VLF & PI & BFO Technologies

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.

* 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.

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 pulsing upward.

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.


VLF Phase Shifting

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 of things:

* 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 shift.

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.

PI Technology
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.

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 echoes: 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 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.

BFO Technology
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 the frequencies.

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 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.

Buried Treasure
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
of factors:

* 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
manufacturer.

* 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.

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 nuggets
* 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.