These guys are all correct.
Here is a sort of strange analogy that I like to use when people talk about this.
Before someone jumps on me, this is really just my explanation of coil detecting patterns and their advantages. No detectors, coils, coins or your feelings will be hurt during this analogy.
With that being said. let's say for conversation purposes that we have two coils (concentric & DD) and they are both 10" and for this example we will say that they are the exact same frequency.
The concentric coil has a maximum detection depth of 8" and a DD that has a maximum detection is 8". Being that the pattern of a concentric coil is conical like one of those paper drinking cups at a water cooler, the concentric coil's maximum depth will only be 8" only in the center of the coil.
The DD coil that also has a maximum depth of 8". It's detecting pattern will be about 1/8" wide nearly the entire length across the center of the coil varying just slightly near the outer edges. Kind of like a hockey stick blade.
Let's say you had 50 pennies scattered on top of a flat 4'x4' surface and your goal is to push all of those pennies off of that surface with the detecting pattern of either coil hovering just under 8" off of the surface and your eyes are closed. Kind of impossible to do it perfectly but in my example we're also all perfect. If we look at the detecting patterns of each coil as a physical figure, it's going to be much easier to push those pennies off with a hockey stick than the point of a cup. Remember we can't see coins when they are underground just like this example with our eyes being closed. The more surface that we have to work with to slide those coins the better. Of course in reality we are running over the coins with the signal and not sliding them.
Granted this is kind of a hair-brained analogy but not far from the truth. If we turned this analogy into reality, the depth of a DD coil in most soil conditions would be deeper than its similar sized counterpart. Sure if the coils were nearly touching the surface that changes things a little with the concentric at least giving you a semi-wide detecting pattern near the top but if you are going for old deep coins you want every advantage that you can get at greater depths. The DD coil besides helping in mineralized ground has a much more forgiving detecting pattern being that it covers more area over the entire depth of the signal. A concentric may be a little easier to pinpoint with and a DD is a little more subject to picking up parallel targets but I think increasing your chances of running over an item with the signal greatly out numbers the ability to pinpoint better. I've often times found trash right next to the treasure signal that I was digging using a DD coil.
It's my opinion that everyone should own the 6" Coiltek coil first and then some flavor of the Minelab 10.5" DD second.
Have fun!
