Most search-coils are round, but some are elliptical. This refers to the overall shape, not to the type of coil construction. In general, elliptical coils provide a broader sweep pattern over the ground, and narrower target response for better pinpointing. Round coils are easier to design and less expensive to manufacture, which is why they’re the most common.
Search-coil type of construction
Words “concentric” and “DD” (or “double-D”) refer to the type of internal coil construction.Most search-coils (whether round or elliptical) are of concentric construction. A concentric search-coil 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 its temperature.
It also provides good discrimination on shallow targets. Some searches (whether round or elliptical) are of DD construction . Double D’s comprise two overlapping D-shaped coils of approximately the same size, one 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 targets responds. Its primary disadvantages from a user’s point of view are multiple responds on shallow targets and poor discrimination of flat iron objects. designing and manufacturing them prone to drift which the design and manufacturing process must minimize.
Manufacturing cost is higher because the coils cannot be would on high-speed winding equipment.
Search coil size
Most standard search-coils are approximately 8 inches (20 cm ) in diameter if round,or approximately 10 inches(25cm in length if elliptical.
Lager search-coils allow covering more area with each sweep and offer a slight increase in depth on medium and large size targets. Unfortunately they are heavier, more difficult to pinpoint with, then to lose small targets, and provide poor target separation.
Small search-coils provide superior target separation (important in trashy areas) and the ability to detect smaller targets (important in gold prospecting). Of course they don’t cober as much ground as a standard size coil.
However (and this may surprise you) small search-coils usually have nearly as much depth capability as standard size search-coils.
What’s on the market and why
The least expensive metal detectors usually come equipped with a round concentric search-coil. The more expensive recent models often come equipped with an elliptical and/or DD search-coil.
Older models, even expensive ones, frequently don’t have a DD search-coil available because DD’s fell out of favor during the 1980′s and 1990′s as the knowledge of how to make DD’s did not advance fast enough to keep up with with the demands of higher performance circuit designs.
Nowadays there seems to be a trend toward DD’s as manufacturers have gained more confidence in their ability to design and make them.
What users tend to prefer
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.