Through the years we have seen a lot of so-called 'progress' in development and design of detectors, but for the most part the starch coils have remained essentially the same for most 'hobby-based' detectors. Either a Concentric or a Double-D with the main differences being the coil's shape and size. Both Concentric and DD can be fitted into a round-shaped housing or a narrow, elliptical type housing. So, for me and my view of the coil types and personal experiences through the years, I reached these conclusions:

** I like both types of coil designs ... IF they work well on the detector they are mounted to.

** I will use either type ... IF they can handle the site environment and existing trash content and type.

** I can enjoy using either type ... IF their have the right size and shape to work in the open-area or confined areas I search.

** And I'll choose either type ... IF their audio and visual response are consistent, to 'typical' / 'reasonable' depth.

That seems pretty simple, but if you hunt where I do, deal with my chosen site challenges, go after the main targets I am searching for, and use the detectors I favor, you can figure out why my overall favorite coil 'type' is a Concentric. Some brands make some of all models using a DD coil, by design They do not have a Concentric, or if they do, the Concentric coil just doesn't work as well . The reverse is also true with some models that were really designed with, and for, Concentric coils, and while some DD coils are available and sometimes work, they just do not work as well in a side-by-side comparison.

While we need to give some consideration to coil types and sizes and shapes, in the end it is the specific detectors those coils are mounted to and how well, or not so well, the detector performance is for the particular application.

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GB_Amateur
Eliptical, round, and OOR describe the overall coil housing envelope but the individual (internal) coil windings can be different. Does that impact the performance? For example, the 8"x9" stock coil on the Tesoro Deltas has a quite small centrally located receive coil (I think...).

Yes, the internal winding can influence the performance or behavior of different coils. In your example here, you are asking about a Concentric coil with an 8X9 outer shape housing the Tx winding, but with a much smaller-size inner Rx winding. That is a Tesoro coil and it works well for most Tesoro models based o their circuitry design. Most can get 'reasonably good depth' but most of us know Tesoro's are not competitive in a depth-of-detection competition.

And every search coil is designed, we hope, by a coil engineer who knows how to match up the best size and shape to tune a coil for peak performance, based upon what the manufacturer considers to be 'peak performance' at that time of design.

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GB_Amateur
DD search coils tend to have the same size and shape T & R windings (although not necessarily the same number of windings in each). That may be for the purpose of symmetry weight balance (and/or other reasons), though. But concentrics can possess left-right and toe-heel symmetry while still having quite different individual shapes for their windings.

Windings differ to make them work. No concern about weight and balance from side-to-side on the DD configuration. The problem I have had with most Double-D coils is that they are not a uniform and consistent-reaction coil from any direction like most Concentric designs are. I am referring to dealing with an individual target without nearby masking metal. A single coin, for example. When there are multiple targets kind of close, and especially when there is any iron-based trash close to the EMF of the coil with the iron target in EMF range of the non-ferrous target, then things get worse for the DD design.

The DD coils work at their best when their sweep presentation is from side-to-side. Concentric coil, due to the uniform EMF all around the coil, can be presented from any direction and still have more consistent performance. Then to make things more of a challenge for a DD coil is that when swept from one side you will have the Tx coil on the outer-edge as the first encounter of the EMF, and then the overlapped configuration in the middle with the Rx coil and then the close Tx coil, and the sweep departs the target with the outer Rx winding. A Concentric coil has the Tx on the full outer-edge and the Rx pretty symmetrical as an inner winding, so that makes a complete sweep across the target more uniform and consistent from either/any direction than a DD coil.

With a single-tone detector, like most Tesoro's, we don't hear a difference. But with our more modern detectors with audio Tone ID and visual Target ID, we can hear and see the difference a lot of the time because a DD coil will typically produce a different Tone and different VDI read-out from the left than a sweep from the right, whereas a Concentric coil of similar size will have a tighter and more consistent audio and visual report.

All things to consider, but in the end it boils down to how a detector's circuitry is designed to perform.

Monte

"Your EYES ... the only 100% accurate form of Discrimination!"
Stinkwater Wells Trading Post
Metal Detector Evaluations and Product ReviewsI'm now 'back home' in Farr West Utah monte@ahrps.org ... or ... monte@stinkwaterwells.com 503-481-8147Detector Outfit: A selection of my chosen makes and models, with the best coils mounted for the tasks I'll take on.F-19 * F-5 * Racer * Racer 2 * V-540 * Relic * Silver Sabre µMAX * Bandido II µMAX * XLTPinpointers: Garrett AT Pro-Pointers .. Headphones: 'Hornet' .. MS-3 Z-Lynk .. ML-80 *** All working well today to make memories for tomorrow. ***