Coil Basics

Steve in PR

Well-Known Member
There are many types of metal detector coils. Every manufacturer either makes their own or buys them from another detector company.

Basically, their construction can be a strand of fine copper wires that are arranged into various designs: round, rectangular, elliptical and square. There are some offshoots for the round design such as the concentric, double-D and spider style.

The technology has evolved since the first experiments conducted by Alexander Graham Bell, the inventor of the telephone, in the 1880’s.

At first, the idea was to locate land mines and the Germans began to make a serious study in the 1930’s.

The design is fairly basic. There are 2 loops of wires or solid copper, one larger than the other. One is used as a transmitter and the other as a receiver. They are enclosed in a plastic, non-conductive or epoxy housing.

The receiver coil might be smaller than the transmitter or nearly the same size, depending on the type of coil. The idea was to create an electro-magnetic field that would bounce off of a metal object in the ground and the receiver will detect the bounced signal and send it to the control box for analysis. This is very similar to the sonar devices you have seen the Navy use to detect submarines, except the Navy uses sound waves.

In my research for this article, there seems to be 2 major concerns: how much ground you want to cover versus how to counteract mineralization in the ground.

For example, an elliptical or rectangular coil will allow you to sweep over more area than a round coil. Some rectangular coils can be as long as 18".

The other side of the coin is ground minerals. Ground mineralization is when there is a high enough presence of iron or other crude minerals in the ground to create false signals. Double-D coils seem to have the upper hand in these conditions. Recent detectors have automatic ground balancing built into the circuitry, eliminating a major headache for people in some areas.

The sensitivity of the coil is determined by the circuitry that you are using. Very Low Frequency (VLF), dual-frequency, multiple or broadband frequency and pulse-induction are the major categories. Most VLF, dual and multiple frequency coils use low frequency signals and will detect well between 6 to 8" deep. Pulse-induction will stretch your depth to 12" but each has its advantages and disadvantages.

I will cover this in more detail another article.