Metal detector coils come in a variety of shapes, sizes and configurations each suited to different metal detecting conditions. Many of these coils are designed for special hunting conditions and it really is not necessary for most beginners to rush out and purchase a large collection of accessory coils. Some gurus like to show off their collection of coils and how wonderful they are.
Keep in mind that they likely only use one or two of the coils for ninety percent of their searches. With time you will come to realize that it is often purveyors and sponsored field testers who encourage owning a large variety of coils and accessories. Do not buy into their hype.
The three most common metal detector coil shapes are round, elliptical, and rectangular. The round coil is the most common coil associated with metal detectors. This shape of coil provides the best depth. An elliptical coil is oval shaped and allows the coil to get into tighter spaces, but at a slight loss of depth when compared to its round counterparts. The rectangular coils tend to be narrow on the sides and very long. These are suited to covering a larger area quickly as they are often eighteen to twenty four inches long.
In addition to shape, metal detector coils also come in a huge range of sizes from as small as four inches to more than two feet. Stock coils usually fall into the eight to ten inch range which is quite adequate for most metal detecting situations. Smaller coils can be useful in trashy areas where a large coil might have more than one piece of metal under the coil at the same time. A smaller coil also tends to be more sensitive to smaller objects that might be missed completely by a larger coil.
There are tradeoffs to using a smaller coil as the small coils typically do not detect as deep as larger coils. I say typically as there is a point at which depth benefits of a larger coil diminishes. This is usually around the eleven to twelve inch range. Coils larger than this can start to lose depth with their main advantage being coverage. However, many detectorists prefer two comfortable swings over one heavy awkward swing.
Finally, metal detector coils come in a variety of configurations. The three most common coil configurations are concentric, DD, and mono-loop. A concentric coil consists of two loops of wire inside the coil, a large outer loop and small inner loop. One coil transmits the signal while the other receives the signal. A concentric coil creates a detection zone that is shaped like a rounded cone. The deeper part of the detection field covers a smaller area. In normal ground conditions, a concentric coil provides extra depth and sensitivity.
A DD or double-d coil consists of two d-shaped loops of wire that slightly overlap on the flat edge of the loop. Again, one loop transmits while the other receives. DD coils tend to handle mineralization better than concentric coils though with a slight loss of depth. Another benefit of a DD coil is that rather than having a cone shaped detection field, a DD coil has a hot strip down the center of the coil that maintains a fairly consistent detection depth across the entire coil, requiring less overlap in coil sweeps.
A mono-loop coil consists of one wire loop that acts as both transmit and receive loop. Like the concentric coils, it is great for depth and sensitivity in mild ground conditions but can decrease in heavy mineralization. Mono-loop coils are commonly found on pulse induction metal detectors.
As mentioned previously, it is not necessary to purchase an arsenal of metal detector coils. For many detectorists, a ten inch stock coil will suffice. If conditions are trashy, a six inch coil can be helpful. There really is no reason for the average detectorist to need more than these two coils. It would be better to spend the extra money on a quality metal detector probe.