New Concentric Co-planar Coil with Strong MF.

[Monolith]

The TX field reduction depends highly on where in the field you look. With conventional design (RX and bucking coil have same size and position), the field "inside" the bucking coil perimeter is very small at the plane of the coil, since it creates equal and opposite total flux to the TX coil in that region. This creates the "balance" where the RX coil sees almost no signal. However, as you go deeper, the TX magnetic field is stronger than the bucking coil field -- which is how metal objects can be stimulated and detected.



Yes, we're probably saying the same thing.

Since the RX coil and bucking coil can be wound on the same form, there is not much chance of shifting relative position to upset the balance. And since the magnetic field within the TX perimeter is not highly variable with position, the balance is not too sensitive to slight shifts there either -- at least a lot less sensitive than with double-D coils, where the tiniest shift changes the balance.

Regards,

-SB

SB,

You are right, the balance of the concentric, co-planar IB coil, is less sensitive to imperfection, than on the DD coil.
There are some parts where my understanding is still a bit vague. For example, the incidence of the 2 opposing fields TX and BU, on a large thin target, like a piece of alu foil, shows a predictable inverse response. I mean to say, that at a certain position within the coil field, the foil produces a negative response, while at a different place the response is positive.
This confirms your statement above.
However, a thick piece of alu plate of the same surface dimension, gives a positive response everywhere. This must have something to do with the deeper eddy currents, while the inverse response if from the skin eddy currents.

But why is that?

It is important to find the answer to this question, because this phase inversion is a problem with discriminating thin, rusty, steel fragments, like remains of sardine cans etc. that litter productive detecting sites.

Maybe I should clarify, that I used a TX pulse specially designed to enhance deep eddy currents, from massive targets, when I came across the problem described above.

Monolith

[simonbaker]

SB,

You are right, the balance of the concentric, co-planar IB coil, is less sensitive to imperfection, than on the DD coil.
There are some parts where my understanding is still a bit vague. For example, the incidence of the 2 opposing fields TX and BU, on a large thin target, like a piece of alu foil, shows a predictable inverse response. I mean to say, that at a certain position within the coil field, the foil produces a negative response, while at a different place the response is positive.
This confirms your statement above.
However, a thick piece of alu plate of the same surface dimension, gives a positive response everywhere. This must have something to do with the deeper eddy currents, while the inverse response if from the skin eddy currents.

But why is that?

It is important to find the answer to this question, because this phase inversion is a problem with discriminating thin, rusty, steel fragments, like remains of sardine cans etc. that litter productive detecting sites.

Maybe I should clarify, that I used a TX pulse specially designed to enhance deep eddy currents, from massive targets, when I came across the problem described above.

Monolith

Those are good questions and likely depend on the details of the actual magnetic field for a particular coil.

I am not too familiar with pulse type MDs, so harder for me to imagine what is going on.

If phase reverses, it makes me think that part of the target is in a region where the bucking field is stronger than the TX field, and part of the target is in opposite case as you move it around.

Not sure if that is easy to avoid! With double-D detectors, I'm sure the target changes phase also from region to region.

Regards,

-SB

[der_fisherman]

Coil Shielding

Yes, there is much influence. Therefore I took the mean radius of the coils and this is a good approximation. Of course, the windings should be wound as tight as possible. I made this by winding a thin isolated wire around the coils. This wire has no potential and is not short circuited. Of course, a nylon thread is more convenient. I hadn't one.

Shielding is most important to RX coil. The TX and bucking coil need not to be shielded and can be neglected. If someone wants a perfect coil, then shield all of your coils.
I have not calculated the inductance of the coils yet. I have to expand the model and this would take some time for calculation. I will support different coil windings for TX and bucking coil soon. This can be better used for particular coil design.

Aziz

Hi Aziz,

I find your comments on shielding to be very, very interesting. I have always shelded both coils, because I did not know any better probably.

Do you feel that even Double D coils only need the Rx coil shielded, or am I totally wrong? I don't want to build a coil and have to throw it away because of building it wrongly.....

I know your post is from 2008, I only read it today and I am hoping that your knowledge and experience since then will enable you to answer my question.

I will read on in the blog as well, and if you have already answered this question, please ignore my post.

Great blog, great diagrams and pictures.

Many thanks

Andy

[der_fisherman]

Gday Qiaozhi,

That's pretty much exactly what i said, not sure about the better seperation of targets though, small coils are definitely more sensitive in the field--each to there own?

It would be interesting to do some testing to check some things out regarding the field strength of say MONO coils of varying sizes.
One would think that with each increase in diameter the field strength would vary, with all things being equal of course. Inductance, Voltage, Amps, etc.

How does that work actually?
It would seem to get the same concentrated field strength as an 8" coil with X Volts & Amps that you would have to increase Voltage & Amps sufficiently to get the same concentrated field strength with every increase in coil diameter?
This is more than likely the reason that larger coils fall in a heap on smaller targets?.

I was just looking at the net & found Garretts opinion on coil size, it's pretty much the same as what we are talking about.
http://www.garrett.com/hobby/techsup...searchcoil.pdf

It appears that the website has been changed, the file will now be found at:-

http://www.garrett.com/hobbysite/pdf...tech_sheet.pdf

regards

Andy

[der_fisherman]

IB coils with and without a Bucking Coil

Firstly this blog was for me VERY interesting indeed. I have read all the posts, truly great.
Now I am going to make a few statements, please let me know if I am possibly correct or not.
1) I get the impression that a "Bucking Coil" makes it simpler to achieve a "balance", without the need for physically moving one or the other of the main RX and TX coils. Possibly "Bucking Coils" are more usefully for manufacturers as they can be set up once the search head is complete.
But the BC reduces the power of the TX coil, reducing the search depth. EG. A negative effect.
2) Double "D" coils are "Nulled" and there is no loss of TX power. Therefore I find that coils without a bucking coil are probably better for the same TX power and probably use less overall battery power, as well as being more sensitive when correctly set up.....
Many thanks to any and all of you that answer my post.

Regards

Andy

[Aziz]

Hi Aziz,

I find your comments on shielding to be very, very interesting. I have always shelded both coils, because I did not know any better probably.

Do you feel that even Double D coils only need the Rx coil shielded, or am I totally wrong? I don't want to build a coil and have to throw it away because of building it wrongly.....

I know your post is from 2008, I only read it today and I am hoping that your knowledge and experience since then will enable you to answer my question.

I will read on in the blog as well, and if you have already answered this question, please ignore my post.

Great blog, great diagrams and pictures.

Many thanks

Andy

Hi Andy,

Generally spoken, shield all your coils (TX, bucking, RX). Double D coil as well (TX, RX).

The transmitter coil should be shielded to avoid interference emissions to other devices.
Note: An antenna consists of an inductor and capacitor (either parasitic or specific).
High electric field at the coil has a high emission potential (power). The shielding reduces the electric field outside of the coil. So the coil do not act as an transmit antenna and transmits electro-magnetic field waves. The transmitter coil should only emit magnetic field nearby the coil. The magnetic field diminishes quickly at distances.

Some tuned LC circuits in metal detectors are sensitive to parasitic capacitances, which can occur between search coil and ground/the operator. This can tune out the LC tank circuit in the transmitter section of the detector. Same applies to RX coil of course.

Note, RX coil is also an antenna.
Fortunately, the "antenna" is not much efficient in the metal detector applications. But it is efficient enough to disturb the operation.

Cheers,

Aziz

[der_fisherman]

Thanks for the rapid answer.
Andy

[simonbaker]

Firstly this blog was for me VERY interesting indeed. I have read all the posts, truly great.
Now I am going to make a few statements, please let me know if I am possibly correct or not.
1) I get the impression that a "Bucking Coil" makes it simpler to achieve a "balance", without the need for physically moving one or the other of the main RX and TX coils. Possibly "Bucking Coils" are more usefully for manufacturers as they can be set up once the search head is complete.
But the BC reduces the power of the TX coil, reducing the search depth. EG. A negative effect.
2) Double "D" coils are "Nulled" and there is no loss of TX power. Therefore I find that coils without a bucking coil are probably better for the same TX power and probably use less overall battery power, as well as being more sensitive when correctly set up.....
Many thanks to any and all of you that answer my post.

Regards

Andy

A couple of points that possibly compensate for concentric coils (using bucking coil):

1. In concentric coils, the Tx and Rx coils are both circular and have the same center, so are in perfect alignment. This may help the effective signal strength compared to the offset IB coils.

2. Double-D coils are not round and likely do not generate as effective a TX magnetic field (for depth) compared to an equivalent diameter concentric TX coil. Double-O coils are round, but may have even more offset alignment causing less efficient use of magnetic field.

3. Concentric coils probably less sensitive to vibration. Vibration can cause phase noise, which can reduce depth in some detector circuits (like TGSL).

4. The deeper you go, the less canceling the bucking coil is to a target. It does not disappear entirely I believe.

I think IB coils have a historic depth advantage over concentric coils using bucking coil, but not as severe as one might think. I agree that mass producing concentric coils is probably easier than IB coils and drives their use, even though the reverse is true for do-it-yourselfers. Concentric coils also are probably easier for the new user.

I would like to hear more discussion on importance of shielding the TX coil. One reason I have heard is to reduce certain ground effects which can modulate the TX oscillator. I'd like to understand better about the "radiative" vs. "magnetic" signal coming from the coil and the significance of it. PI coils must radiate like crazy in comparison.

-SB

[der_fisherman]

A couple of points that possibly compensate for concentric coils (using bucking coil):

1. In concentric coils, the Tx and Rx coils are both circular and have the same center, so are in perfect alignment. This may help the effective signal strength compared to the offset IB coils.

2. Double-D coils are not round and likely do not generate as effective a TX magnetic field (for depth) compared to an equivalent diameter concentric TX coil. Double-O coils are round, but may have even more offset alignment causing less efficient use of magnetic field.

3. Concentric coils probably less sensitive to vibration. Vibration can cause phase noise, which can reduce depth in some detector circuits (like TGSL).

4. The deeper you go, the less canceling the bucking coil is to a target. It does not disappear entirely I believe.

I think IB coils have a historic depth advantage over concentric coils using bucking coil, but not as severe as one might think. I agree that mass producing concentric coils is probably easier than IB coils and drives their use, even though the reverse is true for do-it-yourselfers. Concentric coils also are probably easier for the new user.

I would like to hear more discussion on importance of shielding the TX coil. One reason I have heard is to reduce certain ground effects which can modulate the TX oscillator. I'd like to understand better about the "radiative" vs. "magnetic" signal coming from the coil and the significance of it. PI coils must radiate like crazy in comparison.

-SB

Hi Simon,

really great post, many thanks, but I am not quite convinced in some areas, please don't be offended, we here (and I) need guys exactly like you as when each of us brings a valid point or two to the table, we all benefit from a very full table!!!

I think its a great point for discussion (has probably been already discussed somewhere else here) as to whether concentric or DD coils give the clearest indication of the find's location. I personally prefer the DD as its quite a "tight" detect zone looking from left to right and one only needs to move forwards & backwards to finalize the location. I do believe that although the detection zone is narrower, its still deeper than a comparable concentric coil on the same machine.

Whereas the concentric coils have a "pointed Cone" type of detection area (DDs have a narrow cone left/right) and detect earlier when scanning left & right, but not as deep, because the "energy" of the concentric coil is not focussed as tightly as with DDs.

I think you agree with me up to this point....

Now if the bucking coil, as you and I both seem to agree on, works "against" the transmitted signal (to achieve a "NULL" pickup in the RX coil), even if its effects drop off with depth.

I disagree a bit on that point as I simply don't understand where you are with that as BOTH signals drop off in intensity as depth increases, so how does one decrease less?

I personally feel, when all else is equal, that well made and adjusted DDs have the best depth and pinpointing, and the Concentrics look better and are easier to manufacture in quantity!!

With regard to depth and sensitivity to vibration, I simply cannot follow your line of reasoning at all. If a coil is somehow loosely built, no matter of what type, vibration will be a problem on both.....with regard to depth, for the same power output, the tighter the focus, the deeper the depth for the same power output....but its probably not a huge difference....maybe only measurable in the lab......

But, funnily enough, I personally find the DD easier to fabricate and adjust as a DIYer.....but that's probably just me!! I am sure there are others that think the opposite.

The difference in power usage should not be forgotten either....

Lets see what others will say for either type.....it should prove both interesting and informative!!

Have a great day.....

regards

Andy

PS I would like to learn more about Faraday shielding as well and why its still needed for a TX coil.........especially if a TX coil is fed by a buffered Oscillator, and is not therefore part of the frequency determining components......its something I have long wanted to try and never done......

[simonbaker]

Hi Simon,

really great post, many thanks, but I am not quite convinced in some areas, please don't be offended, we here (and I) need guys exactly like you as when each of us brings a valid point or two to the table, we all benefit from a very full table!!!

I think its a great point for discussion (has probably been already discussed somewhere else here) as to whether concentric or DD coils give the clearest indication of the find's location. I personally prefer the DD as its quite a "tight" detect zone looking from left to right and one only needs to move forwards & backwards to finalize the location. I do believe that although the detection zone is narrower, its still deeper than a comparable concentric coil on the same machine.

Whereas the concentric coils have a "pointed Cone" type of detection area (DDs have a narrow cone left/right) and detect earlier when scanning left & right, but not as deep, because the "energy" of the concentric coil is not focussed as tightly as with DDs.

I think you agree with me up to this point....

Now if the bucking coil, as you and I both seem to agree on, works "against" the transmitted signal (to achieve a "NULL" pickup in the RX coil), even if its effects drop off with depth.

I disagree a bit on that point as I simply don't understand where you are with that as BOTH signals drop off in intensity as depth increases, so how does one decrease less?

I personally feel, when all else is equal, that well made and adjusted DDs have the best depth and pinpointing, and the Concentrics look better and are easier to manufacture in quantity!!

With regard to depth and sensitivity to vibration, I simply cannot follow your line of reasoning at all. If a coil is somehow loosely built, no matter of what type, vibration will be a problem on both.....with regard to depth, for the same power output, the tighter the focus, the deeper the depth for the same power output....but its probably not a huge difference....maybe only measurable in the lab......

But, funnily enough, I personally find the DD easier to fabricate and adjust as a DIYer.....but that's probably just me!! I am sure there are others that think the opposite.

The difference in power usage should not be forgotten either....

Lets see what others will say for either type.....it should prove both interesting and informative!!

Have a great day.....

regards

Andy

PS I would like to learn more about Faraday shielding as well and why its still needed for a TX coil.........especially if a TX coil is fed by a buffered Oscillator, and is not therefore part of the frequency determining components......its something I have long wanted to try and never done......

I enjoy discussing theory in hopes of learning or improving understanding, so never hesitate to question or disagree with my speculations! I always believe the proof is in the pudding, so if theory doesn't agree with practice, I'm interested in finding out why and getting better theoretical models that are useful. I also admit I'm too lame to do the math that would answer many of my own questions.

I see a double-D coil as two eliptical coils side-by-side. Eliptical coils each have a sort of squished magnetic field that will be stronger at shallow depth than a circular coil of similar diameter, but then probably drops off faster because of the smaller width.

It gets complicated to imagine the net sensitivity of two such coils side by side. If the target is directly under the TX coil it gets maximum stimulation, but the RX coil is off to the side and is less sensitive. Vice versa if the target is directly under the RX coil. If the target is in the middle, neither coil is optimally positioned. So that is the double-D's Achille's heel similar to the concentric's need for the bucking coil.

On the negative side for concentric coils, one coil, typically the RX coil, tends to be very small, like 1/2 diameter of TX coil. That is a serious down-side and probably is at least as bad as the offset nature of the IB coils. Combine that with the bucking coil, and the concentric coil does have a serious Achilles heel.

I think the double-D pinpointing is good for surface coins, but only working across the overlap. With the concentric you can scan both side to side and forward-backward to pinpoint. I always thought concentrics were easier to pinpoint with, but that may be a personal feeling.

Now if the bucking coil, as you and I both seem to agree on, works "against" the transmitted signal (to achieve a "NULL" pickup in the RX coil), even if its effects drop off with depth.

I disagree a bit on that point as I simply don't understand where you are with that as BOTH signals drop off in intensity as depth increases, so how does one decrease less?

The idea there is that the bucking coil is a smaller coil than the TX coil, but designed so that the magnetic field of the bucking coil is virtually the same magnitude as the TX coil at the plane of the coils, only opposite direction. This causes near cancellation of the magnetic field at the plane of the coils within the interior of the bucking coil.

But the field of the bucking coil, whose diameter is much smaller, will diminish much faster with depth compared to the TX coil. So as you go deeper, the "field cancellation" goes away and TX coil field will dominate and provided the desired target stimulation. But I agree, there is always some cancellation, perhaps 25% or so that remains.

With regard to depth and sensitivity to vibration, I simply cannot follow your line of reasoning at all. If a coil is somehow loosely built, no matter of what type, vibration will be a problem on both

My speculation there is that with double-D coils, a slight shift of the coils causes significantly more or less flux from the TX coil to pass through the RX coil -- because that is how balancing is done! But with concentric coils, the magnetic field from the TX coil is more uniform in the interior of the coil perimeter, so shifting the RX coil does not make as radical a change in the amount of flux passing through the RX coil.

with regard to depth, for the same power output, the tighter the focus, the deeper the depth for the same power output....but its probably not a huge difference....maybe only measurable in the lab......

I'm not sure what you mean by tighter focus. For the same current and coil inductance, a smaller coil may have a stronger field at zero depth, but the larger coil probably has stronger field at greater depth (circular coils); i.e. the smaller coil field diminishes faster with depth. I need to do the math to confirm the exact quantities there. But in general, the larger the coil, the more depth you can get for a given inductance. There are some trade-offs there so that practically you don't want to go above a certain diameter for practical detection of certain size targets.

When talking about "power" used, it is tricky because a high-Q resonant TX coil will use less power than a lower-Q coil to produce the same field. The bucking coil does limit the Q because it adds wire which decreases the inductance (wound opposite direction) at the same time it increases resistance. So to get the same inductance as an IB coil, you need to wind more wire - more resistance, less Q. The circuit you use to drive the TX coil also affects the power used, so a high-Q coil can be wasted if you don't drive it efficiently. But I do agree that the IB coil should be more efficient power-wise. Using thicker wire helps minimize power loss of course.

I believe on the whole that IB (double-D/double-O) coils are deeper coils than concentrics, but I just wanted to point out some interesting virtues of concentric coils that make them more competitive than expected.

The shielding question is deeper physics for me - I'm aware there are different-order terms in the equation for an oscillating electro-magnetic field, and shielding has frequency-dependent effects, so I'm not clear on the net result. Practice says that shielded coils work better, but I think there is a lot of misunderstanding out there as to why.

I'm actually a big fan of the "mono coil" because of its perfect alignment -- maybe moodz can get one to work well.

Regards,

-SB

[Monolith]

On the negative side for concentric coils, one coil, typically the RX coil, tends to be very small, like 1/2 diameter of TX coil. That is a serious down-side and probably is at least as bad as the offset nature of the IB coils. Combine that with the bucking coil, and the concentric coil does have a serious Achilles heel.

-SB

Hi SB,

intuitively it would seem that the small RX coil will cover only a small search area. In reality this is not so. The area covered is still the area of the TX coil.
Ho does this work?
Look at the TX coil field as a large sphere. Consider this sphere to be jello. Now consider the target to be a bug that fell into the jello. The bug shakes to try to get out. The shaking is shaking the whole sphere of jello, but the movement is stronger near the bug.
If you have the coil sensor balanced within the sphere, it receives the signal directional, but it will sense any movement of the jello.
A large RX coil will be more sensitive for the same amount of inductance, but a smaller coil will generate less residual voltage and therefore can have more inductance to compensate for sensitivity.

Doubts on these fine points of this relationship between TX, BU and RX coils were nagging on me for a long time, so finally I built a few really large coils, like 0.5m, 1.2m and 1.6m. Then I experimented with small and large RX coils, from 300uH to 1200uH.

My findings are along the lines described above. I am sure all this could be proven mathematically and would welcome if somebody would do it for us.

SB, you say the proof is in the pudding. Will jello do the trick?

Monolith

[der_fisherman]

I enjoy discussing theory in hopes of learning or improving understanding, so never hesitate to question or disagree with my speculations! I always believe the proof is in the pudding, so if theory doesn't agree with practice, I'm interested in finding out why and getting better theoretical models that are useful. I also admit I'm too lame to do the math that would answer many of my own questions.

I see a double-D coil as two eliptical coils side-by-side. Eliptical coils each have a sort of squished magnetic field that will be stronger at shallow depth than a circular coil of similar diameter, but then probably drops off faster because of the smaller width.

It gets complicated to imagine the net sensitivity of two such coils side by side. If the target is directly under the TX coil it gets maximum stimulation, but the RX coil is off to the side and is less sensitive. Vice versa if the target is directly under the RX coil. If the target is in the middle, neither coil is optimally positioned. So that is the double-D's Achille's heel similar to the concentric's need for the bucking coil.

On the negative side for concentric coils, one coil, typically the RX coil, tends to be very small, like 1/2 diameter of TX coil. That is a serious down-side and probably is at least as bad as the offset nature of the IB coils. Combine that with the bucking coil, and the concentric coil does have a serious Achilles heel.

I think the double-D pinpointing is good for surface coins, but only working across the overlap. With the concentric you can scan both side to side and forward-backward to pinpoint. I always thought concentrics were easier to pinpoint with, but that may be a personal feeling.




The idea there is that the bucking coil is a smaller coil than the TX coil, but designed so that the magnetic field of the bucking coil is virtually the same magnitude as the TX coil at the plane of the coils, only opposite direction. This causes near cancellation of the magnetic field at the plane of the coils within the interior of the bucking coil.

But the field of the bucking coil, whose diameter is much smaller, will diminish much faster with depth compared to the TX coil. So as you go deeper, the "field cancellation" goes away and TX coil field will dominate and provided the desired target stimulation. But I agree, there is always some cancellation, perhaps 25% or so that remains.




My speculation there is that with double-D coils, a slight shift of the coils causes significantly more or less flux from the TX coil to pass through the RX coil -- because that is how balancing is done! But with concentric coils, the magnetic field from the TX coil is more uniform in the interior of the coil perimeter, so shifting the RX coil does not make as radical a change in the amount of flux passing through the RX coil.



I'm not sure what you mean by tighter focus. For the same current and coil inductance, a smaller coil may have a stronger field at zero depth, but the larger coil probably has stronger field at greater depth (circular coils); i.e. the smaller coil field diminishes faster with depth. I need to do the math to confirm the exact quantities there. But in general, the larger the coil, the more depth you can get for a given inductance. There are some trade-offs there so that practically you don't want to go above a certain diameter for practical detection of certain size targets.

When talking about "power" used, it is tricky because a high-Q resonant TX coil will use less power than a lower-Q coil to produce the same field. The bucking coil does limit the Q because it adds wire which decreases the inductance (wound opposite direction) at the same time it increases resistance. So to get the same inductance as an IB coil, you need to wind more wire - more resistance, less Q. The circuit you use to drive the TX coil also affects the power used, so a high-Q coil can be wasted if you don't drive it efficiently. But I do agree that the IB coil should be more efficient power-wise. Using thicker wire helps minimize power loss of course.

I believe on the whole that IB (double-D/double-O) coils are deeper coils than concentrics, but I just wanted to point out some interesting virtues of concentric coils that make them more competitive than expected.

The shielding question is deeper physics for me - I'm aware there are different-order terms in the equation for an oscillating electro-magnetic field, and shielding has frequency-dependent effects, so I'm not clear on the net result. Practice says that shielded coils work better, but I think there is a lot of misunderstanding out there as to why.

I'm actually a big fan of the "mono coil" because of its perfect alignment -- maybe moodz can get one to work well.

Regards,

-SB

Great and interesting answer, many thanks.

By the way, no coil, once adjusted, can be allowed any movement, which is why many people first saturate the coil with super glue or similar before any form of shielding is applied.

Some big mono coils are filled with builders foam to achieve that.

I personally first, use superglue, then I wrap the coil tightly in insulating tape, pulled as tight as I can, then I take a piece of uninsulated multi-strand wire and wrap this spirally round the coil, leaving a gap of a centimeter or so, then, observing the gap, I wrap the coil tightly with strips of kitchen foil, holding them in pace with a futher layer of insulating tape also as tight as possible.

Finally, I progressively immerse in epoxy resin, adjusting the null each time if it varies, till the whole is immersed in hard epoxy.....but as little as possible as epoxy is heavy....

I usually rout out a piece of plywood to allow the coil to be set in, then coat the whole coil in epoxy as waterproofing, then lightly spray paint to look good, usually black....

If the plywood shape is properly made, it makes a light but strong construction for a search head, I am amazed that I never see any others made that way.....for one offs its great!!

You should look at the detection distances achieved with 25cm DIY coils with both the TGSL and the IGSL on YouTube........really fantastic.

http://www.youtube.com/watch?v=Nx4Oxac0Ufg

http://www.youtube.com/watch?v=MS9Tp2xw7RQ

regards

Andy....

[der_fisherman]

Hi SB,

intuitively it would seem that the small RX coil will cover only a small search area. In reality this is not so. The area covered is still the area of the TX coil.
Ho does this work?
Look at the TX coil field as a large sphere. Consider this sphere to be jello. Now consider the target to be a bug that fell into the jello. The bug shakes to try to get out. The shaking is shaking the whole sphere of jello, but the movement is stronger near the bug.
If you have the coil sensor balanced within the sphere, it receives the signal directional, but it will sense any movement of the jello.
A large RX coil will be more sensitive for the same amount of inductance, but a smaller coil will generate less residual voltage and therefore can have more inductance to compensate for sensitivity.

Doubts on these fine points of this relationship between TX, BU and RX coils were nagging on me for a long time, so finally I built a few really large coils, like 0.5m, 1.2m and 1.6m. Then I experimented with small and large RX coils, from 300uH to 1200uH.

My findings are along the lines described above. I am sure all this could be proven mathematically and would welcome if somebody would do it for us.

SB, you say the proof is in the pudding. Will jello do the trick?

Monolith

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

Regards

Andy

[simonbaker]

Great and interesting answer, many thanks.

By the way, no coil, once adjusted, can be allowed any movement, which is why many people first saturate the coil with super glue or similar before any form of shielding is applied.

Some big mono coils are filled with builders foam to achieve that.

I personally first, use superglue, then I wrap the coil tightly in insulating tape, pulled as tight as I can, then I take a piece of uninsulated multi-strand wire and wrap this spirally round the coil, leaving a gap of a centimeter or so, then, observing the gap, I wrap the coil tightly with strips of kitchen foil, holding them in pace with a futher layer of insulating tape also as tight as possible.

Finally, I progressively immerse in epoxy resin, adjusting the null each time if it varies, till the whole is immersed in hard epoxy.....but as little as possible as epoxy is heavy....

I usually rout out a piece of plywood to allow the coil to be set in, then coat the whole coil in epoxy as waterproofing, then lightly spray paint to look good, usually black....

If the plywood shape is properly made, it makes a light but strong construction for a search head, I am amazed that I never see any others made that way.....for one offs its great!!

You should look at the detection distances achieved with 25cm DIY coils with both the TGSL and the IGSL on YouTube........really fantastic.

http://www.youtube.com/watch?v=Nx4Oxac0Ufg

http://www.youtube.com/watch?v=MS9Tp2xw7RQ

regards

Andy....

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.

I agree, no amount of movement should be tolerated, but even bumping a rock would send shock waves across a coil.

-SB

[simonbaker]

Hi SB,

intuitively it would seem that the small RX coil will cover only a small search area. In reality this is not so. The area covered is still the area of the TX coil.
Ho does this work?
Look at the TX coil field as a large sphere. Consider this sphere to be jello. Now consider the target to be a bug that fell into the jello. The bug shakes to try to get out. The shaking is shaking the whole sphere of jello, but the movement is stronger near the bug.
If you have the coil sensor balanced within the sphere, it receives the signal directional, but it will sense any movement of the jello.
A large RX coil will be more sensitive for the same amount of inductance, but a smaller coil will generate less residual voltage and therefore can have more inductance to compensate for sensitivity.

Doubts on these fine points of this relationship between TX, BU and RX coils were nagging on me for a long time, so finally I built a few really large coils, like 0.5m, 1.2m and 1.6m. Then I experimented with small and large RX coils, from 300uH to 1200uH.

My findings are along the lines described above. I am sure all this could be proven mathematically and would welcome if somebody would do it for us.

SB, you say the proof is in the pudding. Will jello do the trick?

Monolith

Hi Monolith --interesting analogy!

Although the proof is in the pudding, experiments can be deceptive because I believe there is an optimum coil size for a given target at a given depth. This is because although a larger coil will have its magnetic field extend deeper, the field is more spread out and weaker to begin with -- so compared to a smaller coil, it starts out weaker, but then is stronger at a greater depth. 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.

So it depends on the depth of the target what is the best size coil. But since shallow targets are easier to pick up with any coil, it makes sense to optimize for the deeper depths.

For a particular depth, I believe that it probably works best when the TX and RX coils are the same size if possible. I think that is because the equations that calculate the maximum stimulation of the target by the TX coil are just the same as the equations that calculate the maximum stimulation of the RX coil by the target. With concentric design, it is not possible to have equal sized coils, so you live with different sized coils.

The concept of "disturbing the balance" is just different way of visualizing superposition of fields. Really, the target just sees the TX coil simulating it, and the RX coil just sees the electromagnetic fields from the currents in the target. The balancing is just to remove the TX component from the RX reception so it is easier to amplify the target signal. (It also prevents coupling of the low impedance TX coil circuit to the high impedance RX circuit, which would rob some gain).

That is my intuition, but maybe something else happens too! Wish there were time to really do the math and experiments, because this is pretty essential to designing an MD. I'm sure the commercial vendors like Tesoro, Minelab have tried their best to figure this out, so copying their designs is pretty safe. I'd just like to figure it from scratch.

Cheers,

-SB