The words "concentric" and "DD" (or "double-D") refer to the type of your metal detector's internal coil construction. Most searchcoils (whether round or elliptical) are of concentric construction. A concentric searchcoil has a large transmitter coil, and a smaller receiver coil in the center, usually in the same plane. This coil arrangement is relatively easy to manufacture and its symmetry helps to minimize electrical drift due to time and temperature. It also provides good discrimination on shallow targets.

Some searchcoils (whether round or elliptical) are of DD construction. Double-D's comprise two overlapping D-shaped coils of approximately the same size, one being the transmitter and the other the receiver. The advantages of the DD are greater depth in mineralized soil, a broad sweep pattern, and narrower target response. Its primary disadvantages from a user's point of view are multiple responses on shallow targets and poor discrimination of flat iron objects. Designing and manufacturing them is more difficult because their lack of radial symmetry makes them prone to drift which the design and the manufacturing process must minimize. Manufacturing cost is higher because the coils cannot be wound on high-speed winding equipment.

Double-D's are usually preferred for relic hunting and gold prospecting. Concentrics are usually preferred when searching for modern coins in an area where there is also iron and aluminum trash.

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A highbanker is basically a sluice box with mobility and added height. It is mounted on a 4-legged stand that gives the sluice box the correct slope. Instead of being put right in the creek like a sluice, an engine with a water pump and some hoses transports the water up from the stream into the highbanker. Highbankers are also called power sluices. It is a self-contained unit with many uses— prospecting, sampling, and concentrate clean up.

In general, a highbanker or power sluice is extremely efficient at trapping gold, including very fine or "flour" gold. The biggest benefit of having this piece of gold prospecting equipment is the ability to process gold-bearing gravels located a distance away from a water source— in other words, you get to bring the water to the gravel, not the gravel to the water! You can purchase a highbanker with a pump/motor and all hoses and fittings included, or you can buy those items separately.

To run a highbanker, you need a system to pump water to it. Highbanker pumps can be either electric (12 volt battery) or gas driven. Some highbankers are also designed and constructed to re-circulate water so they can be employed in situations where little water is available. Keep in mind that the pumps deliver less water as your equipment location is moved uphill. Pumping uphill increases the resistance on the pump, and as you move uphill, eventually there is a point where the pump will not provide sufficient water to run the high banker. The slope on the highbanker usually ranges from about three to four inches per foot.  Article continued here...