6. How it works
• A (low frequency) metal detector basically consists of a transmitter, a receiver and a
microprocessor (control box) which handles the signals.
Transmitter
• The transmitter is a coil of wire located inside the search coil. By sending an alternating current
through this coil a magnetic field is generated, having a polarity pointing either into or out of the
ground. This magnetic field will induce a flow of current inside nearby metallic objects. These
objects in turn will create a magnetic field with a polarity which is usually opposite to that of the
magnetic field of the transmitter.
Receiver
• The receiver is another coil of wire that is located inside the search coil and which is constructed
in such a way that the magnetic field of the transmitting coil does not induce current in it.
Magnetic fields from nearby metallic objects however will produce an electric current in the
receiver coil.
Phase shift
• The signal received from the receiving coil will usually be delayed versus the transmitted signal
due to the tendency of conductors to impede changes in the flow of current, the so called
inductance. The delay is referred to as phase shift. Metal objects that are primarily inductive
e.g. large thick objects made of good conductors (gold, silver, copper…) will result in large phase
shifts, whereas metal objects that are primarily resistive e.g. smaller thin objects made of less
conductive materials will result only in minor phase shifts. Refer to the table below for a short
overview of the electrical conductivity of some materials.
• Note that detection also depends on the size, shape, depth… of the metal.
Ferromagnetic
• Substances that usually conduct very poorly or not at all can become magnetized when inside a
magnetic field, resulting in a strong signal picked up by the receiver. However, this signal shows
little or no phase shift. Soils containing small grains of iron-bearing minerals will show
ferromagnetic characteristics.
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Weight and Dimensions
