee high to grass hoppers, I thought most people owned one.
Talking of comparing your DD coils with other types, I can only comment on personal experience of owing and using many different brand detectors over the years that to be honest with you even in are uk soils it largely depends on the detector design on whats good or better on which particular machine.
For instance the Whites DFX which is not so popular over here and never was for what reasons im not sure but at a guess I think your avarage detectorist just found them to complicated to use, but if they had grasped the way it operates and had done some good field tests by people like myself with my own custom settings which took a good 18months and hundreds of hours to master then the tables would have turned because even by todays standards of machines on the market there still way past half way to the top end machines.
Coil wise they work at there best with the circular coil they came out the factory which was a 950cm, I have done exstensive tests with the Whites 14x10 DD coil which Whites themselfs told me should give me that extra depth which never happened.
All it did was make pinpointing less accurate and ruin the balance of a perfectly made machine.
So tech boys work that one out because that coil been so much larger should in theory have at least produced more depth .
On the other hand I have a Minelab Quattro which is multifrequency with a 10DD coil factory fitted ,it gives a tad more depth with silver finds, most detectors are good with copper and lead so not worth the mention, but gold its crap, and that no disc and all metals, even my Whites 1v pics up better again with the circular coil.
So my point is between the two types of coils you have been chatting about its horses for courses, I really have come to the conclussion that some work better than others on different machines and yes even maybe the same makes and models because two machine will never be identical due to lots of factors, and thats why different people use different settings, its not just the types of soils etc there detecting on, its also the machines, What do you think?
Ive never made a circular coil for a induction balance machine because they seem so complicated, has anyone here got some uncomplicated instructions to build one say for the TGSL most of us built here because it will be interesting to see the results, Ive made loads of DD coils with very good results, or at least I think so.
May a add ive been reading this thread with great interest like so many others on here, your such a clever bunch I thought I was good but now I feel like a apprentice again
So a big thank you to all of you and as ive said previously if you was girls id give you all a big wet kiss.
On a serious note wish I had known about this forum years ago its been a education and such a learning curve.
Regards
Dave

That is one BIG M.F. of a coil!!!
You must be very fit and strong, but I am not surprised that pinpointing is not it's strong point......its almost 10 meters (30Ft) across....
Can you show us pictures?
Regards
Andy

[Davor]

Aziz,
you mentioned somewhere that you obtained better results with concentric coplanar coils with radius ratios moved from 50% to 80%. Now I just wonder: what would be a radius ratio to to flux ratio (inner circle vs rim between circles) over a range of radius ratios. My gut feeling says that 80% radius should be close to 50% flux. I guess 50% flux could be a geometric sweet spot for these concentric coils, and should lead to a nice turns ratio. Plus it should not be as much affected by ground.

I'm not a fond of elliptic integrals, and your software doesn't mind, so how about it?

[Aziz]

Aziz,
you mentioned somewhere that you obtained better results with concentric coplanar coils with radius ratios moved from 50% to 80%. Now I just wonder: what would be a radius ratio to to flux ratio (inner circle vs rim between circles) over a range of radius ratios. My gut feeling says that 80% radius should be close to 50% flux. I guess 50% flux could be a geometric sweet spot for these concentric coils, and should lead to a nice turns ratio. Plus it should not be as much affected by ground.

I'm not a fond of elliptic integrals, and your software doesn't mind, so how about it?

Yep, I have compared 0.85, 0.8, 0.75 and 0.5 coil radius ratios together (normalized all to same coil inductances, TX (total) = 300µH, RX = 300µH, all induction balanced)

The results were:
The 0.5 radius ratio gives more response in the near detection region and less response at far detection region (good for pin-pointing).
The 0.8 radius ratio gives less response in the near detection region (compared to 0.5) and a better depth detection range (good for large detection depth).

The 0.8 radius ratio worked best for detection depth. The 0.75 isn't much different to 0.8.

BTW, the 0.71 (sqrt(2)/2) is the 50% coil flux area ratio. The sweet spot must be somewhere between 0.7 and 0.8.


Cheers,
Aziz

[Aziz]

Sweet spot at 0.82 (0.82 = sqrt(2/3)) ???
I don't know.
It isn't really relevant. The 0.5 radius radio picks up less EMI noise.
Aziz