3khz or 7.5 for Coins?

[DaveVanP]

I am considering getting an additional coil for my XT705 Gold. It currently has the 5x10 18.75 khz coil. I seem to understand that lower frequencies are best for high conductors (silver/copper), and less sensitive to ferrous. I have a Fisher machine that operates at 7.8, so I was thinking getting a 3 khz coil for the 705 would cover all my bases...

Any experience out there using the 705 at this freq? Recommendations?

 

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[Professor of Engineering]

The 3K Hz coil is fairly deep and hits hard on higher conductors. I like the 18K Hz for jewelry in parks and play areas.

 

[DaveVanP]

About 12 years ago, our town suffered a massive flood, which inundated a refinery, and areas downstream were contaminated with oil in the floodwater. The EPA declared the ground unsuitable for residences, and the refinery made good and bought the land from the homeowners, and paid for moving the houses, or relocating the owners. Now we have nearly two-dozen city blocks (about 20% of the city limits) of vacant land that I have obtained permission to search. The neighborhoods were all built from about 1900, with very little new construction after 1970. Since the houses were demolished - some literally knocked down or bulldozed, there is a lot of iron scrap - nails, iron/lead pipe, and other junk. Afterwards, the ground was leveled, basements filled, etc. The iron clutter is not too dense, but does tend to be concentrated in "patches". My interest is mostly in recovering coins, and was leaning toward getting the 3 khz coil, to better differentiate silver/copper targets from iron. I was considering a 9" concentric, as the soil here is mainly low-mineralized, loamy "riverbottom" land. A larger 15" 3 khz is interesting, but wasn't sure if the added coverage and depth would be any advantage to outweigh slightly less sensitivity to small coins. Before laying out close to $200 (or more) for a new coil, I'd like to get some recommendations.

 

[Jason in Enid]

The lower freq will be better for any kind of round conductor like a coin, especially a silver one. It wont be a huge difference from a 7khz but there is at least a small improvement. For coil design, DD is far superior for high trash density areas, and smaller coils obviously have an even better performance but at the cost of a little bit of depth. Concentric coils are deeper, but that max depth comes at a very small point, which means you have to move forward at a snails pace to make real use of it. Concentrics also have a much harder time in trash and mineralized soil.