How does a metal detector work?

🔵 Of what parts does it consist?

🔵 How does a search coil work?

🔵 What is a VLF, PI, BFO detector?

How does a metal detector work? The principle of metal detectors was discovered as early as the end of the 19th century. In 1937 the first patent was taken. It, like many other inventions, has sprung from military needs. It took off at the beginning of the Second World War when the military wanted to search for mines.

Metal detectors were used on a larger scale for the first time in the battle of El-Alamein. Searching for mines with a metal detector instead of more manual methods greatly improved search results. After World War II, many military detectors were sold out. It became the start of the metal detector hobby we see today. The original principle for a metal detector is the same today.

How does a metal detector work today

Today’s metal detectors are advanced in their electronics part, but the principle of how a metal detector works is quite simple.

1. A current is sent through a coil that generates a magnetic field.

2. When a metal object enters the magnetic field, an electric current is induced (so-called Eddy Current) in the object.

3.These currents generate an electromagnetic field around the object. A second coil reads the changes in the magnetic field.

How does a metal detector work curret send and recive

Different metals have different resistance and inductance. Circuits in the electronics section, phase modulators, compare these different values with the average value for different metals. The electronics section then calculates what is in the magnetic field. What metal it is, how deep it is, and so on.

How does a metal detector work

What does the electronics box contain?

Most detectors today have a display on the electronics section that displays a variety of information. It’s not just info about what you’ve found and how deep it is. There is also information about battery level, memory settings, volume, sensitivity, etc. The electronics box also contains batteries and headphone jacks.

Metal Detector Control box

The electronics part from different manufacturers and different models has varying degrees of water resistance. Some electronics boxes are entirely waterproof, and some models have no moisture or rain protection at all. It is good to investigate when buying a detector. There are simple rain covers that you can buy for your detector. Moisture from a little drizzle or dew is no problem. Most good detectors are durable. Just watch out if you are searching in deep water, so you don’t soak the whole electronics part.

Here are two typical displays. One from Teknetics Eurotek Pro and one from Garrett Ace 200 series.

Metal detectors with no display

Other detectors have a more traditional panel with knobs and buttons. Some control boxes have no display at all but rely entirely on sound as a signal source. 

How does the metal detector search coil work?

The search coil is the sensitive part that usually determines how well a detector works under different conditions. There are mainly two types of search coils among the usual VLF detectors on the market: concentric and double-D search coil. There is also a third variant called mono loop, but it is not as common.

Double-D: The shape of this type of search coil looks like two letters D combined, so-called double-D coil or DD coil. The double-D coil reduces the mineralization reaction and increases the detection depth. On the other hand, the detection area under the loop is smaller than the concentric coil. It is usually slightly more expensive than a concentric search coil.

Concentric Search Coil: Just as the name suggests, this type of coil is concentric, and the advantage of this design is to increase the detection range. Larger coils usually provide better depth and a broader sweep area. Then you can cover more land faster.

On many metal detectors, you can replace your search coil with different models depending on where you are and how you are searching. It is not uncommon to have both two and three search coils to switch between.

Search coils in different sizes

There are coils in many different sizes. For example, if you want to get in between stones, trees and grass stumps, you can use a small search coil. If, on the other hand, you walk on smooth, even ground, you can use a larger coil and thus covers more ground. The coil is usually waterproof and can be lowered a bit into the water. If you go in a few decimeters of water, most common metal detectors are waterproof. But check the manual before dipping it in water. There are also various solutions for how the magnetic field is generated in the coil.

A conventional concentric VLF coil operates as follows:

How does a metal detector work search coil

Sending coil – This is the outer coil loop. Inside the ring, there is an electric coil. Electricity is sent along this wire, first in one direction and then in the other direction, thousands of times every second. The number of times the current direction changes every second determines the frequency of the detector.

Receiver Coil – The inner coil loop contains another wire coil. This coil acts as an antenna to pick up and amplify frequencies coming from the object in the ground. The signal is then sent to the metal detector’s electronics box, where the electronics analyze and process the signal. The signal is then displayed as graphic images, numbers or sounds. Most often, more than one indication is used to show what you have found, how deep it is, etc.

The difference between VLF, PI, BFO detectors?

Metal detectors usually use one of three different basic techniques:

Very Low Frequency (VLF, Very Low Frequency)

Pulse Induction (PI, Pulse Induction)

Beat-Frequency Oscillator (BFO)

VFL Metal Detector

 ✅VLF is the most popular detector technology used today. VLF detectors usually operate at a frequency in the range of 3kHz – 30kHz. Works as described above with two coils, one transmitting and one receiving. VLF is today the most common metal detector and the one that works best as an all-round detector. VLF technology is a good detector that meets most detector needs. A slightly more expensive variant is a multi-frequency detector. It works with several frequencies simultaneously and does a much better job at, e.g. saltwater beaches have better depth and discrimination.

PI Metal Detector

✅PI or Pulse Induction works a little different than a VLF detector. In a PI detector, you only have a single coil that works as both transmitter and receiver. There are also PI detectors that have multiple loops that work synchronously. The coil sends pulses of electricity to the ground creating a magnetic field. When the pulses hit a metal object, the magnetic field is reversed, and the signal is sent to the electronics. PI detectors are generally more expensive than VLF detectors but very good for use on mineralized lands, such as saltwater beaches.

Finds treasures and finds gold

A PI detector can see tiny metal objects in the ground, smaller than a VLF detector can. On the other hand, it will indicate all the time if it is an area with a lot of metal traces in the ground. This sensitivity can make it challenging to use in many places. Unlike the VLF detectors, they have more difficulties in distinguishing (discrimination) between different types of metal and are usually heavier. Other disadvantages of PI detectors are that they draw a lot of battery and are more expensive than VLF detectors. But if you want to search on saltwater beaches, PI technology is what works best. Even when diving, it is the most popular technique. Most commonly gold is found in mineralized soil, so PI detectors are popular as gold detectors and can find gold and metal deep in the ground.

BFO Metal Detector

✅ BFO Beat-Frequency Oscillator metal detectors are the simplest type of metal detectors. A metal detector that you can quickly build yourself. It uses two oscillators that both produce a radio frequency. An oscillator uses a wire coil located in the search coil. The second oscillator uses a small wire coil, and it usually sits inside the control box. The loop in the control box is a reference oscillator. The oscillators are adjusted so that their frequencies are very close to each other. When metal comes close to the search coil, the different frequencies of the coils results in the sound change.

Best use for different detectors?

VLF detectors are the most common and widely used technology. Only because it covers most of the all-around search, it is also the cheapest technology. If it is detecting inland, a good VLF detector includes everything you want to search for. Whether it is a park or a ploughed field, a VLF detector is perfectly adequate. On freshwater beaches, it is also no problem to use a conventional VLF detector. 

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How does a metal detector work on Saltwater Beaches?

If you want to search on saltwater beaches, it demands a little more of the detector. As long as you search way up on the beach, on dry sand, a VLF detector can work. But if you get further down to the water, it becomes more difficult for a single frequency VLF detector. This is where most of the finds are made, in the waterline or a bit out in the water. By adjusting the ground balance and the sensitivity, you can improve the signal quality from the detector even as you approach the saltwater. That is if your detector has adjustable ground balance. Many have preset or automatic ground balance.

Multifrequency and PI detectors

If you plan to search a lot on saltwater beaches, the best options are a multi-frequency detector or a PI detector. Both models are more expensive than regular VLF detectors. But if you want the best, it’s the one that applies. Multifrequency detectors work with multiple frequencies simultaneously. Thus, they can eliminate the effects of mineralization in the soil. But they are expensive and perhaps most for the pros. The PI detectors are not affected by the high mineralization in saltwater beaches. A PI detector is ready from the start to operate on a saltwater beach. It detects deep but has an imperfect separation between the metals. But if saltwater beaches are a significant part of your detector time, it is the obvious choice. 

You want to find GOLD

To find gold, roughly the same conditions apply to saltwater beaches. I can refer to a previous page about finding GOLD with a metal detector.