Well if it's a 10.5" coil then that's out as an answer of coil size. But I see you said it's a concentric. That could be the answer. At the tail end of the detection field a concentric (in general, not always) tends to have a more sharper "point" to it's field where it can more easily separate targets from any angle. However, that's only where they (in general, not a hard fast rule) have a slight advantage over a DD in terms of separation- at the very tip of the field. But, a DD will (in general again, not hard and fast rules for every coil) have better left/right separation over the entire length of the detection field from shallow to deep. Also, while the DD might be more blunt (being a line from tip to tail of the coil in the center) at the very tip, it still can do just as well at separation at the very tip...It's just that you have to be at the right angle. In other words, if the coin and iron are in line with each other and the DD detection line, then they are both in the field of the coil. If you move 90 degrees, however, now just the coin is in the DD's line and not the nail. See what I mean? For that reason I don't even feel a concentric has an advantage at seperation even at it's sharpest point at the tip of the field. It does in a way, but the DD can do just as good at it's tip if you work your way around the target. In general the left/right seperation of a DD coil is much better than a concentric width wise. Beyond all that, DD's have more uniform depth across the entire tip to tail length of the coil, where as a concentric *tends* to have it's best depth right at the very center of the coil and so overlapping sweeps most say is much more critical on a concentric. Some say that at least on some concentrics the deepest part of the field is only about the size of a quarter or perhaps a half dollar or so, where as on a DD it's pretty uniform from tip to tail along the DD line. DD's still will have a slight depth increase in the very center of the coil, but it's not as much of an issue to consider with sweeping as it is on concentrics. Again, in general. I know some argue that the concentric and DD fields can mimic each other more than people suspect on some coils, so please don't beat me up over these general statements. For sure get your hands on a 12x10 if you can because I feel as excellent as the 10" Tornado is, the 12x10 to me has better depth, separation, stability, and seems to hit coins on edge even harder. All these aspects are awesome on the 10" Tornado so I'm not knocking it. Best stock DD coil I've ever used. I just feel the 12x10 sets the bar a bit higher even.
For sure setup is critical for max depth. Like I said, try maxing out sensitivity to where it's just a tad unstable and hunt for a while. Then try dropping it down to say 11, noon, or 2 or 3PM or something and hunt that way. You might find depending on the soil at a site that you seem to get deeper, or at least better target ID, with a lower than max stable setting. Some people have found that to be true, and it's not just a Sovereign thing. It's all about the soil. Even on my Explorers it was wierd but I could hunt with sensitivity right at the edge of stability and find nothing. Then lower it a good bit and hunt the same exact small patch of ground from the exact same angle and start popping silvers. But, not saying that max stable isn't the best depth for most people in most soils. That's always been a general rule of thumb for all detectors so it must have a lot of merit. I would say though that as you play with sensitivity trying changing your sweep speed. Many say the higher you ride it on the Sovereign the even slower you need to sweep. The threshold dropping out too much is a key way to tell if you are sweeping to fast for the given sensitivity level. Also, try finding a super deep target and then play with sensitivity and your general long sweep speed over it (don't wiggle). See which speed and sensitivity setting seems to give you the hardest/best initial "hit" on the target while doing a long general search mode sweep. Try then to hunt at that speed and sensitivity setting at that particular site the rest of the day. Also, when a deep one is found, also trying short sweeping or wiggling over it and play with sensitivity. That alone might clue you off to the right sensitivity setting for max depth if the general hunting sweep doesn't. Notice how stable/easy the ID gets at a certain setting and how good the audio gets. That's probably the best way to adjust if you *suspect* a lower than max stable setting *might* be in order for soil at a certain site.
Keep up the good work. Like I said, be prepared to hate this machine off and on for a while until you can understand what she is saying to you. Also, I look at the Sovereign this way...It's a VERY powerful deep machine. As good as it gets in terms of rough depth power on any machine. It just has more basic controls than some other machines. Not a big deal, because it's easy to set and forget and hunt with less hassle or wondering in the back of your mind what settings on a computerized machine *might* improve performance for a site. I like the no worries/no hassle type of freedom of mind hunting with a Sovereign gives me on most days. It's like an old hot rod with a huge engine. Sure, some newer car might be just as fast, but then you don't have to be a computer expert to fix that old hot rod where as the new car needs a lot of tweaking or specialized knowledge. Not that I'm opposed to computer controls. Miss that on some days and looking at a computer screen on say my Explorers or my QXT Pro. It's just rare to need the odd tweak, and tweaking the wrong way could cause you depth. Far less can go wrong with a machine as simple as the Sovereign. Simply set the sensitivity and threshold, zero our the notch and discrimination, and unleash it on a site.
