In general terms, classic metal detectors consist of a control unit, a shaft and a coil. The principle of operation of metal detectors is based on the creation of a magnetic field around the coil. Reflected from metal objects in the ground, the variable magnetic field is captured by the coil and transmitted to the control module of the device. Simply put, the coil is the "eyes" of the metal detector, and the control unit is the "brains" of the device.

Depending on the purpose and principle of operation, there are several types of metal detectors.

Types of metal detectors

By purpose, metal detectors are divided into two main subcategories:

  • Ground – designed to search for objects in the ground and small reservoirs that do not require complete immersion (for example, in puddles).
  • Underwater – created with the expectation of the possibility of complete immersion under water; also, such models are usually optimized for use on wet, salty soil like sand on a sea beach.

Security metal detectors are included in a separate category. Devices of this type belong to the category of pinpointers – devices of an elongated shape without pronounced coils for pinpoint search over a small area and range. Security models are designed primarily for employees of law enforcement agencies and security services – a compact pinpointer can easily "examine" a person. At the same time, traditional pinpointers are used for pinpoint search for metal objects in the ground, when the information from the metal detector coil is not enough to obtain their exact location.

Pinpointers help to determine the exact location of metal finds in the ground.

There are also metal detectors for demining. We will not discuss them in detail, since these are highly specialized equipment for military use.

In terms of signal processing methods, the most widely used are induction metal detectors, pulse induction models and devices that work on "reception and transmission".

Ground Metal Detectors

Induction models

Metal detectors operating on the principle of relatively low detection frequencies are very popular. They are based on VLF (Very Low Frequency) technology. The technology makes it possible to create fairly reliable devices capable of recognizing metals at shallow depths (15–25 cm on average, less often up to 1 m). Detectors operating on this principle easily distinguish ferrous and non-ferrous metals, as well as non-ferrous metals among themselves (gold, silver, copper, bronze, etc.)

The receiving and transmitting antennas in the design of such metal detectors can be located one inside the other – the corresponding induction coils are called concentric. Another common coil type is the DoubleD. Two elliptical frames for receiving and transmitting are mirrored with an offset along the short side. The signal in the DoubleD coils has the shape of a cylinder, which makes it possible to uniformly scan the area under the coil.

DoubleD coils offer a wider field of view than traditional concentric mono coils.

VLF metal detectors transmit analog signals to the control unit. Their more advanced variety relies on VFLEX technology, a key feature of which is the exchange of digital data between the coil and the “brains” of the metal detector. This results in higher reliability and recognition accuracy.

The principle of operation of VLF metal detectors is similar to the principle of operation of echo sounders – the signal is emitted at one point in space and received at another. That is why such metal detectors need to be diligently moved from side to side, otherwise the coil simply will not respond to the signal.

The coil must be diligently moved from side to side perpendicular to the ground.

Pulse Induction models

Pulse induction metal detectors are equipped with a combined transmitting and receiving coil, the electromagnetic field of which creates eddy currents on the surface of metal products. The principles of detecting metals with their help are tied to the analysis of damped eddy currents from targets. In operation, such metal detectors rely on the principle of PI (Pulse Induction). PI technology is considered the most suitable for adverse conditions such as highly mineralized soils or underwater searches in salt water. Ground metal detectors are usually built according to this scheme.

Metal detectors of the Pulse Induction class are aimed at searching for sufficiently large relics at medium and large depths.

The main drawback of such models is that they do not distinguish between finds by type of metal. Such metal detectors cannot distinguish a coin from a beer cork. Usually, with metal detectors of this type, rather large relics are searched for at depths of half a meter.

Pulse Induction Metal Detectors

"Reception-transmission"

There are also ground metal detectors that detect metal due to coils that are distant from each other or using a large frame with a built-in coil. The work of such devices is usually based on RF (Radio Frequency) technology, without a metal discriminator. As a result, the technology does not distinguish between ferrous and non-ferrous metals. Such detectors are designed to search for large metal objects at great depths (up to 3-6 m). But in this regard, the devices almost do not see small things, which means that coins and other objects smaller than a teaspoon are unlikely to be detected.

Choosing the Right Metal Detector

An important criterion when choosing a metal detector is the operating frequency of the device. In this case, the capabilities of the main unit are implied – the actual operating frequency is determined by the specifications of the installed coil. The general rule in this matter is this: the higher the frequency, the smaller the objects that the metal detector is able to recognize, and the smaller the detection depth. Note that most models support not a single value, but a certain range.

According to detection frequencies, metal detectors can be divided into the following groups:

  • up to 5 kHz – search for large metal objects mainly at great depths;
  • from 6 to 10 kHz – universal equipment that finds fairly small objects like coins at medium depths;
  • from 11 to 15 kHz – search for small objects at shallow depths (coins, jewelry, etc.);
  • from 15 kHz and more – search for the smallest objects (in particular, nuggets) that lie near the soil surface.

Along with the detection frequency, the shape and size of the coil plays a fundamental role in the search. So, oval-shaped models with a diameter of 9-12 inches are considered the most convenient to use – these coils are great for both beginners and experienced diggers. In the standard configuration, metal detectors often come with such coils. The depth and speed of the search also depends on the size of the coil. Small coils (5-9 inches) see small objects better, but are not able to look deep into the ground. Large coils (13-20 inches) hit deep, but do a poor job of indicating the exact location of the subject, and therefore user have to dig a larger hole.

The depth and accuracy of the search depends on the size of the coil.

As for the forms, the following coils are most often found:

  • round – indicate the location of the find with high accuracy, but are sensitive to mineralized soils, and the small width of the captured area requires significant time and effort for search at large areas;
  • oval or elliptical – elongated coils provide a somewhat "stretched" electromagnetic field; on the one hand, this expands the size of the captured area, on the other hand, it slightly reduces the detection accuracy;
  • rectangular – typical for models with PI and RF technologies;
  • butterfly – such coils consist of two ellipses, partially combined and resembling butterfly wings; DoubleD coils are produced in a similar form.

To choose the right metal detector, it is important to decide what and where you plan to dig. To search for artifacts from the Second World War, any ground model will do. If the priority is to search for coins, treasures and non-ferrous metals, you will need a metal detector with a good discriminator. And if you plan to dig in the ground, you need to look at models with the function of ground balance. However, let's not get ahead of ourselves and first analyze the main functions of metal detectors.

Metal Detectors with Discriminator

Functions of metal detectors

So, the function of paramount importance for searches in littered areas is the metal discriminator. Each type of metal has its own conductivity, which is determined by the discriminator. With it, the VLF metal detector can be set up for targeted search for gold, silver, copper, etc. At the same time, the discriminator will help to avoid beer caps, aluminium lugs from cans, foil from cigarette packs, nails, wire and other rubbish.

In metal detectors with a discriminator, specialized search programs are often implemented: jewelry, coins, gold, etc. Note that the identification function works correctly only under the condition of constant movement of the coil with a fairly high intensity. It is harmoniously complemented by the Pin-Point mode, used to clarify the location of the find. The coil in this mode effectively responds to metal objects even when stationary, and objects in the ground can be localized with an accuracy of a couple of centimeters. As a result, you do not have to dig up extra volumes of soil, taking out a find. Discrimination in "sniper" mode does not work.

Often, all modern metal detectors have a display, and some models also have a display with backlit – very useful equipment for a night detective. In advanced search equipment, the display serves to visualize the depth of the dig, detection settings by type of metal, etc. Low-cost metal detectors can be devoid of a display; instead, a pointer indicator or sound indication is used with a built-in speaker. By the way, in models with tone identification, the display is usually not provided at all. But by the tone of the sound signal, you can roughly estimate the nature of the find.

Headphones help to determine the nature of the find by sound.

The VDI / Target ID feature helps to more accurately determine the type of metal. The option implies the display of a numerical value for a certain class of metal. According to data from specialized tables, it is possible to significantly narrow the range of possible objects found.

Ground metal detectors often cannot do without the function of ground balance. However, not every model has it. Correct balance is a prerequisite for effective search. It is advisable to perform it at least once every half an hour – the humidity and composition of soils are constantly changing, and balance allows to quickly adapt to the search conditions in a particular area.

In many models of metal detectors, ground balance can be set both manually and automatically.

The ground balance option can be manual, automatic and combined. Automatic balance, in turn, can be preset or tracking.

Interesting fact. Wet soils have better conductivity, which means that the coil “pierces” deeper and, accordingly, there will be more finds. That is why spring and autumn are considered the best times to dig.

Anti-interference compensates for the effect of the “background” from power lines or radio transmitters, interference from nearby electric motors, other metal detectors, etc. This option is especially important when searching at a high sensitivity level. As for the sensitivity adjustment, it is also often made a separate setting.

Metal Detectors with Anti-interference

Additional accessories

In a quiet place, the beeping of the speaker of the metal detector can be heard very far away. In order not to draw too much attention, you should take a closer look at models with headphone output. It is desirable that the connector is standard – a 3.5 mm audio jack. Then the digger will be unlimited in the choice of headphones. Some models of metal detectors support wireless connection of headphones via Bluetooth. The same short-range communication module can be used to pair the metal detector with a smartphone.

An important comfort factor during long searches with a metal detector is hand support through the armrest. Models with this additional point of support reduce the load on the hand and make operation more comfortable.

The armrest provides a convenient grip of the metal detector and its comfortable holding during long searches.

When searching for valuables in low light conditions or in the dark, a flashlight will come in handy. Usually, metal detectors are equipped with LED light sources, which can be turned off.

Autonomous operation of metal detectors is provided by AA-batteries or “finger-type” batteries, batteries of the “PP3” type, proprietary batteries. The latter option is usually found in the advanced device segment. And if such a battery is non-removable, then buying a spare battery will not work. You can stock up on non chargeable batteries for the future, changing them literally on the go. However, you will need a lot of them (on average 6-8 in the case of "finger-type" batteries).

Some useful tips for working with a metal detector

Suppose you picked up and bought a metal detector. Where should you start? First of all, study the instructions for the device – it contains the most important information. Then it is recommended to listen to how the metal detector sounds on different objects. Dig a small hole and alternately place various metal things in it: a couple of rusty nails, coins from different alloys, gold items, rings, beer caps. So you will learn to recognize the finds by sound. But keep in mind that objects sound worse underground than in an open or freshly dug hole – dense ground gives a different sound picture.

The choice of a place for digging is a topic for a separate detailed article. Often, the location is pre-selected according to old maps, on which you can find roads and buildings that have sunk into oblivion. Riverbeds are also considered good places. Upon arrival at the proposed excavation site, it is recommended to divide the search area into separate sectors. Walking thoughtlessly around the area is a pointless task, because you can miss something really valuable.

Before you go for treasure hunt, you need to select a location and divide the area into separate sectors.

At first, it is better to dig everything, whether it is a “ferrous” sound or a “nonferrous” one. At least, you will fill your hand and learn to hear metals, as a maximum, you will find values even among ferrous metals. When you hear a signal about a find, do not rush to dig right away. Move the coil from all sides, determine the perimeter and add 15-20% of the total area in reserve – this will save the find from damage by a shovel. If you can't find anything, don't despair. The signal could be phantom or the find lies deeper, so try lowering the coil into the hole or scanning the soil with a pinpointer.

With special care it is necessary to carry out excavations in places of former hostilities. Dangerous finds from the World War II and military conflicts of later periods are, unfortunately, not uncommon. Projectiles and other detonating subjects are still in the ground. Also pay attention to prohibitions – digging in protected areas is prohibited by law.

Sometimes dangerous objects are found in the ground. It is not recommended to extract them on your own.

Finally, it is necessary to mention the professional feature of every respected digger – dug holes must be covered. After all, an open pit is not so much ugly as it is dangerous. A person who accidentally enters into it can twist or even break a leg.


Good luck with your digging and have more interesting finds!