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Left-right assymetry of DD coil response

April 12, 2022 09:53AM

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.

Thanks, as always, for the detailed and clear reply. I cherry picked the above because it's something I've wondered about many times but don't recall anyone calling it out. When I sometimes read "only dig when you get a consistent response from multiple directions" I cringe. Sure, the clean hits which occur with no nearby trash and perfect coin orientation will show that. But if a person makes that a requirement they are going to leave some good and otherwise recoverable targets in the ground. (And I think this applies to both DD and concentric, although my experience with the latter is considerably less.)
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