You have to keep in mind that there is no "absolute" answer for all the differences, only "generalities" when comparing the typical <EM>Concentric</EM> wound coils and those that have a <EM>Double-D</EM> (Widescan) design.
First, a quick comment on the commonly used references as stated in your post:
<STRONG>"I had just read somewhere that <FONT COLOR="#ff0000"><STRONG>normal</STRONG></FONT> coil, has <FONT COLOR="#ff0000">signal going into soil like <STRONG>triangle</STRONG></FONT>, the deeper, the less coverage. But is it so for <FONT COLOR="#ff0000"><SMALL>DD</SMALL></FONT> coil? I mean, the deeper, the less coverage?"</STRONG>
• By <EM>"normal coil"</EM> we have the reference to a concentric wound coil. Generally, the Transmit and Receive windings share a similar plane, with one winding placed near the outer edge of the coil and the other a smaller-size winding. These can be configured in round or elliptical shaped coils, open or solid.
• The terms <EM>DD,</EM> <EM>Double-D,</EM> and <EM>widescan</EM> all mean the same. They refer to a search coil design where the Transmit and Receive windings are configured somewhat in the shape of the letter 'D' and a reversed letter 'D' with the straighter sides somewhat curved and overlapping down the center of the search coil. As with a concentric wound coil, they can be configured in either a round or elliptical shape, solid or open.
• The Concentric coil <EM><STRONG>does not </STRONG></EM>have a signal that goes into the ground shaped like a triangle. All too often we see some sort of diagram or sketch depicting a triangular shape area beneath a search coil, usually shown as a "side view." If you visualized this from above the coil a sketch would be more like a "cone-shaped" area, larger near the surface closest to the coil, and constricting as it reached the farthest, narrowest end.
While this IS NOT the shape of the transmitted signal, it <EM>is</EM> a rough concept of the <span style="background-color:#ffff00;"><STRONG>responsive area </STRONG></span>of the generated electromagnetic field created by the search coil windings.
So to answer your question, basically yes, the deeper or farther from the search coil, the smaller the effective portion of the electromagnetic field might be responsive.
• With a Double-D coil, if viewed from the side, you picture a more <EM>responsive area</EM> towards the front and rear from the center of the search coil's generated field. This responsive area runs front-to-rear along the center line of the search coil where the two inner windings are overlapping.
• Common misconceptions regarding the Double-D coil often lead to poor hunting technique afield. Too often a novice becomes easily convinced, through such scetchings or from word-of-mouth, that the effective or responsive area of the field runs to the full "depth" front-to-rear. Due to this misbelief an operator will frequenetly fail to overlap adequately.
Yes, the Double-D coil has a more effective front-to-rear coverage area, but <EM>not</EM> to it's fullest detection depth potential. Dealing in generalities, which you have to do with such a discussion, the Double-D coils has some strenghts in the way of coverage on the side-to-side sweep, but most especially for shallower coins. Those, for example, in the 3" range, and possibly bore with a larger diameter coil.
The uniform shape of the Concentric coil usually has a strength when it comes to accurate pinpointing, as well as sizing and shaping a target.
As for the depth of detection concern, there are too many variables. Sizes of the coils compared. Maker of the coils compared. Ground mineralization. Target size, shape, alloy content and position. And very important is the particular detector the coil in question is affixed to!
Usually, a Double-D coil is considered to provide a little better coverage on the side-to-side sweep, and possibly have a little better ground handling ability in higher mineralized environments.
A Concentric coil requires more overlapping but usually provides a little better depth of detection compared with a similar size D-D coil, and pinpointing is easier to accomplish.
It is always a good idea to have more than one coil for your detector to enhance versatility, and with both concentric and widescan coils available, you might end up with more than two coils for your detector! D)
<EM><STRONG>Monte
