jasong

One other design I'm curious about is a flat, single layer spiral coil wound with variable winding spacing. No spacing between windings at the center leading to more and more widely spaced windings towards the rim. I might be envisioning it wrong, but it seems like denser winding spacing towards the center and less dense towards the rim may have some kind of flux density shaping effect leaning towards emphasizing more flux at depth (vs a standard coil) directly under the coil, at sacrifice of flux towards the edge of the coil and shallow? But yeah, I totally get the no free lunch. Most of physics can be derived from basic thermodynamic principals, I try to always look at systems from a conservation of energy viewpoint.

Thanks yeah that was what I was envisioning. I figured there would be a definite increase in flux density for near surface (small nugget) detection. Wasn't clear on if there might be deeper effects. The density increase also seems to be kinda "too near" the surface to be useful for general use coils, but for crumbing/dinking coils it might be interesting. That anti-Helmholtz configuration is interesting. Haven't looked at that before, but that seems kinda like achieving similar affects to the Halbach array. That's too bad the flux decreases at depth compared to a single TX though. It definitely seems possible to increase the small/shallow target response. I wonder if there exists some strange or clever design that can trade decreased flux density towards the surface for increased flux density at depth? (well, a design that is actually usable by a human arm anyways 😅)

I'm envisioning the opposite of a ferrite core, more like an exterior casing to concentrate flux such that the amount of magnetic field energy is maximized towards the ground and minimized towards open air. Since half of the field of a coil radiates upwards and thus is not particularly useful for detecting, it seems to make sense to put it where it's more useful - if possible? Here is a cutaway diagram. Would this not have any benefit, at least for near surface flux density concentration? At the field strengths of PI machines I'd guess something like fairly thin 24 gauge steel would be more than enough, saturation is probably ok since the goal would just be some improvement, not capturing all the upwards flux. Is there a way to use that other half of the "wasted" (using that term loosely) magnetic field energy more usefully downwards if this doesn't work? Sort of like the idea behind a Halbach Array, except whatever the EM version would be? Weight alone doesn't have to be a deal breaker. For instance if a hypothetical 12" coil with flux shaping components weighed the same as a 20" standard coil, can it be made to obtain similar depths? If so, then it'd be worth it due to maneuverability with all other things being equal.