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Guide to Metal Detectors
Types of Detectors
Equipment and Accessories
General Setup
General Troubleshooting
Using Your Metal Detector
Glossary
 

 RadioShack Guide to Metal Detectors

Types of Metal Detectors

There are three main types of metal detectors sold through RadioShack:
BFO Metal Detector Beat-frequency oscillation is the most basic type of metal detector. The BFO type or (two-button system -- off/on/tuning and volume control) transmits a beat frequency radio signal that is balanced by tuning the detector until a slight sound is heard through the speaker or headphones. This type of detector uses two coils: one large coil in the search head and a smaller coil in the control box. Each coil is connected to an oscillator which generates a steady frequency, with each coil having a slightly different frequency. This difference in frequencies will cause a steady "beat" which can be picked up by the receiver in the control box. Any metal or mineral in range of the signal will interfere with the frequency in the search coil, causing a change in the duration and tone of the beat frequency.

Very low frequency metal detectors also use two coils (a transmitter and a receiver). However, in a VLF metal detector, both coils are in the search head, which has an outer and inner loop. The outer loop contains the transmitter coil, and the inner loop contains the receiver coil. Alternating current is passed through the transmitter coil, with the frequency corresponding to the number of times per second that the current changes direction. This current generates a magnetic field perpendicular to the direction of the current, which changes polarity as the current changes direction. If the coil is parallel to the ground, the magnetic field will move down and up as the current changes direction. This magnetic field will cause any conductive objects in range to generate magnetic fields in the opposite direction of the transmitter's magnetic field. The receiver coil, which is designed to ignore the transmitter's magnetic field, will pick up the secondary magnetic field and relay the signal to the control box, which interprets the signal for depth and type of metal. The depth is based on the strength of the secondary magnetic field; the type of metal is determined by using phase demodulators to analyze the difference in frequencies (phase shift, or phase response) caused by differences in resistance and inductance. This determination is called discrimination; many metal detectors allow you to discriminate (or filter out) objects above a given phase shift. When there are set filters for given segments of phase shift, it is called notching. VLF Metal Detector
Advanced detectors can also, depending on circuitry, show type of object, approximate depth, coin denomination, etc. but when detecting it is best to keep your eyes on where you are walking and depend on your ears for the different sounds from the detector. Over time the human ear usually becomes a more accurate detector than meters or displays.

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