Geotech

Jason: Yes, the anti-Helmholtz is a bit like a Halbach array. I once looked into a Halbach array for a medical detector. I don't know any way to trade off near-field for far-field, as far-field always follows near-field. "There is no free lunch." Chet: I'm using FEMM which is free and there are some Youtube instructional videos. After going through it a couple of times and setting up a few "starting point" coils, I can now do a sim in about 5 minutes.

Any time you place ferrite near a pancake coil it causes the flux lines to bend more. This is why mineralized ground reduces depth, it increases the flux curvature and reduces flux density at depth. Placing a ferrite shell on top of the coil certainly does what you envision (reduces the top-side field); at the same time it increases the bottom-side flux density very close to the coil but this advantage is quickly lost and at depth the flux density is reduced. Here is an FEM plot: Here is the same coil with the ferrite set to air: It's possible the ferrite-shelled coil would be good for shallow tiny nugget detection. Another (lighter) possibility is to design an anti-Helmholtz coil, where an anti-phased bucking coil placed just above the TX coil to "focus" the field downward: Unfortunately it tends to also reduce flux density at depth compared to just a single TX coil. .