i'm gonna stick my neck on the line here ,but i have had the concentric coil for a couple of weeks now and getting used to it and its ways i.e only dig the REALLY high pitched signals and after a day out with rockwolf purely testing different coil set ups i am willing to say that the concentric coil on its own with no messing about KNOCKS OUT 95% OF HOT ROCKS(COKE) i haven't dug any for two weeks and the only piece we could find today to give a good signal was about six inches in diameter.i am impressed finds rate as per normal no hot rocks and no weight penalty .the one to get