Search coils for metal detectors: advantages and disadvantages

The search coil is the “eye” of a metal detector: it largely determines how deep the detector reaches, how well it detects small objects and how smoothly it runs on difficult ground. There are several types of coil, each with its own advantages and disadvantages.

1. Concentric coils

Concentric coils consist of two or three round windings placed one inside the other. The outer winding transmits the electromagnetic field, the inner one receives the signal reflected by a metal object.

Advantages

• High sensitivity: the field is concentrated in a cone under the centre of the coil – small objects are detected well.
• Precise pinpointing: the target is usually right under the centre of the coil.
• Good separation of objects lying close together on clean ground.

Disadvantages

• More prone to interference on mineralised ground: on iron-rich or salty ground they run less smoothly.
• Narrower search path: the field becomes narrower with depth – you have to overlap your sweeps more.

2. Double-D coils (DD coils)

DD coils consist of two overlapping D-shaped windings. The search field is a narrow strip that runs along the whole length of the coil.

Advantages

• Stable on difficult ground: they work much more steadily on heavily mineralised fields and on saltwater beaches.
• Wide search path: the field covers the whole length of the coil – you cover more ground with each sweep.
• Even depth across the whole width of the path.

Disadvantages

• Pinpointing is a little harder: because of the wider path the target is not always right under the centre – a pinpointer helps.
• Weight: DD coils are often slightly heavier than concentric coils of the same size.

3. Mono coils

Mono coils have a single winding that alternately transmits and receives. They are mainly used on pulse induction (PI) detectors, which work with short magnetic pulses rather than a fixed search frequency.

Advantages

• Great depth: especially for large and deep objects.
• Unaffected by mineralisation: ideal for goldfields and extremely mineralised ground.

Disadvantages

• Hardly any discrimination: PI detectors with a mono coil usually also report iron – you dig more junk.
• Lower sensitivity to very small objects close to the surface.

4. Elliptical coils

Elliptical coils are concentric or DD coils in an elongated, oval shape.

Advantages

• Agile: ideal between stones, in dense vegetation and on uneven ground.
• Good separation of objects lying close together, e.g. among iron junk.

Disadvantages

• Slightly less depth than a round coil with the same area.

Frequency and coil

On VLF and multi-frequency detectors, the frequency determines which objects the detector responds to best:

• Low frequencies (approx. 3–7 kHz): good for large and deep objects and for highly conductive metals such as silver and copper coins; less sensitive to very small objects.
• Medium frequencies (approx. 7–15 kHz): a good compromise between depth and sensitivity – the classic all-round range for coins, jewellery and relics.
• High frequencies (from approx. 15 kHz): very sensitive to small objects such as gold flakes, thin coins and small items; in return, less depth on large objects.

Multi-frequency detectors transmit several frequencies at once and so combine the advantages – more on this in multi-frequency in detail.

Summary

Which coil is right depends on the terrain, the ground and the objects you are looking for: concentric coils work precisely and suit clean ground, DD coils bring stability and a wide search path on mineralised ground, mono coils on PI detectors deliver great depth on goldfields, and elliptical coils are the agile choice for dense terrain. You will find coils for your detector under search coils; our search coil guide explains which coil fits which detector.