DD coils for the T2

[pull tab mel]

The 11" and 5".....Do these detect in the cone shape while using?

 

[Elton]

No.they shoot down the middle pretty much ..with some side pickup per my tests.

 

[TerraDigger]

Think of the shape of a football with the pointed ends cut off. A DD field is very similar to the concentric field, only narrower in width and a little bit more physical coverage at bottom of the field (edge of detection depth in ground).
DD fields are not a straight knife-edge running the length of the coil like a certain australian manufacturer likes to portray them. First Texas elongated DD coils will provide a narrower field than a round DD. Thats why they were designed that way. Dave J is a very smart engineer

 

[Monte]

such thing as a "Cone Shape" to the transmitted field about a search coil. Search coil dynamics have long been a very misrepresented or described feature, and this lends itself to that Double-D Vs Concentric type of question.

The 11" and 5".....Do these detect in the cone shape while using?

Fact: The Teknetics T2 is only supplied with a standard 11" elliptical Double-D search coil. There has been an available round 5" D-D coil, and now a 15" elliptical DD coil is available. Note that the ONLY search coils designed to function on the T2 (as well as the G2) are Double-D internal windings. There are no Concentric wound coils available from Teknetics.

First, as mentioned below, there is a misrepresentation about the functional or responsive electromagnetic field shape that exists about a Double-D coil. All too often they have been described as producing an equal response from each side of the coil.

Response: This is not quite correct due to the overlapping design of the Transmit and Receive windings and many times a close-proximity target can produce some rather strange responses.


Second, it has also been stated that a D-D coil will project (send or transmit, if you will) a "knife-edge" straight down from the coils center axis running from the front of the coil to the back of the coil, and to an equal distance at depth to match the length of the search coil.

Response: Again, this is also not true. For one thing, between different manufacturers detector designs, coil designs and how their circuitry works, some models will be more reactive to shallower targets ahead of or behind the search coils length. Also incorrect is that there is a "knife-edge" field that is "transmitted".

Instead, the projected EMF (electromagnetic field) will usually have a wider (left-to-right) responsive area nearer to the search coil than at the reaches of the detector/coil depth range or responsiveness, and the responsive portion of the EMF will also taper in toward the coils center from the front and rear such that the "hot spot" or strongest response (presuming the target is a coin and it is laying flat to the coil) will be in line with the center of the coil.. If all is in the operator's favor, with almost any decent search coil, the strongest response will be in-line with the center axis of the search coil, if you imagine a line running from above the search coil straight down into the ground.


Third, we can also often read comments that suggest a Double-D coil is the best coil to use in a highly mineralized environment, and that a D-D coil might also be less affected by outside FMI (electromagnetic interference).

Response: While there will be some conditions where you might see a difference in performance between a same-size D-D and Concentric coil in some challenging ground environments, there are more real differences in performance between the two types. Besides, many people can boldly state that they have read or heard, but do they really know just what "highly mineralized ground" really is?

Do they know not only the Ground Balance Phase Number but also the amount or content of the Fe3O4? I've hunted ground that seemed like ti would be mellower due to a lower Ground Phase read-out, but the Fe3O4 ferrous content was much greater and caused more performance impairment.

Also, while I have hunted for gold nuggets in several states using a variety of detector brands and search coil sizes and designs, my largest gold nuggets, deepest gold nuggets, and the most gold nuggets have been located when using a round Concentric search coil, not a Double-D design. I know several people who professionally nugget hunted in the late '70s and during the '80s and '90s and they, too, had better success in their areas with the combinations I used.


Okay, so what about the Concentric search coils projected EMF? What is the difference in its 'shape' or projected field? Here is where we find the second batch of misconceptions about the transmitted field compared with the reactive or responsive or functional portion of the search field.

Now, remember, I am not a design engineer. I am not a master search coil developer. I am not a book-learned smarty-pants about how they make them or the variances from one coils size and the placement of their internal windings to another. Nope, I am just a very dedicated and avid detectorist who has worked with a few detector manufacturer's engineers during my 47 years of detecting, including one of the sharpest to design search coils, and I've listened to their descriptions of what really happens.

The Concentric search coils do not 'project' or 'transmit' or 'send' an electromagnetic field from the outer Transmit winding that aims toward the center of the coils axis to form a 'V' or 'Cone Shaped' EMF. No, instead the generated EMF that flows out and around and about the Transmit winding is, or should be, relatively uniform all about the search coil. the strongest portion of the EMF, where the lines of the magnetic field are the tightest or closest, produce a greater response from targets located closer to the search coil. As the EMF field gets farther from the transmit winding, they will start to bend outward from the center axis.

Depending upon the size and placement of the inner Receive winding, and the detector's electronics to respond to the changes the detector/coil sense, your greater distance of response will generally be at a point where the EMF lines bend apart from each other. Since the Concentric coils internal windings are generally positioned uniformly about the coil, the functional responsive area of the EMF (not the transmitted 'shape') will somewhat appear to taper farther from the search coil. The sides of the functional portion of the EMF are not straight and 'V'-like but can be more bowl-shaped when viewed from the side.

So, my lengthy and perhaps confusing response gets to this point. The Concentric search coils do not project or transmit a "cone-shaped" EMF, and the reactive area, if viewed from the site is tapered toward the greatest responsive point, but is also not in a true "cone shape." Neither the Double-D or Concentric search coil designs have a great visual display or imaginary suggestion of their EMF or the functional/responsive portion of the field (from a side view) from most manufacturers.

Sadly, it seems that like some other detector related topics, some makers either try to fool shoppers, or they just tend to avoid offering a valid description. Maybe it's because it can be a complex thing to explain, or maybe it's just easier to let the average consume think all the common descriptions are fine.

Sorry to ramble, and pardon me if my descriptions have left you (any of you) confused about this topic. Now, if you think I have misled anyone, try this out. Offer up a clear and specific post on a few general metal detecting Forums, not anything brand-specific, and then direct a letter to each and every manufacturer and ask them to do two things:

1.. Respond with a written description of what an EMF is that is generated about the search coil of most of the TR (VLF) detectors we are using, like the T2, etc., and then

2.. Ask them to please post a photo of the side-views of both a Double-D and Concentric search coil EMF that shows the magnetic field patter about the search coil. Then ....

3.. Request that they draw in a close representation of the responsive potion of the EMF, from a side-view, for a small target, such as a US 10

 

[Dave J.]

Monte posted that just to find out how long it would take me to cruise the forum and find it, whaddayawanna bet? And of course that remark about engineers, that's just Monte's "Dave bait". I bit, here goes.

* * * * * * * *

Monte, that's just about as good a non-engineer explanation of searchcoil response as I've seen. Wouldn't necessarily agree with every word of it, but I'm not gonna nitpick those little details. The big picture is correct.

Now on that business of providing a pictorial representation of searchcoil field: In principle it could be done, but I've never seen it done. For the transmitter coil field, there are three space parameters. For each location in space there is a vector with three orientation parameters and one magnitude parameter, total of four. Multiply them and you have a 7-dimensional system. Now convolve that with a receiver coil: you've got a 14-dimensional system. You won't be drawing a picture of that on a piece of paper anytime soon. Not on 100 pieces of paper either.

But when you've somehow drawn it, it's still not telling you very much about target response. The target occupies a space trajectory in time, that totals 4 dimensions. The target itself has two obvious response dimensions: amplitude and phase. We're up to 6 dimensions. Now consider the fact that the amplitude and phase vary with the orientation of the target with respect to the transmit field as well as to the receiver response equivalent field. For the simplest magnetic shape, a ring, that orientation has three dimensions, so we're up to 9. But there are all these different shapes that behave differently, so throw in at least a couple more shape parameters, we're up to 11 target parameters now. Does masking matter? Throw in another target with its own 11 dimensions.

So far we haven't said a thing about what the electronics do with the received signals. The actual machine response depends on what the guts do and how you have the controls set. The actual user response (dig or don't dig, the response that matters most) depends on variables of knowledge and attitude and skill.

Even the "engineer perspective" can go more than one way. I can prove based on actual lab measurements that a DD searchcoil is better in high mineralization. I can also prove using the same kind of data, that a concentric is better in high mineralization. It all depends on how you interpret the data. I generally prefer to say that a DD is better in high mineral, but if someone wants to argue that they've found a lot of gold with a concentric, well, they may have a point. The original GB and later the GB2 have found a lot of gold, and those machines use only concentrics.

* * * * * * *

There is no accurate picture of the searchcoil field. If it did exist, it wouldn't do a customer any good anyhow, because it doesn't take into account the characteristics of the target or how the machine is being used. What can be offered is broad rule-of-thumb generalities which suffice to explain a lot of what customers actually see happening in field use. Convenient fictions. There is no secret inside dope truth to be known and told. The only truth available is what actually happened when you swung that particular machine at that particular time over that particular target-- that particular swing. It really happened and you saw it happen. The next sweep over that same target may produce a different response, without having changed searchcoils meanwhile. And that's the truth.

--Dave J.

 

[pasttom]

While this has been VERY helpful and I now know why I prefer FT machines, I believe the original question sought a yes or no answer.
Why is life never that simple?

 

[Pops]

Your question provoked some very interesting comments from some highly experienced and respected people in the metal detecting world. Most of the technical talk went over my head but from experience and conjecture on my part there is not a true cone shape but more bowl shaped. It makes sense to me that the detection field generated by the coil would be rather uniform with the deeper area being the center of the coil but not concentrated there like a true cone as the signal would gradually decrease in strength as it moved away from that center area. But, when you factor in any impedence of the signal like mineralization or metal, the bowl shape distorts where the impedence is located.

 

[Monte]

I guess my initial reply could have simply been a "No, it's not cone shaped" and then leave it at that. However, doing so would have suggested that the DD coils don't but the Concentric coils do. I prefer not to confuse facts, although some of my replies might cause some to wonder.

Monte posted that just to find out how long it would take me to cruise the forum and find it, whaddayawanna bet?

Well, I will admit that on this forum there was an excellent chance that you would pop in and correct me, or just enlighten the topic. naturally, that's exactly what you did.

And of course that remark about engineers, that's just Monte's "Dave bait".

I am sure you know that there are really just two detector manufacturer engineers who I would expect to have the knowledge to reply and the desire to join the discussion ... and naturally, you would be the one I'd expect to reply. 'Thank You!'

I bit, here goes.

* * * * * * * *
Monte, that's just about as good a non-engineer explanation of searchcoil response as I've seen. Wouldn't necessarily agree with every word of it, but I'm not gonna nitpick those little details. The big picture is correct.

Again, a 'Thank You'. Both for first complementing me on the post in general, and also for not nitpicking the little details. Like I said, I am just into using and learning these great products and how they work, not the textbook educated guy.

Now on that business of providing a pictorial representation of searchcoil field: In principle it could be done, but I've never seen it done.

Nope, all we really have seen are the goofy suggestions form many sources that 'suggest' a cone shape field, and there have been a couple of examples from that other Texas manufacturer about DD and Concentric coil fields.

For the transmitter coil field, there are three space parameters. For each location in space there is a vector with three orientation parameters and one magnitude parameter, total of four. Multiply them and you have a 7-dimensional system. Now convolve that with a receiver coil: you've got a 14-dimensional system. You won't be drawing a picture of that on a piece of paper anytime soon. Not on 100 pieces of paper either.

But when you've somehow drawn it, it's still not telling you very much about target response. The target occupies a space trajectory in time, that totals 4 dimensions. The target itself has two obvious response dimensions: amplitude and phase. We're up to 6 dimensions. Now consider the fact that the amplitude and phase vary with the orientation of the target with respect to the transmit field as well as to the receiver response equivalent field. For the simplest magnetic shape, a ring, that orientation has three dimensions, so we're up to 9. But there are all these different shapes that behave differently, so throw in at least a couple more shape parameters, we're up to 11 target parameters now. Does masking matter? Throw in another target with its own 11 dimensions.

So far we haven't said a thing about what the electronics do with the received signals. The actual machine response depends on what the guts do and how you have the controls set. The actual user response (dig or don't dig, the response that matters most) depends on variables of knowledge and attitude and skill.

Even the "engineer perspective" can go more than one way. I can prove based on actual lab measurements that a DD searchcoil is better in high mineralization. I can also prove using the same kind of data, that a concentric is better in high mineralization. It all depends on how you interpret the data. I generally prefer to say that a DD is better in high mineral, but if someone wants to argue that they've found a lot of gold with a concentric, well, they may have a point. The original GB and later the GB2 have found a lot of gold, and those machines use only concentrics.

For me, to be sure, this is the type of response to posts that I always find helpful, and I am sure many other readers will also. It goes a long way toward helping learners understand that there is a lot more to how these things work that making a toaster to brown some bread.

There is no accurate picture of the searchcoil field. If it did exist, it wouldn't do a customer any good anyhow, because it doesn't take into account the characteristics of the target or how the machine is being used. What can be offered is broad rule-of-thumb generalities which suffice to explain a lot of what customers actually see happening in field use. Convenient fictions. There is no secret inside dope truth to be known and told. The only truth available is what actually happened when you swung that particular machine at that particular time over that particular target-- that particular swing. It really happened and you saw it happen. The next sweep over that same target may produce a different response, without having changed searchcoils meanwhile. And that's the truth.

--Dave J.

A 'spot-on' comment here, too, as you've helped people understand that each coil sweep or target presentation can vary. It can change based upon the target position, coil approach and speed and other variables. That's why many avid detectorists like to listen for and recover even a single, one-direction response. Also we don't always anticipate or require a perfect visual report of the targets possible identity. Just way too many variables.

Thanks, again, for the excellent reply about the search coil transmit field parameters and such technical stuff! My "Thank You" is certainly an expression for the many who appreciate your long-time efforts and forum contributions.

Now, it's time to go fire up an Omega and find some stuff!

Monte

 

[pasttom]

While its true the first question was very basic, I guess it was a bit like the old saw, "Have you stopped beating your wife? And a yes or no would not have done justice.
I want to say that wading through the exhaustive and exhausting answers of Monte and Dave was beyond my mental pay grade, it was very helpful. As one who is a bit like a kid at a magic show every time I MD, amazed that it works at all and that there is so much stuff sleeping under foot. It is more than nice to see behind the curtain once and awhile to see the mechanics of magic and appreciate a bit more the efforts behind making it all possible. Thanks, guys! Most educations are priced beyond common reach and you give away what cost you years of study and labor to obtain. Bless you! I guess this is why this is such a great web site!
Tom

 

[Big Boys Hobbies]

We all appreciate all that you guys do on here for us!

 

[pull tab mel]

Great info.......