In Reply to: Do Round Coils give the Best Performance? posted by Eric Foster on May 15, 1998 at 19:30:00:

Larger coils do indeed go deeper, but the facts are a bit misleading. Larger coils will find larger objects at greater depths, but lose sensitivity to smaller objects. As a class project we built a 24 inch round coil that would reach great depths, but would not respond to a coin lying on top of the ground.

Most of the oval or rectangular coils are of the so called Double D, or widescan, construction, and while the depth pattern of the magnetic field (matrix) is more consistent from front to rear, the bowl shaped matrix does not penetrate the soil as deeply as the cone shaped signal from a round concentric design of the same approximate surface area. Some mistakenly believe the widescan coil is a deeper seeking design, but the apparent depth is only because less overlapping is required. Our experiments under controled conditions have shown that on coils of the same approximate surface area the point of the cone shape concentric pattern is 1/2 to 1 inch deeper than the pattern from an equivalent widescan coil. I'm sure there are those who will take exception to that statement but the facts are indisputable. Coils of concentric design are also more sensitive to smaller objects than are widescan designs, as evidenced by the widescans ability to handle more difficult soil conditions. Prospecting detectors use widescan designs for that reason. Another misconception is that the widescan coil only has a narrow strip down the center of the coil that is receptive to signals. As opposed to a concentric coil with both transmit and recieve windings being of circular design and approximately the same diameter, the widescans transmit and recieve windings each occupy different sides of the coil with the straight sections in the center of the coil overlapping slightly. The half of the coil with the recieve windings is receptive, not just the center overlapping sections. That is why widescans coils perform better in areas with difficult soil conditions or where there is an abundance of trash. This means there are less ground minerals or trash objects under the receptive section of a widescan coil at a given time than are under an equivalent concentric design. Hopefully this will clarify rather than futher confuse the issue of coil design.

Richard Schneyweiser, Phd.