I'll take a stab at this, but it's not a cut and dried simple answer or explanation.
Where to start isn't even easy, but I think that soil conditions is as good a place as any. So first, it depends on where in the world you are, just how mineralized your ground is, and whether you're dealing with a predominance of magnetic or conductive minerals. There are parts of the country like Mississippi, So Cal, and the PNW where conductive minerals seem to be a big issue, and many in those areas tend to prefer HF(18.75kHz) coils, as do folks hunting beaches everywhere. Other parts of the country are heavily laden with magnetic minerals, mostly places where the glaciers gouged the ground up like the Great Lakes region and spotty places where the glaciers dumped ground they picked up. It's been my experience that LF(3kHz) coils do very well in those conditions as well as milder ground. In those conditions an HF coil is frustrating to say the least, and an MF DD is a much more acceptable alternative. Then there is that magical mild soil that blankets a good part of the central U.S. where there is a mostly even blend, and quite often not highly mineralized. There the MF(7.5kHz) and LF coils really shine....the LF for it's depth and almost violent response to high conductors, the MF for it's depth and honesty.
Now here's where coil selection get's a little more complicated.
The frequency choices that we have to select from most often have more to do with the target response than what they will or will not see. George Payne did extensive testing years ago and concluded that a silver dime responds best to a frequency of 2.7 kHz. Nickels and gold jewelry respond best around 16.5 kHz. All other US coins are most responsive to frequencies under 10 kHz.
With that in mind, you need to consider what coins you're most interested in finding, and how they will respond. Also, there is an ID honesty factor that has to do in part to how target segment bin widths are assigned to the different frequencies (remembering that this is an all digital machine, and programming is part of the equation). With HF coils the target segment bin widths are wider at the lower end of the scale, where the low conductors like gold and nickel are, and narrower at the high end of the scale where copper and silver are. This means that a low conducting target will be more likely to fit within a segment neatly, where a high conductor is apt to hit in two or three bins at least partially, so the low conductor hit will be solid and steady, and the high conductor will be less consistently accurate. Inversely, a 3kHz coil is exactly opposite, and will be less consistent or accurate on low conductors, and will deliver clear clean response on high conductors. The MF coils have virtually uniform bin widths from top to bottom, making them the most honest universally, and the best general purpose frequency.
This illustration shows the difference in bin widths between the three frequencies.
Lastly there is coil size and configuration.
Bigger coils will see deeper in mid to mild conditions, all else being equal, up to about 15" (I have an 18" loop for my Explorer, and I'm not convinced that it runs any deeper than a 15"). A DD coil of any frequency will do better than an equivalent size Concentric coil in highly mineralized conditions. It's compensating for a much smaller total volume due to it's looking at a narrower slice of ground. In mild soil a CC has the depth advantage, but lacks separation where there is heavy trash density. Even a DD can have separation issues where trash density is high, because separation is not just side-to-side, but also heel-to-toe. Your friend's 5x10" DD separates very little better than a 10.5" DD due to length heel-to-toe, but is depth limited by it's width in comparison to the 10.5".
Of the twelve coils available for the Xterra there is no redundancy. Four are concentric, and the rest DD.
There are five HF coils: 6" DD, 5x10" DD, 9" CC, 10.5" DD and 15" DD. the 5x10" and 10.5" are NOT waterproof, but are water resistant, meaning that they are fine in wet grass, taking the occasional wave splash, or getting hosed off.
There are four MF coils: 6" CC, 9" CC, 10.5" DD, and 15" DD. Only the 10.5" is water resistant, the rest are waterproof.
Leaving the three LF coils: 6" DD, 9" CC, and 15" DD. All are waterproof.
Personally, I'd like to see an MF DD smaller than the 10.5", and an LF DD between the 6" and 15", but that takes some lobbying. So if those are of interest to anyone, they should contact both Minelab and Coiltek to make their desires known.
I hope that this helps! If not, please feel free to ask any questions that you may have.
Also, I'd recommend that you go to the FAQFAQ pinned thread in this sub-forum to access links to videos, articles, and other info.
GL & HH!
