I had to laugh as I recently purchased an F2 in a 3 coil package, one being the DD Coil. It has been so hot here,
I have not had the ambition to put this machine to real use yet but I did run it for a bit the other day with the DD coil and it produced out of all the melted aluminum from peoples old camp fires, some coins and yes, those rusty old battle caps showing as Quarters. Please understand that I have not spent enough time with this machine to relate the number display to specific metalic items but the segment for a Quarter did display for some of the caps I dug.
Now the only other machine I have that has an assortment of coils is a Whites GM V-Sat used for gold nugget hunting. It is strictly an ALL METAL machine but I have two DD coils and one that is a elliptical concentric. With either of these machines, the nature of the DD coil is it will sort of speaking, see much easier through the Iron content of soil than will a concentric or elliptical concentric. My experience with the elliptical concentric has been with hot rocks for example, that it will respond to those on or partially exposed to the surface. Now my two elliptical DD coils, one a standard size and the other smaller, have a tendency to respond not only to surface and partially exposed, but to hot rocks that are buried. I'm not going to go into the details of how to know a Hot and Cold rock but the DD coil has the ability to allow more power to be used in detecting without so much of it being reflecting back and overwhelming the machine than do the concentric unless the soil is mostly void of iron and then the concentric by design will go deeper.
One example I once read was you are on the road at night in the fog and a rabbit is in the road. When your lights are on dim (less power, your Gain or Sensitivity), not so much of the light reflects back off the fog (iron content) and you can see the rabbit. If you turn on your bright lights (turn up the power, your Gain or Sensitivity), more of the light is going to reflect back off the fog (the iron content) so the rabbit is going to be harder to see. The concentric are looking at a whole pie (soil with iron content) where an elliptical DD is only looking at a long slice of it.
The only explanation I can come up with on some bottle caps is, the metal content or possibly an outer micro cladding is something other than steel that evades iron rejection. Being they are food/drink related, they may even contain a low grade of stainless steel that would eventually rust. The thing about all the different machines indicators, bells , and wistles is, sooner or later, some metallic item is going to fool them all. I suspect though if a guy plays around enough with the numerical identity feature which I have not spent enough time with yet, you may see a number trend and relate it to a specific sound in relation to coil swing pass.
So far, I'm thinking the F2 is as capable a machine as any other and personally, I enjoy and mostly use elliptical DD coils more often than concentric and I don't use a hand held pin pointer even though one came with the F2. Thinking back, have been detecting since about 1982, some bottle caps have just been a part of the detecting thing and it's my belief that DD coils have the potential of discovering them more often but then I'm kind of a dig all guy anyhow. I don't care how much a person spends on a machine, they all sooner or later can be fooled and miss a good target.
