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 "antenna"
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 above.
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)
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.
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 pulsing upward.This causes
the detector to sound off.