Mention the words metal detector and you'll get completely
different reactions from different people. For instance, some people think
of combing a beach in search of coins or buried treasure. Other people
think of airport security, or the handheld scanners at a concert or
sporting event. If you work in construction, you might think about looking
for buried cables or pipes.
Photo courtesy Bounty Hunter Metal detectors can provide hours of enjoyment
when you use them to search for buried treasure.
The fact is that all of these scenarios are valid. Metal-detector
technology is a huge part of our lives, with a range of uses that spans
from leisure to work to safety. The metal detectors in airports, office
buildings, schools, government agencies and prisons help ensure that no
one is bringing a weapon onto the premises. Consumer-oriented metal
detectors provide millions of people around the world with an opportunity
to discover hidden treasures (along with lots of junk).
Learn about metal detectors and the various technologies they use. Our
focus will be on consumer metal detectors, but most of the information
also applies to mounted detection systems, like the ones used in airports,
as well as handheld security scanners.
Anatomy of a Metal Detector
A typical metal detector is light-weight and consists of just a few parts:
Stabilizer (optional) - used to keep the unit steady as you
sweep it back and forth
Control box - contains the circuitry, controls, speaker,
batteries and the microprocessor
Shaft - connects the control box and the coil; often
adjustable so you can set it at a comfortable level for your height
Search coil - the part that actually senses the metal; also
known as the "search head," "loop" or
Photo courtesy Garrett Electronics Garrett GTI 1500 metal detector
Most systems also have a jack for connecting headphones, and
some have the control box below the shaft and a small display unit
Operating a metal detector is simple. Once you turn the unit on, you
move slowly over the area you wish to search. In most cases, you sweep the
coil (search head) back and forth over the ground in front of you. When
you pass it over a target object, an audible signal occurs. More advanced
metal detectors provide displays that pinpoint the type of metal it has
detected and how deep in the ground the target object is located.
Metal detectors use one of three technologies:
Very low frequency (VLF)
Pulse induction (PI)
Beat-frequency oscillation (BFO)
In the following sections, we will look at each of these technologies
in detail to see how they work.
A Discriminating Taste Very low frequency (VLF), also known as induction balance,
is probably the most popular detector technology in use today. In a VLF
metal detector, there are two distinct coils:
Transmitter coil - This is the outer coil loop. Within it is
a coil of wire. Electricity is sent along this wire, first in one
direction and then in the other, thousands of times each second. The
number of times that the current's direction switches each second
establishes the frequency of the unit. For example, some models have a
frequency of 6.6 kilohertz (KHz). This means that the current changes
direction 6,600 times per second.
Receiver coil - This inner coil loop contains another coil of
wire. This wire acts as an antenna to pick up and amplify frequencies
coming from target objects in the ground.
Photo courtesy Bounty Hunter This LandRanger metal detector from Bounty
Hunter uses VLF.
The current moving through the transmitter coil creates an
electromagnetic field, which is like what happens in an electric motor.
The polarity of the magnetic field is perpendicular to the coil of wire.
Each time the current changes direction, the polarity of the magnetic
field changes. This means that if the coil of wire is parallel to the
ground, the magnetic field is constantly pushing down into the ground and
then pulling back out of it.
As the magnetic field pulses back and forth into the ground, it
interacts with any conductive objects it encounters, causing them to
generate weak magnetic fields of their own. The polarity of the object's
magnetic field is directly opposite the transmitter coil's magnetic field.
If the transmitter coil's field is pulsing downward, the object's field is
The animation above demonstrates VLF technology.
The receiver coil is completely shielded from the magnetic field
generated by the transmitter coil. However, it is not shielded from
magnetic fields coming from objects in the ground. Therefore, when the
receiver coil passes over an object giving off a magnetic field, a small
electric current travels through the coil. This current oscillates at the
same frequency as the object's magnetic field. The coil amplifies the
frequency and sends it to the control box of the metal detector, where
sensors analyze the signal.
The metal detector can determine approximately how deep the object is
buried based on the strength of the magnetic field it generates. The
closer to the surface an object is, the stronger the magnetic field picked
up by the receiver coil and the stronger the electric current generated.
The farther below the surface, the weaker the field. Beyond a certain
depth, the object's field is so weak at the surface that it is
undetectable by the receiver coil.
How does a VLF metal detector distinguish between different metals? It
relies on a phenomenon known as phase shifting. Phase shift is the
difference in timing between the transmitter coil's frequency and the
frequency of the target object. This discrepancy can result from a couple
Inductance - An object that conducts electricity easily (is
inductive) is slow to react to changes in the current. You can think
of inductance as a deep river: Change the amount of water flowing into
the river and it takes some time before you see a difference.
Resistance - An object that does not conduct electricity
easily (is resistive) is quick to react to changes in the current.
Using our water analogy, resistance would be a small, shallow stream:
Change the amount of water flowing into the stream and you notice a
drop in the water level very quickly.
Basically, this means that an object with high inductance is going to
have a larger phase shift, because it takes longer to alter its magnetic
field. An object with high resistance is going to have a smaller phase
Phase shift provides VLF-based metal detectors with a capability called
discrimination. Since most metals vary in both inductance and
resistance, a VLF metal detector examines the amount of phase shift, using
a pair of electronic circuits called phase demodulators, and
compares it with the average for a particular type of metal. The detector
then notifies you with an audible tone or visual indicator as to what
range of metals the object is likely to be in.
Many metal detectors even allow you to filter out (discriminate)
objects above a certain phase-shift level. Usually, you can set the level
of phase shift that is filtered, generally by adjusting a knob that
increases or decreases the threshold. Another discrimination feature of
VLF detectors is called notching. Essentially, a notch is a
discrimination filter for a particular segment of phase shift. The
detector will not only alert you to objects above this segment, as normal
discrimination would, but also to objects below it.
Advanced detectors even allow you to program multiple notches. For
example, you could set the detector to disregard objects that have a phase
shift comparable to a soda-can tab or a small nail. The disadvantage of
discrimination and notching is that many valuable items might be filtered
out because their phase shift is similar to that of "junk." But,
if you know that you are looking for a specific type of object, these
features can be extremely useful.
A Quickened Pulse
A less common form of metal detector is based on pulse induction
(PI). Unlike VLF, PI systems may use a single coil as both transmitter and
receiver, or they may have two or even three coils working together. This
technology sends powerful, short bursts (pulses) of current through a coil
of wire. Each pulse generates a brief magnetic field. When the pulse ends,
the magnetic field reverses polarity and collapses very suddenly,
resulting in a sharp electrical spike. This spike lasts a few microseconds
(millionths of a second) and causes another current to run through the
coil. This current is called the reflected pulse and is extremely
short, lasting only about 30 microseconds. Another pulse is then sent and
the process repeats. A typical PI-based metal detector sends about 100
pulses per second, but the number can vary greatly based on the
manufacturer and model, ranging from a couple of dozen pulses per second
to over a thousand.
Photo courtesy Garrett Electronics This Garrett metal detector uses pulse
If the metal detector is over a metal object, the pulse creates an
opposite magnetic field in the object. When the pulse's magnetic field
collapses, causing the reflected pulse, the magnetic field of the object
makes it take longer for the reflected pulse to completely disappear. This
process works something like echos: If you yell in a room with only a few
hard surfaces, you probably hear only a very brief echo, or you may not
hear one at all; but if you yell in a room with a lot of hard surfaces,
the echo lasts longer. In a PI metal detector, the magnetic fields from
target objects add their "echo" to the reflected pulse, making
it last a fraction longer than it would without them.
A sampling circuit in the metal detector is set to monitor the
length of the reflected pulse. By comparing it to the expected length, the
circuit can determine if another magnetic field has caused the reflected
pulse to take longer to decay. If the decay of the reflected pulse takes
more than a few microseconds longer than normal, there is probably a metal
object interfering with it.
The animation above demonstrates PI technology.
The sampling circuit sends the tiny, weak signals that it monitors to a
device call an integrator. The integrator reads the signals from
the sampling circuit, amplifying and converting them to direct current (DC).The
direct current's voltage is connected to an audio circuit, where it is
changed into a tone that the metal detector uses to indicate that a target
object has been found.
PI-based detectors are not very good at discrimination because the
reflected-pulse length of various metals are not easily separated.
However, they are useful in many situations in which VLF-based metal
detectors would have difficulty, such as in areas that have highly
conductive material in the soil or general environment. A good example of
such a situation is salt-water exploration. Also, PI-based systems can
often detect metal much deeper in the ground than other systems.
A Steady Beat
The most basic way to detect metal uses a technology called beat-frequency
oscillator (BFO). In a BFO system, there are two coils of wire. One
large coil is in the search head, and a smaller coil is located inside the
control box. Each coil is connected to an oscillator that generates
thousands of pulses of current per second. The frequency of these pulses
is slightly offset between the two coils.
As the pulses travel through each coil, the coil generates radio waves.
A tiny receiver within the control box picks up the radio waves and
creates an audible series of tones (beats) based on the difference between
If the coil in the search head passes over a metal object, the magnetic
field caused by the current flowing through the coil creates a magnetic
field around the object. The object's magnetic field interferes with the
frequency of the radio waves generated by the search-head coil. As the
frequency deviates from the frequency of the coil in the control box, the
audible beats change in duration and tone.
The animation above demonstrates BFO technology.
The simplicity of BFO-based systems allows them to be manufactured and
sold for a very low cost. You can even make one at home following the
instructions on this page. But these detectors do not provide the level of
control and accuracy provided by VLF or PI systems.
Metal detectors are great for finding buried objects. But typically, the
object must be within a foot or so of the surface for the detector to find
it. Most detectors have a normal maximum depth somewhere between 8 and 12
inches (20 and 30 centimeters). The exact depth varies based on a number
The type of metal detector - The technology used for detection is a
major factor in the capability of the detector. Also, there are
variations and additional features that differentiate detectors that
use the same technology. For example, some VLF detectors use higher
frequencies than others, while some provide larger or smaller coils.
Plus, the sensor and amplification technology can vary between
manufacturers and even between models offered by the same
The type of metal in the object - Some metals, such as iron,
create stronger magnetic fields than others.
The size of the object - A dime is much harder to detect at deep
levels than a quarter.
The makeup of the soil - Certain minerals are natural conductors and
can seriously interfere with the metal detector.
The object's halo - When certain types of metal objects have
been in the ground for a long time, they can actually increase the
conductivity of the soil around them.
Interference from other objects - This can be items in the ground,
such as pipes or cables, or items above ground, like power lines.
Photo courtesy Bounty Hunter The control unit for a Bounty Hunter Tracker IV
Hobbyist metal detecting is a fascinating world with several
sub-groups. Here are some of the more popular activities:
Coin shooting - looking for coins after a major event, such
as a ball game or concert, or just searching for old coins in general
Prospecting - searching for valuable metals, such as gold
Relic hunting - searching for items of historical value, such
as weapons used in the U.S. Civil War
Treasure hunting - researching and trying to find caches of
gold, silver or anything else rumored to have been hidden somewhere
Many metal-detector enthusiasts join local or national clubs that
provide tips and tricks for hunting. Some of these clubs even sponsor
organized treasure hunts or other outings for their members. Check out
LostTreasure.com for more information on clubs.
In addition to recreational use, metal detectors serve a wide range of
utilitarian functions. Mounted detectors usually use some variation of PI
technology, while many of the basic handheld scanners are BFO-based.
Photo courtesy Garrett Electronics A Garrett walk-through metal detector
Some nonrecreational applications for metal detectors are:
Airport security - screen people before allowing access to
the boarding area and the plane
Building security - screen people entering a particular
building, such as a school, office or prison
Event security - screen people entering a sporting event,
concert or other large gathering of people
Item recovery - help someone search for a lost item, such as
a piece of jewelry
Construction - locate pipes and cables underground
Archaeological exploration - find metallic items of
Geological research - detect the metallic composition of soil
or rock formations
Photo courtesy Garrett Electronics A Garrett Super Scanner handheld metal detector
Manufacturers of metal detectors are constantly tuning the process to
make their products more accurate, more sensitive and more versatile. On
the next page, you will find links to the manufacturers, as well as clubs
and more information on metal detecting as a hobby.