Actually there's a myth about concentric coils. The old snow-cone shape detection field that everyone thinks is not entirely true. It's more like a bowl at the end of the detection field before you lose the signal completely. I read this and did some testing to confirm it. Also the DD coil is not a wiper blade with equal detection depth from front to back. The depth tapers off at the ends. As far as coverage there's very little difference. The advantage with DD coils is in minerals, with good ground you will find deeper stuff and more of it with a concentric coils. That's why for years the companies sold their detectors with concentric coils. . Take the minelab exteras for example. I will say that minelab prefers to put DD coils on their multi frequency detectors. I don't know why.