VLF detectors can come with DD or concentric coils. This is similar to but not quite the same thing as DD versus mono on a pulse induction detector. There are three basic coil types in wide use, Concentric, Double D (DD) and Mono. A VLF detector transmits and receives simultaneously and so there must be both a transmit coil and a receive coil. A pulse induction (PI) detector can alternate between transmit and receive and so one coil can be employed as both transmit and receive in an alternating