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A little over a year ago, Minelab authorized Coiltek Manufacturing to produce the 15-inch All Terrain 7.5 kHz DD coil for the X-TERRA. Since its release, the medium frequency 15-inch X-TERRA coil has been proving its capabilities to coin-shooters and cache hunters across the globe. However, recognizing that there are many other forms of “treasure hunting”, Coiltek continued working with Minelab in an effort to develop additional coils for the X-TERRA lineup. These efforts have paid off in the form of three addition X-TERRA coils. They include a new 15-inch All Terrain Double-D at 3 kHz, a 15-inch All Terrain Double-D at 18.75 kHz and a small 6-inch Double-D at 3 kHz.
Although any of the three X-TERRA frequencies (3 kHz, 7.5 kHz and 18.75 kHz) are capable of detecting anything metal, those of us familiar with the X-TERRA recognize that certain coil characteristics provide specific benefits. In part:
- Low frequency coils are better suited for higher conductive targets.
- High frequency coils are more sensitive to lower conductive targets.
- Larger coils will detect larger targets more deeply than a smaller coil.
- Smaller coils are more sensitive to small targets.
- Concentric coils provide more accurate Target ID than a comparable size Double-D coil.
- Double-D coils separate targets better than a comparable size concentric coil.
- Low frequency coils typically detect deeper than comparable sized high frequency coils.
- 3 kHz coils are compatible with the X-TERRA 70, X-TERRA 505 and X-TERRA 705.
- 7.5 kHz coils are compatible with all X-TERRA series detectors.
- 18.75 kHz coils are compatible with the X-TERRA 50, X-TERRA 70, X-TERRA 305, X-TERRA 505 and X-TERRA 705.
The waterproof, 15-inch All Terrain Double-D coil at 3 kHz weighs 28.8 ounces. You many notice that Coiltek has introduced a series of inner spokes to the All Terrain design. These spokes provide more stability to the outer perimeter of the coil, reducing the possibility of false signals due to coil flexing. I found the 15-inch All Terrain Double-D coil at 3 kHz performed quite well in areas with moderate levels of magnetic mineralization, hitting on both copper and silver coins quite well.
The waterproof Coiltek 15-inch All Terrain Double-D coil at 18.75 kHz weighs 25.8 ounces. With frequency characteristics to be best suited for lower conductive targets, this particular coil should become extremely popular with beach hunters and prospectors as this coil is extremely sensitive to gold, jewelry and lower conductive targets. All large diameter coils cover a lot of ground with each sweep. So don’t let your sweep outpace your stride! And as with any Double-D coil, overlap your swaths by one-third to reduce the possibility of passing up targets.
The water-resistant 3 kHz “Digger” weighs a mere 12.8 ounces. At 3 kHz, it provides excellent detection of both silver and copper coins. With the inherit separation characteristics of a small Double-D coil, it is fully capable of locating those old coins in and around adjacent trash. As someone who enjoys detecting around old rural homesteads, the small size and solid bottom design allows me to “work” the coil in amongst the crop stubble and weeds, without getting hung up. It sounds off “loud and clear” on highly conductive targets such as copper and silver coins. And it provides a very distinctive broken audio on deeply buried iron. This “Digger” coil will be seeing a lot of “action” in the many places I enjoy using my X-TERRA.
You can read the full test results of the COILTEK 15” All Terrain X-TERRA 7.5kHz coil here.
It appears that the 18.75kHz coil is the best coil for searching salt water beaches for low conductivity items. Pure gold at 4.1x10+7 S/m, is only about 1/3 less electrically conductive than silver at 6.3x10+7 S/m. Relatively speaking, does this difference constitute high verses low electrical conductivity?
If I were to go with the 18.75kHz coil, for homestead coins, relics and cache hunting, what is the relative reduction in deep target locating capability, compared to the 3kHz coil, maybe 10-20%?
The 18.75kHz coils are good for locating gold, but is it less effective than the 3 or 7.5 kHz coils for locating high conductivity silver and copper?
I live in Delaware, where there is little or no gold prospecting opportunities, but I have in close proximity, the two extremes -- salt water beach hunting in highly mineralized beach sand, for gold, silver, copper coins and gold jewelry (likely less conductive than pure gold), and colonial homestead hunting in low soil mineralization, for gold, silver, copper coins, relics and (hopefully) caches. Maybe the ideal scenario would be the 18.75kHz coil for the beach and the 3kHz coil for homestead hunting???
I would greatly appreciate your thoughts/comments as to what you might recommend for me.
With those numbers in mind, your suggestion of buying both coils may be the best. Use the LF coil in places where you hunt for deep silver and copper coins, relics and caches, knowing that individual pieces of small gold jewelry will not be detected as deeply as with the HF coil. Without a doubt, for small gold jewelry on the mineralized salt beaches, the HF version will hunt deeper than the other two frequencies. I also found the HF coil to be a bit more "stable" in areas with higher levels of mineralization. However, if the budget doesn't allow for buying both and I could only chose one to begin with....I'd opt for the 3kHz coil. Although it isn't as "deep" on small gold jewelry or nickels, it offers excellent depth on copper and silver coins. Accepting what we know about target response and metallic content, the difference in depth between the three All Terrain coil frequencies became increasingly apparent as the target size increased. The LF coil really shines on those larger targets, regardless of metallic content. And, although the LF coil may not detect those smaller gold coins as deep as the HF coil, rest assured it is capable of hitting a $5 gold piece at 11-inches. And in my testing of the coils, that was within an inch and a half of what the HF coil was capable of doing.
Thanks again for the question. I hope I've been able to provide the information you need to make your decision. If not, let me know and I'll give it another shot! HH Randy