Types of Coils

Some of the basic coil types:

The most common description for metal detector coils are: concentric, elliptical, and Double D.

Concentric Coil
A concentric coil is probably the most commonly used coil. Different manufacturers refer to these coils with different names, however a concentric coil is a concentric coil! These coils are round, some being very thin in height, some thicker, some with open holes in the center, making them shaped like a flattened donut, and some are solid.  A few of the manufacturers have migrated into designing their concentric coils into a design which has caused them to be referred to as “spider coils” because the bracing of the coil’s structure makes the overall appearance of the coil to resemble a spider’s web.

Elliptical Coil
This style of coil also gets its name from the shape of the coil. Instead of the round shape of the concentric coil, it is merely has elliptical shape. Now if the coils are different only in shape, you are probably thinking why would one want to buy an elliptical coil over the concentric coil?

Let’s assume we have a 8”concentric coil (or round coil). Let’s also assume that the coil covers a path of 8” wide each time the coil is passed over the ground in the sweeping motion. Now, let us also assume the coil weighs 12 ounces.

Overhead view of a concentric coil

By making the coil less in width and longer in length the coil will become elliptical in shape. Lets suppose the 8” concentric coil mentioned above were to be reshaped by decreasing its width by one inch and increasing the length of the coil by 1”.  The coil would be elliptical in design, and would be, let’s say, a 9” x 7” coil. By doing so, the coil has increased the amount of ground it will cover per sweep of the coil by one inch.  We have thus an elliptical coil with the same size windings as was in the 8” concentric coil, and the same weight as the 8” concentric coil, but have a bonus of 1” more ground cover per sweep. Had the coil width been decreased by 2”, the length could have been increased by 2”, making the coil an elliptical 10” x “6” coil, thus creating a coil with two inches more ground coverage per sweep of the coil, with the same overall weight of the 8” coil. (The quoted measurements of the coil were just for an illustration, not exact measurements.)
The major advantage of the coil is the ground coverage versus the weight, the major disadvantage is that it usually takes a little more getting accustomed to the coil to achieve the precise pinpointing of the target as compared to the concentric style.

Overhead view of an elliptical coil

Concentric Spider Coil
A concentric spider coil has the same windings features as the concentric coil mentioned previously. The major difference between the two is appearance. However, it is my personal opinion that the spider coils seem to be a little more sturdier, and when bumping the coil into small twigs and objects while in the woods relic hunting, the amount of false signals are quite a few less than the normal concentric coil. I think this might be because the windings are held in place within the coil a little better since most of them have the windings imbedded in a layer of epoxy. This is just my personal opinion.

Overhead view of a Concentric Spider Coil

Overhead view of the windings of the Double D coil