As shown above, two types of coils are found within the DD-coil: First is the Transmit Coil (TX) which transmits a magnetic signal. Second is the Receive Coil (RX) which receives information about distortion in the magnetic field (such as when a target is present) and sends a signal to the control housing. These two inner coils overlap in the center of the DD-coil as an elliptical shape (shown in red above).
The configuration of the DD-coil significantly decreases interference from the ground. Specifically, the DD-coil arrangement cancels out signals from minerals in the ground that would normally cause interference in a concentric coil. This is explained in more detail below based on differences in the detection fields between the two coils.
The concentric coil detects signals under the ground in a triangular shape, with the base at the top (running from one end of the coil to the other) and the point at the bottom. This area is known as a positive detection field. The positive detection field of the DD-coil only involves the center section (in the shape of an “I”) and searches the ground like a narrow chisel. The rest of the coil (the loop of each “D”) is a negative detection field, meaning that it provides a cancelling effect. It is this features that allows it to cancel out most mineralized signals from under the ground.
The positive detection field of the concentric coil is more likely to pick up all three objects at once if those objects are towards the top of the detection field. Also, keep in mind that because the positive detection field of the concentric search coil narrows to a point under the ground, it will not detect targets as well towards the bottom of the detection field compared to the DD-coil.
The DD-coil is less sensitive than the concentric coil in non-mineralized ground because it has a smaller positive detection field. However, the DD-coil will greatly outperform concentric coils (e.g., more depth, better target separation) in mineralized ground and salt-water because it will not yield as many false positive results (e.g., a signal indicating a target is present when it is only mineralized ground). This is a great feature of DD-coils because false positives (known as “falsing”) are very frustrating for metal detectorists.
DD coils can make it more difficult to tell bottlecaps from actual targets without learning the audio signs of a bottle cap. Some people find pinpointing with a DD-coil challenging but with practice it becomes much easier. It is best done by using the pinpoint feature on the machine, mentally remembering the area where the strongest pinpoint signal comes from in a north-south direction and an east-west direction (or vice versa), and marking the ground at the mid-point of where those strongest signals come from in each direction (e.g., like finding the midpoint of a cross). Given that the positive detection field of a DD-coil is in the middle if the machine, you may need to turn the coil on a 45-degree angle to detect targets next to a tree or solid non-metal object. Overall, I highly recommend a DD-coil for most detecting activities.