Information on How a Metal Detector Works

Metal detecting is practiced by millions of enthusiasts worldwide.

Metal detection is a popular pastime in the contemporary world. Once only an activity of the rich, the modern availability of cheap high quality metal detection equipment means that millions of people practice the hobby globally. Metal detectors send signals into the ground to search for metal objects. A metal detector makes a sound if a metal object is detected.

Radio Wave Transmission

Basically metal detection involves the transmission and reception of radio wave signals. A metal detector consists of: a battery which acts as the power supply; a transmitter electronic oscillator which generates a radio signal; an antenna which transmits the signal and receives the returning signal; a sampling circuit and an integrator which produces the sound. The battery powers the oscillator which produces a radio signal, this signal current travels from the transmitter oscillator through a wire to the antenna which is located at the base of the metal detector. The antenna receives the returning signal from the ground. The returning signal is measured by the sampling circuit and the integrator produces a sound expressing the measured signal.

Electromagnetic Field Generation

As the radio signal current circulates through the antenna of the metal detector, an electromagnetic field is created which flows out in all directions. When the magnetic field moves across a metal object the magnetic field is interrupted. The metal object interacts with the signal being produced by the metal detector and produces its own magnetic field. This in turn produces an opposite current in the coil which induces a signal indicating that metal is present.

Iron Mineral Magnetic Field Distortion

Iron minerals are non-conductive minerals which redistribute the lines of the electromagnetic field being generated by the detector. This upsets the balance of the transmitter and receiver, basically interrupting the signal current and hindering the accurate detection of metal. Good metal detectors alert the user to the presence of iron minerals. The detection of iron minerals is a major problem for the users and manufacturers of metal detectors alike.

Target Size

The size of the metal target determines the success of detection. Larger targets can be detected more easily and at deeper distances than smaller targets because they interrupt the magnetic field more substantially and thus send back a greater number of return currents. An object with twice the surface area of another will produce twice the detection signal of the smaller object, but this does not mean it can necessarily be detected from twice as far as the smaller object; the detection signal loses strength rapidly over distance.

Pulse Induction Detectors

Pulse induction detectors differ from standard modern metal detectors. Pulse induction metal detectors transmit repeated pulses toward the ground which generates an answering pulse from the target object. The pulse is measured by a sampling circuit which sends it to an integrator which generates a sound. From the characteristics of the measured pulse it can be determined whether the object reflecting the pulse is metal or not. Unlike the other types of detector, pulse induction detectors are capable of ignoring mineralization in the ground and conductive salts. However PI detectors cannot discriminate between different types of metal.

References

• How Metal Detectors Work: How They Work
• Molecular Expessions: How a Metal Detector Works
• Hobby Hour: Metal Detectors