Paragraph 3 of page 99 of The Minelab Explorer & E-trac Handbook by Andy Sabisch
Another interesting characteristic of Double-D coils is that unlike concentric coils , smaller coils do not have an appreciable loss of detection depth when compared to the stock coil. This means you can still find deeper targets with a small coil while eliminating the possibility of missing something due to target masking. Larger Double-D coils are also not prone to the typical loss of sensitivity to smaller targets that one sees on larger concentric coils. Many users of the larger coils such as the 15 Coiltek WOT or the new Detech / EXcelerator S.E.F. coils consistently report finding small artifacts or even coins on edge at impressive depths

Do you think I get as good depth on my SEF 6x8 as the 11 Pro coil. Interested in opinions.