Hello. I own a teknetics Delta 4000 (it uses same coils as Omega). It has the stock 8" concentric coil. I recently purchased a 5" DD coil for it and I'm very pleased with the results in trashy areas and tot-lots. Now, I'm thinking about purchasing the 11" DD coil, but I'm not 100% sure of what improvement this coil will make over the stock 8" coil. I would appreciate your input, if you own or have used both coils...Thanks!