I imagine I need to figure this out for myself - maybe it's different for each coil but I notice that the deeper the coin the more I miss it when I dig. Shallow? I can probe it right away. Six inches or more and I often miss it. Where is the sweet spot on a DD coil in your experience?
Sent from my Pixel 2 XL using Tapatalk
If a coin is not flat then the pinpointing will be off based on the angle of the coin and the depth. Think of the face of the coin like a spotlight (Bat signal!). If the coin is at a 45 degree angle and only 1 inch deep then it won't make any noticeable difference. If it's 8 inches deep then it will pinpoint off by a few inches.
But, to answer your question about sweet spot, remember that you're using a DD coil and not a concentric coil. The detection field of a standard sized DD coil is like a butcher clever with an inch thick blade running from the back to the front. Or, an inch thick rectangular pane of glass. It has maximum depth almost all the way from the back to the front. Very little depth at the sides of the coil.
That pane of glass or blade has a middle, and it's just forward of where the shaft meets the coil, but there's not much use in thinking of it as a "sweet spot". Not in the same way a concentric coil has a depth/sensitivity sweet spot in the center bottom of the detection field shaped like an upside down cone.
Pinpointing with a DD coil is all about using that "blade" (or pane of glass if you prefer) to find the edges of the target and then centering the retrieval on that shape.
With an AT Pro coil a typical unmasked coin target should appear and disappear at the tips of the triangle shaped holes at the front and back of the coil. That's where the detection field "blade" starts and ends, and at near maximum depth. (You can think of the bottom edge of the blade as being a bit curved or rounded off at the front and back edges.)
In clean ground, if you can hear the target in one part of the detection field you should be able to hear it all the way from front to back. If you can't, then it's nearby discriminated out metal masking it and not a lack of depth or loss of sensitivity away from the middle.
To determine where to dig I
focus on a spot on the ground where I think the target is located. Then, as I waggle/swing and pull back or push forward I don't follow the coil with my eyes. I focus on my spot and see if the blade passing through it and the audio sync up. If it doesn't, I adjust the point on the ground based on what I saw and waggle/swing again. When the blade passes through the spot and audio all match up, then I've got my spot. I only do a 90 degree check if I'm trying to determine the separation between two or more nearby signals.
Visualizing the shape of the detection field is the key to getting the most out of the DD design, such as teasing out partially masked targets. You can find partially masked deep targets using just the toe of the coil--the front of the blade--or even the heel.
