Quote
Monte

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.
Monte