I used both Double-D coils back then as well as Co-planer / Concentric types, and it is usually a lot easier to use 'EPR' techniques with a Concentric coil, and especially easier with a smaller-size coil. Something in the 5" to 7" diameter. The DD coils often need to be worked a bit more 'off-the-edge' of the coil while a Concentric type can often start to respond within an inch in from the outer edge

Also, the problems with DD coils are two fold. One is that they might require working the coil more to the front or to the rear to get the EPR effect, plus many DD coils for todays detectors are bigger-size. In the 9" to 11" diameter, and in an iron plagued site, such as a lot of bottle caps in old picnic groves, etc., those bigger coils might get worked back or forward and encounter one or more other pesky Bottle Caps and defeat the ability to audibly classify that problem trash.

Most places where Bottle Caps are encountered, they are either on or near the surface or down to maybe 2"-3", and with the right detector and coil and technique, I can audibly classify ±90% of those I encounter.

Monte

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