Question for Monte

[markg]

I think I have a fair understanding of what goes on, but, Monte, here is where your experience will come in.
Do you care to expand on the differences of a regular concentric coil verses a DD coil?
Can you go into some detail about the transmitting and receiving coils? What goes on in the ground?
Maybe how the machine processes the information and gives the user a response?

 

[markg]

n/t

 

[Monte]

I am but a lowly detectorist. One who has spent ample time making detectors and coils work, not making detectors and coils. I've had the pleasure of working with and/or engaging in conversation with some of the most knowledgeable engineers who have brought us fine equipment from various manufacturers over the years.

We can all reflect on product literature that has been supplied for nearly half a century of hobby-related products and end up very confused. Seldom have I seen any manufacturer provide us with an accurate depiction of what a generated electromagnetic field might look like as it flows, uninterrupted, about a search coil. Some early Garrett literature (late '60's or early '70s perhaps) and more recent Tesoro catalogs have provided a decent suggestion as to how the field appears.

What I have gained through my association with various engineers, as well as plenty of personal experimentation since I built by first detector from a kit in early '65, simply helps me get an idea of what happens and that helps me personal when out detecting, as well as when doing a seminar or in a casual discussion as I try to eliminate technical lingo and explain things in "human speak" so us average folks can understand it.

I think I have a fair understanding of what goes on, but, Monte, here is where your experience will come in.

Certainly, it would be good to hear your take on what happens. It would also be great if a savvy design engineer, such as Carl at White's, could pop in here and clear things up ... in "human speak" terminology.

Do you care to expand on the differences of a regular concentric coil verses a DD coil?

By "regular" concentric coil I presume you mean the typical or standard concentric, right? Quite often I read posts where folks refer to any round coil as being concentric, but it applies to even the slightly "out-of-round" shapes, such as Tesoro's 8X9 coil, or some pronounced narrow elliptical designs. Equally, a Double-D coil (aka D-D or Wide-Scan) can be housed in a round shaped housing, an elliptically shaped housing, or more pronounced shaping such as the 10X12 SEF and other SEF Excelerator coils from Detech.

If I may, let me suggest a simple description of the differences as I see them:

Concentric: A design with both the Transmit and Receive windings placed on a similar plane and having the inner Receive winding spaced relatively uniform from the Transmit winding all around the coil. The generated electromagnetic field forms in a uniform shape all about the search coil, and as the coil is swept side-to-side and encounters a single target, the effects that metal type has on the EMF is more-or-less uniform on both sides as the coils crosses the target. The result is that a concentric coil will generally provide more accurate and consistent discrimination characteristics, and due to the uniform EMF pinpointing, and detuning to pinpoint, is usually much better than with a D-D designed coil.

Double-D: A design with both the Transmit and Receive windings placed on a similar plane, with the exception of a little overlapping of the Transmit and Receive windings. These two windings are usually the same approximate shape, but the Transmit winding is positioned on one side (left or right) and the Receive winding on the other side. The result is that the generated EMF isn't a uniform position all about the entire search coil housing as we envision it, but is off-center, so to speak. Therefore, as the coil is swept side-to-side and encounters a single target, the effects that metal type has on the EMF is interpreted or 'read' somehwat differently the coil crosses the target. The result is that a wide-scan coil will generally provide less accurate and more inconsistent discrimination characteristics that a concentric coil. Perhaps not as radically different in some cases, but they will not be as consistent, Also, due to the elliptically-shaped EMF pinpointing, and detuning to pinpoint, is usually not as easy or precise as with a concentric designed coil.

Can you go into some detail about the transmitting and receiving coils? What goes on in the ground?

Not really beyond what little I've already suggested. Not all concentric coils are made the same and not all wide-scans are, either. You could have two 8" concentric coil with a Transmit coil of similar size and outer placement, but the two coils might use different sized inner Receive windings and that can make a big difference in field performance.

Ground conditions, as well, can have a different influence on the coil's EMF making their in-ground performance differ a bit from a nice-and-proper, in-air designed intent. Perhaps even more challenging is knowing which particular make/model detector the coil is mounted to because I've found the detector and settings are more critical to how things work than simply a coil size or type. As an example, I worked with White's new 10" DD coil on an M6 and MXT and liked how it worked in an open field that was part of a large renovation of two old school ball fields. Then I got the Vision/V3 with the stock 10" DD coils and didn't like the performance at the same site nearly as well. I switched coils around and in the end had 4 different 10" DD coils to use in evaluating a Vision, a Spectra V3, the M6 and two MXT's. It wasn't the coil that made them function differently, it was what they were affixed to and how that particular detector worked.

Before all my evaluations were done, by late June of last year, I had used and compared several specimens of the 10" DD, 6X10 DD and so-called 4X6 DD as well as the 6

 

[markg]

Thanks again
One thing I learned from you years ago, was to run the discrimination as low as possible to help with masking.
mark