The size of a coil can influence the detection depth and sensitivity of a detector. The larger the coil, the deeper it tends to detect, but will have less sensitivity to small targets. The smaller the diameter, the more sensitive it becomes, but with reduced detection depth. Smaller coils will also be lighter in weight, easier to control and may be chosen for their ability to negotiate difficult terrain or undergrowth. Smaller coils also have an advantage in areas of high trash, due to their smaller detection footprint.
The most common coil shapes are the conventional solid Round coils, the Elliptical shaped coils and the Open-web coils. The main reason for the change in shape is to conform to a physical requirement, i.e. the elliptical coil can be pushed around bushes or between stony country easier than round coils. Open-web coils help cut through water when detecting in water and are lighter for their size. Conventional round coils are often more stable, higher performing, and are particularly popular when gold prospecting.
The three most common types of coil windings are Concentric, Double-D (DD), and Monoloop. The difference between these coil types is the pattern the wire is wound within the coil.
A Concentric coil has an inner circle and an outer circle wire winding. Its search pattern is cone shaped and can be useful for accurately pinpointing the target. Concentric coils tend to be noisier in highly mineralized ground and require more overlap of sweep for thorough ground coverage.
Double-D (DD) Coil
A Double-D coil has two overlapping wire windings in the shape of two D’s (one reversed). The characteristics of a Double-D coil are stability (especially in heavily mineralized ground), good depth, sensitivity and a very thorough search pattern.
When used with a GPX detector Double-D coils (as opposed to monoloop coils) are able to discriminate between ferrous and non-ferrous targets (when Iron Reject is activated). They are also more stable when used on wet salt beach sand and in electrically noisy environments.
Monoloop coils are a special style of coil for Pulse Induction (PI) technology detectors (SD & GPX Series). These coils have one winding of wire around the circumference of the coil, which is used to both transmit and receive. The signal pattern of the Monoloop coil is cone shaped, requiring more overlapping. In extremely heavily mineralised grounds they can be more difficult to ground balance, however they tend to provide slightly better depth than the Double-D coils.