A VLF machine & a nickel at 15" in saltwater/sand matrix? Sov, Excal, CZ 20/21 "are capable" but it's pretty situational, the greater the mix the more difficult it becomes. 15" is pushing the practical limit, especially in stock set ups. Excal & Sov users frequently upgrade their headphones to aid in the detection of deeper responses, and this is probably more important then the actual coil used. The send signals, while weaker then those of a PI machine usually aren't the issue, they are still penetrating deep enough, but rather it's the returns that are too weak due to the weaker send. This is also the whole concept behind amplifiers, providing the ability to increase the strength of these weaker return signals, thus increasing the ability to hear these deeper/weaker responses. But this is just part of the problem because as the saltwater/sand mixture increases it becomes harder for the machine to separate conductive metals from the increasingly conductive mix. Most quality VLF saltwater machines can reach the nickel at 15" but the problem is that many of them lack the ability to tell you they've found it. So, even if a larger coil gets you deeper that shouldn't be taken to mean that it's always going to be able to tell what's down there. I recently had OBN install a set of his Skullies on my Excal, same machine, same coil, but an increase in depth simply because those weaker responses can now be heard, add an amplifier to this and the ability to detect deeper responses will be increased even more. The send signals are already getting there, which is why larger targets can be heard at greater depths, because these larger, denser targets produce stronger return signals. If your signal wasn't already getting there then these larger targets couldn't be heard either. I'm not a techie guy so maybe someone else can explain this better then I have.
