Everyone wants to get that extra advantage with their metal detector. One of the ways to get extra out of your machine is to get a situation specific detector coil. When you have the right metal detector coil for the right job your good detecting finds can go drastically up. You wouldn't want to use racing slicks on a mud course at the track. Same applies to metal detectors. Now what kinda coil is right for me? Smaller coils are usually more sensitive and get better separation. Bigger coils get better depth but can mask close targets. Each metal detecting situation is different and certain coils have advantages different places. Most of the time the stock coil will give you the best results for that specific metal detector. Metal Detector manufacturers spend many hours trying to get the best all around coil for that detector. Right now the two main coils are the (DD) and the concentric or also named (mono loop) coil. The concentric coil puts out a snow cone-shaped field (shaped like a snow cone), the DD field is more like the "knife blade" of the concentric (shaped like an I). The DD coil separates much better and gets more even depth especially in mineralized ground.
The pictures above show the detection fields of the two coils, the concentric and DD coils. The lines show the search field penetration of the two different metal detector coils. From the picture you can see why as was said why the DD coil can separate better than the concentric or mono loop coil. One of the bad things about the DD coils is sometimes bottle caps are harder to tell from good targets. With a little practice and keen ear to the audio one can tell the tell tale signs of a cap. Usually falsing at the tip or heel of the coil is a great indicator. I personally LOVE the DD coil. I feel the advantages of depth and separation greatly outweigh the negatives of digging a few screw caps. I also will prefer a smaller coil over a bigger coil in most situations. Yes bigger coils get more depth but the main trouble I see is not the depth it is the target mashing of trashy areas. I have found more good targets with the stock coil or smaller coils. If you have any questions about metal detector coils or detectors in general please contact me. I would be more than happy to make you great deal and answer all your questions.