Hi Kurt, I don't know what part of Nebraska you live in. But I live in Western Iowa and the soil here is moderately mineralized as well. And, like you, I look for old coins at old sites. Personally, I like the 7.5 kHz and the 3 kHz coils better than the higher frequency coils. I have both the 18.75 DD Elliptical and the 18.75 round DD, and seldom use them. For the type of hunting I do and the places I hunt, the Elliptical coil does not achieve the depth of the concentric 9-inch coils in any frequency. And, although the 10.5-inch round DD coil does perform better (for me) than the elliptical, I don't like the additional weight at the end of the rod. Again, the mineralization here does not warrant using a DD coil for stability. Don't get me wrong. Both DD coils and higher frequency coils have their place. And many posters to this forum have made some excellent finds. But for the places I hunt and the settings I use, I prefer the LF and MF concentric coils.
Theoretically, the lower frequency coils will detect higher conductive targets better. And, higher frequency coils will detect lower conductive targets better. To a coin hunter, that means lower freqs will hit silver and copper better than higher freqs. And, for jewelry hunters, the higher freqs will do better than the lower freqs on gold and platinum. Lower freqs will go deeper on large objects and higher freqs are more sensitive to small targets. DD coils will perform better in highly mineralized soil due to the design of the windings. A concentric coil will hunt deeper than a DD coil of comparible size, if the soil mineralization does not inhibit the concentric's ability to penetrate the ground.
For coin hunting in old fields that were once picnic grounds, chautauquas, race tracks etc, I like the 3 kHz coil. I hunt in 4-tone, all metal most of the time. And I simply ignore the low tones. I am convinced that the 3 kHz coil provides me a more definite audio response to low tone iron than even the 7.5 kHz coils. And far better than the 18.75 kHz coils. This more defined audio response allows me to separate targets better and spend less time checking and rechecking those "iffy" signals. With that said, if someone took the tags off of my LF and MF coils, based on depth, sensitivity and separation characteristics, I'm not sure I could tell them apart.
JMHO HH Randy
