Ok, just viewed an air test of the Garrett 9×12″ concentric coil vs the Garrett Stock 8.5 x 11″ Double D coil. He used the two coils on the AT Max. The test was done on an individual’s metal detecting channel on Youtube. Full disclosure requires me to let you know that I like most of his videos and being well filmed and an accurate representation of the metal detecting hobby with little or no hype.
Now here is where I fault his air test. The air test shows the concentric coil delivering better results on deep targets. Well that is a given due to the different coil designs. The DD coil will deliver a full 11″ swath at max depth whereas the concentric coil (using a cone pattern) will deliver only an inch or so swath at maximum depth. So now lets figure in the hunting time looking for deep objects. It is 11 to 1 ration in favor of the Double D coil. So a 2 hour hunt of a specific area with the Double D coil will turn into to a 22 hour hunt using the concentric coil.
On the upper beach or volley ball court or areas where the targets are only 2″ deep, then the coils are basically equal. On a search for deep coins or jewelry, you could not possibly consider using a concentric coil.