Metal Detector Coil Sizes and Types: DD vs Concentric vs Mono

In the DD vs concentric coil comparison, a DD coil is usually the better choice for mineralized ground and wide, even coverage, while a concentric coil offers a cone-shaped field that is easy to pinpoint and performs well in mild soil. Mono coils are mostly used on pulse induction detectors, where they give strong sensitivity and depth but more ground noise in hot soil. Coil size matters as much as type: small coils separate targets among trash, large coils cover more ground and reach deeper on larger targets.

The search coil is the part of the detector that actually "sees" the ground, and changing it can transform how a machine behaves. This guide explains how each coil type works, where it excels, and how to choose the right size and type for your hunting.

How a search coil works

A search coil contains windings of wire. On most detectors, one winding transmits an alternating or pulsed magnetic field into the ground, and another winding (or the same one, on mono coils) receives the response. The way those windings are arranged determines the shape of the detection field below the coil, which in turn affects depth, coverage, pinpointing and how the coil handles mineralized soil.

Concentric coils

A concentric coil has a transmit winding and a receive winding arranged as rings around the same center. The detection field is shaped roughly like a cone (often described as a snow cone) pointing down from the coil, strongest under the center.

• Strengths: Easy pinpointing because the strongest response is directly under the coil center. Good target ID stability and depth in mild soil.
• Weaknesses: The cone narrows with depth, so deep targets are only detected under a small central area. In mineralized ground, the coil sees a large volume of ground signal, which can cause noise and reduce depth.
• Best for: Parks, lawns and fields with mild soil; coin and jewelry hunting.

DD (double-D) coils

A DD coil has two D-shaped windings that overlap down the middle of the coil. The detection field forms where they overlap, producing a narrow, blade-like field that runs from the front to the back of the coil.

• Strengths: More even width of coverage at depth than a concentric coil of the same size. Better stability and ground handling in mineralized soil. The narrow blade can help separate targets on trashy ground.
• Weaknesses: Pinpointing takes a little practice, because the target sits beneath the centerline rather than at a single central point. Signals can be slightly broader along the front-to-back axis.
• Best for: Mineralized and variable soils, gold prospecting with VLF detectors, relic hunting, and general all-round use.

Our guide to pinpointing a target fast includes a technique for finding the target along a DD coil's centerline.

Mono coils

A mono coil uses a single winding that both transmits and receives. Mono coils are mainly used on pulse induction (PI) detectors.

• Strengths: High sensitivity and good depth, particularly on PI machines in milder ground and on beaches.
• Weaknesses: In highly mineralized ground, mono coils can be noisier than DD coils on the same PI detector. They generally do not support the kind of target discrimination a VLF with a DD or concentric coil offers.
• Best for: PI beach hunting, deep target work in mild soil, and some gold prospecting where the ground allows.

For more on the electronics behind these differences, read pulse induction vs VLF metal detectors.

DD vs concentric coil vs mono: side-by-side

Note that these are general tendencies. Coil design, frequency, the detector's electronics and the ground all influence real performance, which is why the same coil type can behave quite differently on two machines.

Coil sizes: small, standard and large

Small coils (roughly 6 inches and under)

Small coils are lighter and separate targets well in heavy trash, iron-infested homesteads, and tight spaces between rocks, roots and fences. They often handle mineralization better than larger coils and are very sensitive to small targets close to the surface. The trade-off is less ground covered per swing and less depth on larger targets.

Standard coils (roughly 8 to 11 inches)

Most detectors ship with a coil in this range. It is a balance of coverage, depth, weight and target separation that suits most sites.

Large coils (roughly 12 inches and up)

Large coils cover more ground per sweep and can detect larger targets at greater depth. They are popular on open fields, beaches and cleared prospecting ground. The trade-offs: more weight, less sensitivity to very small targets, poorer separation in trash, and often more susceptibility to ground noise and electrical interference. Examples in our catalog include the UIG MT18 large-diameter coil for compatible UIG detectors.

Elliptical coils

Elliptical coils combine the reach of a longer coil with a narrower width, making them easier to push between rocks and vegetation. Many gold-prospecting coils use this shape.

Solid vs open-web coils

Open-web (spoked) coils are lighter and pass through water and grass with less drag. Solid coils are sturdier and easier to slide over rough ground. Electronically there is little difference between them; choose based on weight, durability and terrain.

How to choose: coil selection checklist

1. What is your main ground? Mineralized or variable soil points to a DD coil. Mild soil gives you the choice.
2. How much trash? Heavy trash favors a small coil. Clean open ground favors a larger one.
3. What size are your targets? Small gold and small jewelry favor small to standard coils; large or deep targets favor large coils.
4. What detector technology? VLF detectors use DD or concentric coils; PI detectors often use mono or DD coils.
5. How long are your sessions? Weight adds up. Try the balance of a large coil before committing.
6. Is it compatible? Coils are generally model-specific. Use genuine, compatible coils and confirm fit before ordering.

A second coil is often the most cost-effective upgrade for a detector you already own. The UIG DD13, for example, is a DD coil for compatible UIG detectors. If you are unsure which coil fits your machine, contact us before ordering.

Coils on long-range and multi-system detectors

Many professional multi-system detectors ship with several sensors: a conventional search coil for close-range detection and pinpointing, plus antennas or imaging sensors for long-range or 3D modes. The principles above apply to the conventional coil. Long-range antennas and imaging sensors work differently, and their range figures are manufacturer-stated maximums under ideal conditions.

Frequently asked questions

Is a DD coil deeper than a concentric coil?

Not always. In mild soil a concentric coil can match or beat a DD of the same size on some targets. In mineralized ground the DD usually performs better because it handles ground signal more effectively and keeps a more even field at depth.

Which coil is best for gold prospecting?

For VLF gold detectors, a small to medium DD or elliptical DD coil is the most common choice because prospecting ground is often mineralized. PI prospectors use both mono and DD coils depending on ground conditions.

Does a bigger coil always find deeper targets?

Bigger coils tend to reach deeper on larger targets but are often less sensitive to small targets. For small gold or small jewelry, a larger coil can actually miss targets a smaller coil finds.

Can I use any coil on my detector?

No. Coils are designed for specific detector models and connectors. Use a genuine, compatible coil and check compatibility with the seller or manufacturer before buying.

What is a mono coil used for?

Mono coils are mainly used on pulse induction detectors for depth and sensitivity in milder ground and on beaches. In heavily mineralized ground, a DD coil on the same PI detector is often quieter.