Got the DD 18kHz one. I hope what I have been reading is right about that coil. The 6" concentric seemed too close to the 9" stock coil and I really liked the 4x6 DD coil on my DFX for cellar holes. I just wonder if the 18kHz freq. will be alright for coins as that's what I mostly go for. I'm not a gold digger, but if there were some gold around, I'd change my tune real quick. Ron