first off, welcome to the X-Terra forum. You could search through all the past posts concerning coil differences, and get some good information from lots of knowledgeable folks. But here are some of my opinions in a couple paragraphs. All three frequencies and any of the X-Terra coils will find copper, silver, gold and any other metallic target. However, lower frequencies are more responsive to higher conductive targets (such as silver and copper). And higher frequencies are more responsive to lower conductive targets such as gold (as well as iron and aluminum can slaw). Larger coils typically hunt deeper than smaller coils. And smaller coils are more sensitive to small targets (like small nuggets or jewelry). Concentric coils produce a magnetic field that I refer to as an inverted melon. Some people call it cone shaped. But it is more like the shape of the bottom of a mixing bowl. In other words, it doesn't go down to a point. It gradually gets smaller in diameter as the depth increases. That is why I advise people to overlap their swaths by at least 1/3. Preferably by 1/2, if you want to get the deeper coins. A DD coil separates targets better and is better suited for hunting in highly mineralized soil conditions, due to it's design characteristics. If you are interested in the technical nuts and bolts, let me know and I would provide more information. But for the most part, I've found that if I can ground balance my 9-inch concentric coil with a GB setting of 28 or higher, it works great. However, if I am in a place where the concentric gorund balances with a reading of less than 28, I usually switch over to one of the DD coils. In addition to working better in highly mineralized soil, a DD coil separates targets better. The reason is because the receive section of the coil is directly down the middle of the coil, from the front tip to the back tip. They call them DD coils because if you drew a capital letter D, and then drew another one backwards, on the left side of the first one with the straight portion being shared by both letters, it would look similar to the two windings are in a DD coil.
I hunt for old coins at old sites. My favorite spots are old homesites and "long forgotten public areas" that existed 100+ years ago. Today, many of those sites are farm fields. If I am hunting an old homesite, I'll put on my 9-inch concentric coil at 3 kHz. Like I said, all three frequencies will find coins. But I like the 3 kHz for two reasons. One, it is more responsive than the other two frequencies to silver and copper coins. Two, as such, it is less responsive to that old deep iron (nails, pieces of implements etc) that is found around many of the places I hunt. I hunt in multiple tone mode and usually all metal. When I go over a piece of deep iron, the harmonic audio tells me that it is not something I am interested in retrieving. If I happen to get into a trashy area where I am having 5 or 6 "hits" per sweep, I'll usually get out the small 6-inch DD coil at 18.75 kHz. Now, 18.75 kHz is the last frequency (of the three) that I want for hunting around lots of old iron. I'd rather have a lower frequency in a small DD coil. But, they don't make one. So I am willing to sacrifice some of the discrimination information for better target separation that the DD coils provide. And the smaller DD coils are better than the larger DD coils for separating targets, due to a smaller footprint. In other words, they don't have to analyze nearly as much soil (or as many targets) at one time, compared to a larger coil. Granted, the smaller coils won't hunt as deep. But I am convinced there are more old coins still hiding out there due to being masked by adjacent targets than due to extreme depth.
My soil around here is moderately mineralized. Very seldom to I find an area that the GB number is less than 30. As such, the concentric coils do very well for me. Therefore, I don't need the additional weight of the 10.5 DD coils. In moderately mineralized soil, the concentric coils will hunt deeper than a similar sized DD coil of the same frequency. Your coil choices will have a lot to do with where you hunt and what it is you are hunting for. Coil prices.........FindMall is sponsored by some very reputable dealers. And I'm sure any of them would appreciate your business. There are links on their banners at the top of each forum. Most of their dealer websites have product pricing.
HH Randy
