Your choice of coil will depend on the sites you hunt and the targets you are hunting for. By design, DD coils are better suited for areas with higher mineralization. In a DD coil, the two coils are positioned so that most ground signal in the receive coil is eliminated. As well, the DD coil incorporates smaller diameter windings for the transmit and receive coil. This factor makes the DD coil have less antenna effect, resulting in less noise being detected and amplified into your ears. On the other hand, if you are in moderately mineralized soil, you will likely get more depth from a similarly sized concentric. I hunt in a part of the Country that, for the most part, has moderately mineralized soil. In deciding which coil to use at a specific site, my unofficial "rule of thumb" is to use concentric coils if the GB reading I obtain is mid 20's or higher. If the GB reading is less than say 28, I use the DD coils. My exception to that rule is the 6-inch DD coil at 18.75 kHz. I don't use it for the frequency. I use it simply because I believe I get better target separation with the DD design than I do the concentric of the same size.
As to comparing frequencies and determining which may provide the best service for you...... the theory is that lower frequencies provide better response to higher conductive targets such as copper and silver. Higher frequencies provide a better response to lower conductive targets such as gold. Now, that is NOT to say that any one frequency of X-Terra coil is not good for all three. I am convinced they are all more than capable of detecting coins made of silver, copper or gold. (nugget hunters will likely chose the 18.75 kHz coils for the reason mentioned). With that stated, I don't chose my coil based on what they might be best at detecting specific targets. (in my case, old coins) As I said, ALL of them do a sufficient job. This may sound odd, but I chose mine for the response they provide on targets that they are not designed to be best at detecting. Let me explain.....since the 3 khz coil is best suited for silver and copper, it makes sense to me that it isn't as sensitive to lower conductive targets such as aluminum can slaw and small bits of deep iron. So when I am hunting coins at most of my favorite sites, I use the 3 kHz concentric. If I could get a 3 kHz X-Terra coil in a 6-inch DD design, I'd use it. (are you listening Minelab Santa?) If I were hitting the beaches searching for gold jewelry and coins, I'd likely put on one of the 18.75 kHz coils. As mentioned before, Nugget hunters are finding the 18.75 kHz coils to outperform the other frequencies. And with both examples, the size and shape of the coil is left to the task at hand and the "comfort factor" as to size and shape. By task at hand, remember the elliptical coil covers a lot of ground in a lightweight package. But it is not waterproofed by Minelab and being a solid coil, it is more boyant than the "spider" coils. For all around hunting, coins, jewelry and relics, the 7.5 kHz does just fine. And again, depending on the mineralization at the specific site, I'd make the decision to use either the DD or concentric.
As a final note, I believe any of the coils made for the X-Terra will serve coin shooters and relic hunters well. I made my personal choices by comparing them head to head in the places available to me. As I said, I like the 3 kHz for what it doesn't do as much as for what it does. It likes silver and copper much more than it does aluminum and iron. Since it only comes in a 9-inch concentric, that is the one I use in wide open spaces. I like the 6-inch DD for target separation in trashy environments. The 18.75 kHz may not provide the audio "sound" that I would prefer for rejecting aluminum and iron. But since it only comes in 18.75 kHz, that is the one I use. My reasons for chosing a specific coil may not be the same as yours. I chose the frequency that works best for me on old coins in open areas. And I chose the size and design for those times when target separation is my primary concern. But again, any of them will provide proficient "detectibility" for 90% of the users. They're so close in performance that if someone put another frequency coil on my X-70 when I wasn't looking, or if someone switched the 6-inch concentric with the 6-inch DD, I might not even notice! HH Randy
