I don't have a major difference in performance when it comes to 'audibly classifying' some problem ferrous or magnetic targets such as Bottle Caps [size=small](the crimped-on "crown caps" as they are also described)[/size] or round washers, etc. You refer to the using the 'edges' of the coils to help determine a Target ID or Target Type. that is a technique that I named "Edge-Pass Rejection", or EPR, and I have been using the EPR technique since the mid-to-late '70s and instructing EPR in my seminars or presentations since '81. It is not a new discovery that seems to be spoken of on more forums in just the more recent years.
But there are two things to consider. One is the size of the search coil used, and the other is the type of search coil used. Most search coils used are smaller to ';standard' size, and until more recent years that 'standard' would have ben 8" or so. In more recent years we have seem 10", 11" and 12" round coils or elliptical coils such as 7X11, 8X11 and now 9X12 as a 'standard' size. Most of the coils used as 'standard' today have been/are Double-D types whereas they used to be mostly Concentric designs.
As a rule, EPR will be effective and noticeable from about the edge of the search coil to perhaps about 1" inward, toward the center, of a Concentric coil, however it will most often be noted from the coil edge to about an inch or so out beyond the edge of a coil with a Double-D winding. Also consider that with a smaller to 8"/9" coil, EPR will be an effective technique to use because the EMF is a bit tighter than with a larger-size coil. Also, classifying a Bottle Cap or similar ferrous-based object is easier when it is located relatively shallow, such as surface to perhaps 3" or so, and is less effective as the coil-to-target increases. It's more difficult with a larger-size coil, especially a DD design, to get a good negative response on the ferrous-based target.
Remember, too, that using the EPR technique is a method to help classify a Bottle Cap or other ferrous-based target. A good non-ferrous target usually responds positively from dead-center f the coil's center axis out toward the edge of the coil and weakens near the coil's edge. Ferrous targets are more easily recognizable using this technique. I use two XP ORX models and they don't come with a "horseshoe" to try and assist with iron type objects. Also, you're trying to compare two different size and shape coils on their ability to classify two targets that are one being a pesky ferrous-based object and the other a non-ferrous object. The two coils will perform differently and the two objects will have a different influence on the EMF based upon their metal type.
Personally, dealing with them and a 5X9½ and 9" round coil set, both DD designs, I can understand their behavior and I don't have a problem with either of them when encountering Bottle Caps or Tabs.
Monte
