One of the questions I get asked most frequently from new metal detector owners is, “What type of coil should I use?” This is a great question, but a difficult one to answer since there are many variables involved. Unfortunately there is no perfect coil. A particular type may work really well in one area, but perform poorly in another. Serious detectorists are aware of this and choose their coil accordingly. Before we go into the methods used for choosing the right coil, let’s first discuss what a coil is and what role it plays in metal detection.

The coil, also referred to as a search coil, loop, head, or foot, is the device connected to the bottom of a metal detector. They are most often made of plastic and can be round, elliptical or square, and come in a variety of sizes. Some are made exclusively for use on land; others are designed to be used underwater. The coil is basically an antenna, and its primary job is to set-up, transmit and receive the electromagnetic field. As electrical current circulates through the metal windings inside this antenna, an invisible electromagnetic field is generated and flows out into the surrounding medium (i.e., air, rock, soil, etc.). Whenever this field comes into contact with a piece of metal, tiny circulating currents called “eddy currents” begin to flow across the surface of the metal object. This phenomenon causes a distortion or a power loss in the original electromagnetic field, and in turn, triggers an audible or metered response alerting the operator that he/she has just crossed over something metal. All metal detectors need some type of coil in order to function.

The way in which the wires are wound inside the coil will greatly influence its performance. The pattern of windings or “configuration” helps detectorists classify their coils. The most popular names in use today are either Monoloop or Double D. Monoloops consist of a single circular winding of wire which acts as both the transmit and receive coil. Monoloops have excellent sensitivity and depth capabilities, but may behave erratically in highly mineralized ground. Double D coils consist of two separate windings of wire that resemble two letter D’s placed back-to-back. One winding transmits, while the other receives. They do not penetrate as deeply as the monoloops, but they are able to ignore a vast majority of ground mineralization. It is for this reason that Double D coils are the preferred choice for working highly conductive or “noisy” soils, such as those found in the Australian Outback. They are stable and quiet and keep the operator from having to dig many false signals generated by both soil and hot rocks. Monoloop coils offer superior depth and sensitivity over DD coils, but are better suited for milder ground conditions, like those found in most of North America.

Now that we have a better idea of what a coil is, how it functions, and the types of configurations, we can now discuss what factors influence coil selection. There are five important factors which must be taken into consideration. These are: 1) ground mineralization, 2) terrain, 3) overburden depth, 4) relative target size, and 5) operator experience. 

Ground Mineralization: Metal detectorists often refer to highly mineralized ground as being “hot”. When they speak of the ground in this manner they are simply referring to the various conductive and magnetic properties it exhibits, not temperature. Many soils, especially those found in the goldfields, are composed of rock, soil, sand and clays that contain small grains of iron-bearing minerals. These minerals are predominately the iron oxides: magnetite, hematite, limonite, maghemite and lepidocrocite. All of these oxides exhibit a varying degree of ferromagnetism. Ferromagnetic substances can be magnetized by being exposed to another electromagnetic field, like the one produced by the coil. Any material that will allow current to flow will set up a secondary magnetic field. This often causes the detector to receive a signal from the ground itself, which may be much stronger than the signal produced from a metallic target. It is this natural property that causes the ground in highly mineralized areas to become noisy or “hot”.

If the area you are detecting causes the detector to become unstable or unusually erratic and can not be remedied by re-ground balancing, then chances are it is heavily mineralized. Soils that are stained a dark red or orange are typically noisier than those brown or grayish in color. Even though some depth penetration must be sacrificed, these areas are best worked with a Double D coil. On the other hand, if a satisfactory ground balance is achieved and the detector is running fairly smooth in the area you have selected, definitely opt for a Monoloop coil. Personally, I always try a monoloop coil first, and then if the ground proves too noisy, I switch over to a Double D.  

Terrain: By terrain I am referring to the characteristics of the land you are metal detecting. The presence or absence of brush, trees, grass, and rocks, as well as the gradient of the land will largely dictate what size and shape of coil you use. Areas that are wide open and flat with relatively few obstructions will allow us to swing a much larger diameter coil. Areas that are narrow, rocky or clogged with vegetation will force us to use a smaller diameter coil. For flat open spaces with relatively little trash, I recommend using the largest coil you can swing, such as a 16”, 20” or 25”. For tight spots such as a narrow brush filled gully, I would suggest something in the 8”, 10”, 11”, or 14” range. I also find myself choosing round coils for open ground, and elliptical coils for tight work where the going is rough. Elliptical coils are also better for getting into tight nooks & crannies such as those found beneath rocks and under cactus.  

Overburden Depth: Coils come in a wide array of sizes and will most definitely affect the performance of your metal detector. The smallest I have ever seen measured not much bigger than a doughnut, others are so massive they must be carted around on a trolley! Generally speaking, smaller diameter coils find smaller targets, but do not penetrate very deeply. Large coils will locate bigger targets at greater depths, but aren’t well suited for hunting small targets.

Small coils (i.e., 14” or less in diameter) should be used to hunt shallow areas, where the operator knows most of the targets will be fairly close to the surface (1-foot or less). Large coils (i.e., 16” or more in diameter) should be used to hunt areas where the operator knows most of the targets are likely to be deep (1-foot and beyond). I should also mention that that round coils tend to offer slightly better depth than elliptical coils, whereas elliptical coils tend to offer slightly better sensitivity. If you are having trouble determining the overburden depth in your area, I suggest you select a larger coil to ensure you are not only getting maximum penetration, but ground coverage as well. 

Target Size: A detectorist must select their coil based not only on ground mineralization, terrain, and overburden depth, but also on the relative size of the desired targets. This is especially crucial for the electronic gold prospector since gold nuggets tend to be fairly small. To give you an example, about two years ago I discovered a virgin nugget patch in the mountains of Arizona using my metal detector. After many weeks of hard work, my wife and I pulled over 1,000 nuggets off it. It was an incredible find, but almost all of the nuggets were tiny; not weighing more than 1-gram. Actually, the very first piece I unearthed which pointed me in the right direction only weighed a mere 0.2-gram. I tell this story because had I been swinging a very large coil I would have likely missed that first nugget and ultimately the patch. If a person is seeking tiny objects I always recommend choosing a smaller diameter coil. They may not penetrate as deeply as their larger relatives, but their ability to “sniff” out minute fragments of metal is unmatched. When you need maximum sensitivity, think small coils.   

Operator Experience: This category is included because the amount of detecting experience a person has, should to some extent, influence their coil selection. We have already discussed the “mineral canceling” properties of the Double D coils. They are smooth, stable, quiet and ward off excessive false signals caused by both soil and hot rocks, and are less likely to frustrate a new detector owner. It is no coincidence that most metal detectors are supplied standard with a Double D coil in the 10” to 12” range. Monoloop coils offer distinct advantages but they are more susceptible to ground noise, and therefore take a bit more practice. If you are new to the hobby, I suggest running a Double D coil during your initial break-in. Once you have got a few weeks experience under your belt, definitely consider adding a monoloop to your coil arsenal.

Many people I encounter want to swing big coils for the added depth and ground coverage, but also, well…because they look cool. Large diameter coils can and do work wonders on supposedly hunted out sites, but they are not without some disadvantages. They are more awkward to swing, somewhat heavier, and more difficult to pinpoint with. Like the monoloop coils, I suggest that a newcomer to the hobby stick with coils 14” in diameter and smaller until they have had a chance to become more familiarized with their machine. Once they are able to swing the machine for several hours without becoming overly fatigued and have mastered pinpointing; then it is time to seriously consider a larger coil.

No one search coil can do it all. What works beautifully in the deserts of Nevada will most likely flounder on a Civil War battlefield in Georgia. The coil is an integral part of any metal detector; having the potential to enhance or hamper its effectiveness. Seasoned detectorists know this, which is why they change their coils to suit their environment. They experiment often to learn what each is capable of, and typically have a collection of coils differing in both size and configuration - you should too. Time spent learning how different coils influence your metal detector’s performance will only help to increase the odds of a success hunt. I wish all of you the very best of luck with your detecting!

For additional information on metal detecting, please visit the author’s website at www.ArizonaOutback.com.

References:

Garrett, C. (1995). Modern Metal Detectors. Dallas, TX: Ram Publishing Company.

Rowan, M. and Lahr, W. (1995). How Metal Detectors Work. Sweet Home, OR: White’s Electronics, Inc.