When you start to get the hang of metal detecting and find you are at the point where you feel it’s time to invest in a new coil you may find that there are different types of coils available adding to the confusion of what to buy. There are two main types of coils for metal detecting, the concentric coil (there are also similar ones called mono coils) and the double d coil (commonly called a DD coil). Many beginner metal detectors come packaged with a concentric coil and the more expensive powerful detectors usually come with a DD coil.
Both types have two coils called the TX coil and RX coil. The TX coil transmits a magnetic field in the surrounding space and the RX coil receives or senses the distortion within the generated magnetic field. The diagram below shows the location of the TX and RX coils in both types.
Due to the location of the TX and RX coils the resulting magnetic fields are different. The concentric coil sends out a cone shaped magnetic field whilst a DD coil emits a more straight field near the center of the coil, you can think of it as a knife going straight down the middle. This makes it easier to separate detections and achieve a greater depth. The diagram below helps illustrate what happens.