A
Anonymous
Guest
Some of my earlier comments regarding large coils have elicited some questions, which I feel might be best answered here. Where circumstances permit, larger coils DO detect deeper. HOW much deeper is dependant on operating mode, gain setting, soil mineralization, soil moisture content, the coil's internal configuration, discrimination setting (if any) and target size. Since all of those factors are inter-related, frankly there are really no "simple" answers. At best, we can speak in generalities. Generally, a detector goes deeper in the all-metal mode of operation. Thus, if you are in an area where targets are few, and far between, yet very deep (plowed fields are a good example) then all-metal operation can give you greater depth of detection then would teh discrimination mode. This practice works well on very older (Colonial, Civil War) farm sites, including those which have grown over with new growth timber. Also, higher power settings can give greater depth, however, the degree of soil mineralization MAY influence the optimum gain setting permissable. Where soil mineralization is low, high levels of gain can be employed. Where the soil mineralization is very high, a severe reduction in gain may be necessary to maintain stable operation. Regrettably, I face this situation in the Mojave Desert. Since larger searchcoils "see" more ground, they naturally "see" more mineralization. The detector's circuitry perceives "more" mineralization as "higher" mineralization, thus reducing the maximum permissable gain. For example, here is a hypothetical case: with a 10" concentric coil, you might be able to employ a gain setting of 10. With a 12.5" concentric coil, you might be able to use a gain of 8. With a 15" coil, a gain of 6. These are all hypothetical figures, not intended to imply any existing machines or coils, but I think you get the point. The real issue is, Would a 15" coil operating at a gain of 6 get more usable depth then the 10" coil operating at a gain of 10? Maybe. But then again, maybe not. High soil mineralization is the real "depth killer" here. This is the "point of diminishing return" that I lightly touched on in a previous post. You don't necessarily get TWICE the depth with a 15" coil as you would with a 7.5" one; likewise, you won't necessarily get 50% more depth from that 15" coil as you would from the 10" one. Look at it this way: you might get a dime to 8" with the 10" coil, optimally tuned, but with the 15" coil, optimally tuned, you might get the dime to 9, maybe 10, inches. Yes, you did get a depth increase, but not by a factor of 50%; more like 15% to 20%. In fact, you might not even get that. How so? I really like DD coils for their ability to reject iron better then concentrics, and, that they handle "bad" ground better then concentrics. But, this is at a price: depth. When operated at the same gain setting, concentric coils of comparable diameter get better depth than DD's. Thus, in many cases, a 10" concentric MAY get the same, perhaps better, depth as does a 12.5" DD. It all depends on the (several) factors involved. So, it is best to keep an open mind on such matters, and, depend on your own testing, in your own soil, under your own conditions, to make your final decision on what coil to use. An expensive, and time-consuming, practice, to be sure. But well worth it if you want to get the truly maximum depth from your detector. HH jim