With the 2-inch difference in depth, I would have expected a larger margin between the GB of the 10.5 DD and the 9-inch concentric. 27-29 compared to 29-30 isn't much of a spread for so many differences in the two coils. So now you have me thinking......Hmmmmmm.... with so many variances in the two coils, why did they GB so closely? On the other hand, maybe having so many variances is
exactly why they GB'd so closely! I wonder if the DD design of the 10.5 was able to "neutralize" any additional ground interference it may encounter (due to it's enhanced surface area), as compared to the smaller concentric coil? Darnit! I really wish the GB numbers you used were anything but those you said so I could have some substance to work with!
28? 28??? As I have said before, 28 happens to be my "magic number" for determining when I switch from a concentric to a DD coil. I don't have any scientific evidence to back it up, just practical experience. But I usually consider ground that allows me to balance at 28-29 (or a higher number) to be decent ground for my concentrics. If it is 28 (or a lower number) I will put on the DD. So by you saying 28, I can't say which one I would have chosen. At a 28, it would probably depended more on whether or not I ate my Wheaties that morning, than the coil design!
DD vs Concentric...... JMHO
If we were able to compare a 10.5-inch DD to a 10.5-inch concentric, we could observe more variables between the two designs than we can by having two different sized coils. It just stands to reason that bigger coils go deeper. And if we had a 3 kHz DD to compare to a 3 kHz concentric, we could observe more variables between the two designs that with coils of different frequencies. Again, lower frequencies are capable of deeper depths on larger targets. But we have what we have. So the best way for me to analyze which coil performs the best is to do exactly like Chuck has done. Put them head to head and "dance with the one who brung you"! For those who are still trying to decide which accessory coil might work best for you, I'll try to outline the differences between DD and concentric.
DD coils were designed specifically to reduce ground interference. For those not familiar with the construction of a DD coil, think of it as two capital letter D's. One is facing the correct way and the other one is reversed. In other words, they are back to back with the flat portions overlapping down the center and the curved portions making up the rounded shape of the coil. One D is the transmit winding and the other D is the receive winding. The field produced between the transmit winding and the receive winding where they overlap down the middle is called the positive detection field. That is the part that has to pass over a target to detect that target. The field produced by the transmit and receive windings located at the "outside edge" of the coil is a negative detection field. The cancelling effect of this negative field is what allows the DD coil to function better in mineralized soil.
A concentric coil is constructed with a Transmit coil surrounding a receive coil. Think of it as a donut with the transmit windings going around the outside. And the receive windings going around the inside, adjacent to the hole. (I know, that is a dumb analogy
) The detection field is the area between the two windings that penetrates into the ground in what some consider to be a cone shape. When the windings are made as large as the coil shell can accomodate, you get the largest detection field and the greatest depth. Unfortunately, when used in mineralized soils, you also get the most ground interference. And the larger the coil, the more ground effects you have.
So, when you compare the performance of a DD coil and a concentric coil, there are many variables. Ground mineralization, adjacent targets and interference factors to name a few. At the end of the day, in a perfect world with no mineralization, a concentric coil will hunt deeper than a DD coil of the same size. Unfortunately, 1. we don't live in a perfect world and we don't hunt where the mineralization is zero; 2. Minelab doesn't make a 9-inch DD or a 10.5-inch concentric for comparison; And 3. when you throw the three X-Terra frequencies into the "which coil is deeper" mix, nothing will surprise me! JMHO
Thanks again Chuck
HH
Randy
to analyze whether the frequency of the coil, the size of the coil or the design of the coil were the most contributing factor for the difference in depth of detection.