Critterhunter
New member
First, read this excellent article on the differences between DD and concentric coils. It has some nice drawings to illistrate the differences between the detection fields generated by these two types of coils, along with illistrations as to why a DD will unmask targets better due to the nature of it's detection field. The only thing I disagree with is his statements about not really needing smaller or larger coils to enhance unmasking ability or to get deeper. I find having an extra small coil along with a larger coil than what came with a machine stock has been very handy for me in certain situations (a tiny coil for heavy trash or a larger one than stock for extra deep coins).
http://www.ohiometaldetecting.com/2/post/2009/09/shes-right-you-really-dont-need-an-additional-searchcoil.html
Here's my theory on coil dynamics, as well as recovery speeds, and how these things relate to unmasking abilities...
One thing about unmasking ability...You can't change the laws of physics. Think of the detection field as a beam from a flashlight. It can't go around corners. If trash is directly over the coin, or even a good deal off to the side of the coin but much shallower than the coin (how far off to the side it can be depends on how much difference there is in depth between the two targets), the coil signal will hit the trash first and never have the ability to reach the coin to see it.
The flashlight analogy isn't exactly the best way to describe the beam, because a flashlight can hit a closer object and still have part of it's beam go right past the side of it and continue on...Hitting the further away object as well. A coil's detection field tends to be more broad the closer it is to the coil, and if any of this beam first hits the shallower object it in effect collapses the field and doesn't allow any part of it to continue further on to see the deeper object. IE: It can't shoot a straight beam of detection right past the shallower object that part of it is hitting and down further to the deeper target. That's why the greater distance between depths means the two can be somewhat further off to the side of each other and still in effect the deeper target never gets hit by the detection field.
Even a small but shallower item such as a staple (read a test about this) will stop the detection field from reaching the deeper coin. That's where even a fast recovery machine really has no better ability than a slow recovery machine (so long as you are sweeping the slower machine slow enough to make it's recovery between targets a non-issue).
Where the real ability lies is in just how sharp the detection field is of the coil in order to be able to separate targets. Something like an SEF 12x10 has a much tighter left/right detection field than a typical DD coil, making it laser like in it's ability to separate two targets sitting right next to each other. I'm fairly impressed in the abilities of this coil in that respect. It feels like you are using a scalpel to slice between targets.
Now, when a coin and a piece of trash are laying at the same depth and so close that they are even touching each other, resulting in both being washed in the magnetic field from the coil at the same time....That's when it's up to the machine's internal processing to be able to separate the two in some way if it can. I've used some machines that were poor at this, and would either discriminate out the combined targets, or at least severely average the signal to somewhere halfway between them on the conductivity scale. Other machines seem better at making distinctions between the mixed signal, and won't discriminate or at least won't severely average the two targets on the conductivity scale...At least as often as other machines tend to.
Ask yourself this...Would you rather use a fast recovery machine that's coil is putting out a blunt and wide beam into the ground, or would you rather use a slow recovery machine that is using a coil with a very sharp detection field that can "light up' one target while not the other in it's detection field? Don't get me wrong, fast recovery has it's advantages with the ability to swing faster without missing targets, but slow recovery is for the most part a non-issue so long as you keep your sweep speed at a proper slow speed. That, combined with trying to "sniff" around targets with proper coil use (wiggling and such) to see if there are other objects also present, makes it less of an issue.
That's were some (including me) are big fans in the separation ability of DD coils versus concentrics. The DD detection pattern is somewhat like a line going from the tip to the tail of the coil, giving it better left/right separation over the entire depth range. While a concentric tends to have more of a blunted or more broad detection field, at least at shallower depths, and so tends to have it's best separation ability at the deepest part of the signal where it narrows towards a point. Of course these are very broad statements, as various coils have very different detection patterns and qualities that somewhat overlap each other when comparing one style (DD) to the other (concentric), so these aren't hard and fast rules or correct by any means in all respects. But one of the big perks of DD coils is that, because of their general detection field, they ride on and "see" (again, in general) less ground matrix than a concentric...Which means they are submitted to and washing in less ground matrix than a similar sized concentric. Translating into machines being able to handle rougher ground matrix's than is typical for similar sized concentric. Again, not a hard and fast rule....But in general.
One of the odd things I've noticed is that a small (5.5") DD coil seems to hold onto it's depth abilities better than a small concentric. I'm simply amazed that I'm getting 7 to 9" on a dime with such a little DD coil while my 8 to 10" concentric coils on other machines could only muster around 7.5" in my soil. That just blows my mind. I know that (again, generally speaking) a concentric of the same size as a DD will tend to have slightly better depth (even though it usually is only one small point at the center of the field that reaches deeper than the DD), but this rule seems to reverse it's self when the coils get smaller than say about 6"
I know I'm way off topic and have rambled enough. Sorry...
This is all my theory anyway, and so I welcome any different views on all this that don't agree.
Correction Edit...
Probably a better way to say that is....That the closer to the coil the shallower trash target is, the further off to the side the deeper coin can be and still be masked...Because the coil's signal is more broad (even on a DD, but worse on a concentric) closer to the coil, so it has more chance for the field to first hit the trash target and thus not reach the deeper coin. So the closer the trash is to the coil, the more off to the side that trash can be in relation to the coin and still block the signal.
Think of the detection field like a soap bubble. The first thing it hits causes the bubble to burst, and it won't squeeze past that object and still be able to hit something deeper.
Again, would like to hear opinions that differ on all this.
http://www.ohiometaldetecting.com/2/post/2009/09/shes-right-you-really-dont-need-an-additional-searchcoil.html
Here's my theory on coil dynamics, as well as recovery speeds, and how these things relate to unmasking abilities...
One thing about unmasking ability...You can't change the laws of physics. Think of the detection field as a beam from a flashlight. It can't go around corners. If trash is directly over the coin, or even a good deal off to the side of the coin but much shallower than the coin (how far off to the side it can be depends on how much difference there is in depth between the two targets), the coil signal will hit the trash first and never have the ability to reach the coin to see it.
The flashlight analogy isn't exactly the best way to describe the beam, because a flashlight can hit a closer object and still have part of it's beam go right past the side of it and continue on...Hitting the further away object as well. A coil's detection field tends to be more broad the closer it is to the coil, and if any of this beam first hits the shallower object it in effect collapses the field and doesn't allow any part of it to continue further on to see the deeper object. IE: It can't shoot a straight beam of detection right past the shallower object that part of it is hitting and down further to the deeper target. That's why the greater distance between depths means the two can be somewhat further off to the side of each other and still in effect the deeper target never gets hit by the detection field.
Even a small but shallower item such as a staple (read a test about this) will stop the detection field from reaching the deeper coin. That's where even a fast recovery machine really has no better ability than a slow recovery machine (so long as you are sweeping the slower machine slow enough to make it's recovery between targets a non-issue).
Where the real ability lies is in just how sharp the detection field is of the coil in order to be able to separate targets. Something like an SEF 12x10 has a much tighter left/right detection field than a typical DD coil, making it laser like in it's ability to separate two targets sitting right next to each other. I'm fairly impressed in the abilities of this coil in that respect. It feels like you are using a scalpel to slice between targets.
Now, when a coin and a piece of trash are laying at the same depth and so close that they are even touching each other, resulting in both being washed in the magnetic field from the coil at the same time....That's when it's up to the machine's internal processing to be able to separate the two in some way if it can. I've used some machines that were poor at this, and would either discriminate out the combined targets, or at least severely average the signal to somewhere halfway between them on the conductivity scale. Other machines seem better at making distinctions between the mixed signal, and won't discriminate or at least won't severely average the two targets on the conductivity scale...At least as often as other machines tend to.
Ask yourself this...Would you rather use a fast recovery machine that's coil is putting out a blunt and wide beam into the ground, or would you rather use a slow recovery machine that is using a coil with a very sharp detection field that can "light up' one target while not the other in it's detection field? Don't get me wrong, fast recovery has it's advantages with the ability to swing faster without missing targets, but slow recovery is for the most part a non-issue so long as you keep your sweep speed at a proper slow speed. That, combined with trying to "sniff" around targets with proper coil use (wiggling and such) to see if there are other objects also present, makes it less of an issue.
That's were some (including me) are big fans in the separation ability of DD coils versus concentrics. The DD detection pattern is somewhat like a line going from the tip to the tail of the coil, giving it better left/right separation over the entire depth range. While a concentric tends to have more of a blunted or more broad detection field, at least at shallower depths, and so tends to have it's best separation ability at the deepest part of the signal where it narrows towards a point. Of course these are very broad statements, as various coils have very different detection patterns and qualities that somewhat overlap each other when comparing one style (DD) to the other (concentric), so these aren't hard and fast rules or correct by any means in all respects. But one of the big perks of DD coils is that, because of their general detection field, they ride on and "see" (again, in general) less ground matrix than a concentric...Which means they are submitted to and washing in less ground matrix than a similar sized concentric. Translating into machines being able to handle rougher ground matrix's than is typical for similar sized concentric. Again, not a hard and fast rule....But in general.
One of the odd things I've noticed is that a small (5.5") DD coil seems to hold onto it's depth abilities better than a small concentric. I'm simply amazed that I'm getting 7 to 9" on a dime with such a little DD coil while my 8 to 10" concentric coils on other machines could only muster around 7.5" in my soil. That just blows my mind. I know that (again, generally speaking) a concentric of the same size as a DD will tend to have slightly better depth (even though it usually is only one small point at the center of the field that reaches deeper than the DD), but this rule seems to reverse it's self when the coils get smaller than say about 6"
I know I'm way off topic and have rambled enough. Sorry...
This is all my theory anyway, and so I welcome any different views on all this that don't agree.
Correction Edit...
Critterhunter said:
Probably a better way to say that is....That the closer to the coil the shallower trash target is, the further off to the side the deeper coin can be and still be masked...Because the coil's signal is more broad (even on a DD, but worse on a concentric) closer to the coil, so it has more chance for the field to first hit the trash target and thus not reach the deeper coin. So the closer the trash is to the coil, the more off to the side that trash can be in relation to the coin and still block the signal.
Think of the detection field like a soap bubble. The first thing it hits causes the bubble to burst, and it won't squeeze past that object and still be able to hit something deeper.
Again, would like to hear opinions that differ on all this.
