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There are a variety of searchcoil sizes and shapes and configurations. The correct one to use depends on the environment it will be used in as well as the targets being sought. A change in any one of these variables may require a different searchcoil. Sometimes changing the search coil on your metal detector is the next best thing to buying a whole new machine! It is possible that merely changing the coil on your detector will lead you to a host of new targets.

CONFIGURATIONS
In  addition  to  searchcoil  sizes  and  shapes,  there  are also a variety of searchcoil configurations available, with each proving to be  the best choice depending on hunting applications and ground conditions. The configuration of a searchcoil refers to the arrangement of the Transmit Coil (TX) and Receive Coil (RX) within the searchcoil shell. The most common configurations are: Concentric, Mono, Imaging, and Double-D.

Concentric
The concentric configuration consists of a TX coil and RX coil which are usually circular. The advantage of this configuration is that both the TX and RX coils are wound as large as possible within a given searchcoil diameter. This provides the largest possible detection fi eld and greatest detection depth, making the concentric coil potentially the most sensitive confi guration available. In addition, concentric coils also provide the most symmetrical detection field, allowing ease in pinpointing and consistency in target identification. For these reasons, they are the most commonly used searchcoil and will provide the best overall performance in most environments. Unfortunately, this configuration is the most susceptible to interference from ground minerals, which results in substantial loss of performance when used over heavily mineralized ground.

Mono
A mono-coil is available only on Pulse Induction detectors, such as Garrett's new Axiom Metal Detector, and is a variation of the concentric configuration. The mono-coil can be manufactured with the TX and RX coils located together or as a single coil acting as both TX and RX. The detection and performance characteristics of the mono are essentially the same as the concentric in that it provides the maximum possible sensitivity, but suffers some performance in mineralized ground.

Imaging
An Imaging searchcoil is an enhanced version of the concentric configuration that features an additional RX coil. This extra coil provides the detector with additional target information necessary for true target-depth perception and true target-sizing capabilities. With this additional sizing information, the detector can more fully characterize a target and for the first time distinguish between trash and good targets of the same conductivity (e.g. a quarter vs. a soda can). Only the Garrett GTI metal detector offers this technology.

DD Search coils have two smaller “D” shaped coils housed inside to penetrate heavily mineralized ground that is commonly encountered while gold prospecting and relic hunting. The  Double-D  configuration  is  designed  to significantly  reduce  ground  interference  and, thereby,  recover  the  performance  lost  by  a concentric  coil  over  mineralized  soil.  With  the Double-D, it is the arrangement of the TX and RX coils that produce a canceling effect of ground signals. This  configuration  is called  DD  because  both TX and RX coils are in the shape of a “D”. The positive detection fi eld of the DD runs beneath the overlapping center section from front-to-back. The remaining portion of the coil actually produces negative (i.e. canceling) detection fields. It is this canceling field that allows the DD coil to maintain performance over mineralized ground. Because of its small positive detection field, the DD is inherently less sensitive than a concentric searchcoil of the same size, over non-mineralized ground. The Double-D will, however, significantly outperform  the  concentric  coil  over  mineralized ground. For this reason, it is highly recommended when hunting over mineralized ground commonly found when prospecting and relic hunting.

It’s important to know the benefits of using various size, shape and configurations of coils because simply changing the search coil on your metal detector is the next best thing to buying a whole new machine! Garrett is the global leader of ground search metal detectors, pinpoints, coils, and other accessories. Browse Garrett detectors and more.

Part 1 covered search coil size, shape and depth.

The searchcoil is a vital part of your metal detector.  It generates a magnetic field and senses metallic targets in the surrounding environment. The size and depth of the magnetic field is determined by the shape and size of the searchcoil. Understanding the  purposes  behind  the  various  sizes and shapes of searchcoils will empower you  with  the  ability  to  choose  the  best coil for each application.

HOW SEARCHCOILS WORK
Searchcoils generally consist of two internal sets of coiled wires, a Transmit Coil (TX) and a Receive Coil (RX). Mono coils can be different in that one coil acts as both the TX and the RX. When the detector is turned on, the TX coil generates a magnetic field in the surrounding space. When a metallic object is within this generated magnetic field, it will create a distortion in the magnetic field. The RX coil will sense this distortion and send a signal to the control housing. A searchcoil’s detection pattern is determined by the combination of the TX’s generated field pattern and the RX’s sensing field pattern.

SEARCHCOIL DEPTH
The detection depth of a searchcoil, as a rule-of-thumb, will be approximately equal to its diameter, for a coin-sized object. However, as a searchcoil’s size increases and its field pattern becomes larger, the field pattern becomes less concentrated and begins to miss small objects. For a coin-sized object, this effect becomes noticeable when using searchcoils larger than about 15" in diameter. Since the field generated by a large searchcoil is larger, deeper and less concentrated than a small searchcoil, it is the best choice when hunting for targets that are usually large and deeply buried, such as caches or relics. there is a direct relationship between the size of a magnetic fi eld and the size of a coil. The bigger the coil, the larger the magnetic fi eld. Therefore, larger searchcoils generally detect deeper than smaller searchcoils.

SEARCHCOIL SIZES

• Smaller Search Coils (less than 6" in diameter): Great for “trashy” areas, really allowing you to sift through metal debris. These coils are best for tighter places and the smallest, shallow targets. Because the magnetic field of a small search coil is concentrated within a small volume, it is the best choice for hunting in areas with a lot of metal debris. This allows you to maneuver through and around trash to locate good targets, especially when searching in tight places where large searchcoils cannot go. In addition, because of their concentrated detection field, small searchcoils are the best for detecting very small objects. However, a small searchcoil provides less coverage per sweep; more scans will be required to cover a search area.
• Medium Search Coils (6-11" in diameter): Usually come standard with your metal detector, medium search coils are designed for general use in finding the broadest range of targets in the broadest range of environments. For general-purpose hunting, which typically includes coins and coin-sized targets, a medium sized searchcoil (8 - 9") is the best choice. Because a medium sized searchcoil provides the best combination of magnetic field concentration, detection depth and capability to detect the greatest range of target sizes within the detection area, it is standard with most detectors. In addition, it’s lightweight and easy to maneuver.
• Large Search Coils (over 11" in diameter): The larger the search coil, the deeper it will penetrate the ground. Great for finding caches or relic hunting, finding deeply buried, large targets. Large  searchcoils  generate  wider  and  deeper  magnetic  fields  than small coils and consequently provide greater depth and coverage. This means that scanning a search area requires fewer sweeps with a large searchcoil than with a smaller coil. This larger scan area, however, can become a problem in trashy areas where the search coil is detecting several targets at once.

SEARCHCOIL SHAPES
Generally, search coils are circular or elliptical in shape. An elliptical searchcoil  is  more  maneuverable  than  a  circular  searchcoil  and  its narrow width actually provides greater coverage than a circular coil due to its elongated length. However, a circular searchcoil has slightly more detection depth and sensitivity in non-mineralized soil, so it is still the most commonly used shape.

It’s important to understand the pros and cons of using various size, shape and configurations of coils because simply changing the search coil on your metal detector is the next best thing to buying a whole new machine! Most metal detectors come with a standard search coil that is good for general use – finding the broadest range of targets in the broadest range of environments. But search coils come in all shapes and sizes, and it's possible that merely changing the coil on your detector will lead you to a host of new targets in that “hunted out” area.

Garrett is the global leader of ground search metal detectors, pinpointers, coils, and other accessories. Browse Garrett detectors and more.

Part 2 — search coil configurations